Assembler is the most obvious language that you'll consider using as you probably won't need to buy any other tools to use it. More than likely the manufacturer will provide an assembler for the chip and you won't need any books as all the instructions are in the datasheet and you just start coding.
This route to programming is very easy but you may be setting yourself up for problems later on as:
It is trivial to write short programs.
It becomes progressively more difficult to write large programs.
It seems to be the best option as it gives the fastest code.
It seems to be the best option as it gives the smallest code.
Assembler: Fast & small code
There is no doubt that assembler gives the fastest and most optimized code (your brain is better at optimization than any compiler!) but assembler is difficult – typically you'll spend ten times as long writing assembler as you would writing in a high level language.
Assembler / Compiler Trade off
This is the trade off; to write the fastest most optimized code or to get the task solved more quickly.
Another problem with assembler is that to do even the most trivial task you have to think about every aspect of the code and all implications on registers and register flags.
Even making a microcontroller perform the most trivial task is difficult e.g. for making a loop in assembler you need to think about which register to use and which instructions all the while thinking about how those registers should not interact with the loop register/other registers etc.
Assembler: Problem - changing the target
Another difficulty is when you change from one microcontroller to another (even in the same device family) the assembler instructions may be changed e.g. more instructions to improve microcontroller performance. So you will have to learn an entirely different instruction set when moving either to a new target microcontroller or moving to a different device within the same family i.e. code re-use is not possible unless you stay with one microcontroller (or devices with a similar internal architecture).
High level languages
HLL: Retarget
Retargeting code to another microcontroller is easier since the HLL will know the details of the new target i.e. instruction set, fuses etc. All you need to worry about is the specific differences between the different microcontrollers (in the same family this will be setting up the internal peripherals).
The important point is that the HLL takes care of the assembler code needed to do the job.
HLL: Easy to understand.
The most useful aspect of a high level language (HLL) is that the language is written in a form you can easily understand – there are no cryptic assembler commands that you have to remember and most commands are made up of several machine code instructions – saving you coding effort (often there are built in libraries of code e.g. LCD driver, Serial port driver, I2C driver etc
So the HLL makes it easy to write code as it generates the correct assembler for the target microcontroller.
HLL : Whitespace
You can also make use of white space (areas of no code) to separate out the various operations within the program – typically assembler code is just one great big list that is really very difficult to read – I know there are comments but you need to comment almost every line so that someone else can
understand the code.
HLL: Task splitting
One of the best features of a HLL is that you can split tasks into separate functions that you can concentrate on them individually (as the HLL takes care of local variables etc.). For assembler even when using a call instruction you have to take care of preserving the register state – in the HLL it's all done
for you.
HLL: Code re-use
Once you learn the HLL you will find it easy to read code written by other people and you will be able to re-use code that you have already written whereas with assembler you will constantly need to analyze the code to see if it fits in with your new functions.
The only decision then is which high level language? There are really three contenders BASIC, C and Pascal – these are the most popular languages and for popular microcontrollers there will be an HLL compiler for each one. I'll just list the advantages and disadvantages of each
BASIC Advantages
Very easy to learn and use.
A BASIC compiler will produce code that runs fast as a C compiler.
Many in built functions (depending on compiler).
Very popular – large user base with many example programs.
BASIC Disadvantages
Non standard language.
If using an interpreted HLL will run very slowly.
Note: Because the language is not standardized it will be difficult to move code to a new processor target type.
Pascal Advantages
Easy to learn and use.
A Pascal compiler will produce code that runs fast as a C compiler.
Many in built functions (depending on compiler).
Pascal Disadvantages
Not as popular as C – so not as many compilers.
A bit wordy – it was originally intended as a teaching language.
Not as flexible as C.
C Advantages
Compiled language - always runs fast.
Standardized language (ANSI)- easier to port to different compilers / target devices.
Many compilers available.
Many in built functions (depending on compiler).
Very popular – large user base with many example programs.
Used in many different industries.
Usable at the hardware level as well as higher abstraction levels (although C++ is better for very abstracted programming models).
C Disadvantages
Hard to learn at first.
Strong type checking means you spend time pleasing the compiler (although this protects you from making errors).
You can find more information from the website here and how to build a website like it here.
There are three ways to program a PIC microcontroller
Using normal programming hardware (high volt programming HVP).
Low volt programming (LVP).
Bootloading.
The first two methods use the programming port of the PIC microcontroller labeled ICSP (In Circuit Serial Programming).
This port is shared between the existing pins of the microcontroller and after programming the pins revert back to normal microcontroller operation.
Note: To make ICSP work correctly you have to consider the effects and requirements of the ICSP programmer e.g. for HVP a high voltage exists at the Vpp pin (your circuit must be able to handle the high voltage - up to 13V). Also the loading for the other signals PGC and PGD must not be too high i.e. don't put an LED on these pins that uses 20mA - if you did the voltage levels would not be high enough at the inputs to the PIC for programming.
It's fairly easy to design for ICSP use by using isolation resistors to normal circuitry and choosing not to use heavy loads on these pins.
ICSP provides 6 connections from the pic ICSP programmer to your board as follows :
VPP - (or MCLRn) Programming voltage (usually 13V).
Vcc - Power (usually 5V).
GND Ground (zero volts).
PGD - Data usual port and connection RB7.
PGC - Clock usual port and connection RB6.
PGM - LVP enable usual port and connection RB3/RB4.
PIC Micro: High Volt Programming
To use the first method a hardware interface is needed or 'PIC programmer' to interface between the programming software (usually running on the PC) and the PIC chip. This hardware takes its information from the PC via one of three interfaces either:
The RS232 COM port
The Parallel port
The USB port
You choose the interface you want to use and then choose an appropriate PIC programmer. The PC then communicates with the hardware generating the serial (ICSP) signals to translate the PIC hex file into a serial data stream suitable for the target microcontroller.
Note: Almost all PIC microcontrollers use the ICSP interface so once you have a HVP you can program virtually any PIC microcontroller. e.g. you can program 12F675, 16F84, 16F88, 16F877(A), 18F2550, 18F452 etc.
There are several programs for programming PIC micos e.g. ICPROG and many different hardware programmers.
PIC Micro: Low volt programming (LVP)
LVP is exactly the same as HVP except:
The Vpp voltage is set to the normal supply voltage.
The PGM pin indicates programming mode.
Note: In this mode you can not use the PGM pin for anything else it is dedicated solely to LVP control.
Devices are manufactured with PGM mode enabled and the only way to turn off the PGM mode is to program it using an HVP programmer.
Note: Some PIC microcontrollers can only use the HVP method since for the LVP method you have to sacrifice one pin - PGM - (to tell the PIC Micro either that it is being programmed (high volts e.g. 5V) or that it is not being programmed (0V) ) and some PIC micros only have 8 pins e.g. 12F675. For this chip the PGM pin is not available so HVP is the only way.
The real benefit of using the LVP mode is that you can program several PIC Micros on a board without having to individually program each one - you could daisy chain each extra micro to a master micro which would then program each one in turn - and this is only possible since the Vpp signal is a normal logic level in LVP mode.
PIC Micro: Bootloading
Bootloading uses any available interface to load a program into program memory. It requires a bootstrap program to interpret the interface data and translate it into program memory instructions.
Note: Note only the newer devices that are capable of programming their own memory can use this method.
Typically a serial port is used for bootloading and the PIC micro bootstrap program will wait for a set time after power up listening on the serial port for a reserved word that tells the bootstrap program to start i.e. it listens for sequence of characters that is not normally used on the interface
Once it receives this sequence it enters bootstrap mode where a hex file is transmitted to the microcontroller over the interface. It interprets this and programs the memory of the microcontroller and then starts the program.
There are two issues with this method:
You have to program the bootstrap code using HVP or LVP.
It uses up some of the microcontroller resources.
Once programed it provides a convenient way of using the device as you won't need programming hardware anymore and one major benefit is that you can re-program a device without undoing the equipment e.g. if you boxed up you project you could still re-program it using the serial port!
You can find more information from the website here and how to build a website like it here.
The 12F675 is one of the smallest PIC Microcontrollers - it's a tiny device with 8 pins but it's packed with peripherals and it even has a built in 10bit ADC which can read analogue inputs from 4 pins.
It has the following internal peripherals:
1. Two timers.
2. An analogue comparator.
3. 10 bit ADC.
It also has an internal oscillator and internal reset circuit. This means the device uses minimal external components to make it work (other devices require an external crystal oscillator). Of course it also has the usual internal programming memory, EEPROM and RAM needed for programming.
Ideas for projects:
1. 4 channel volt meter.
2. Multi channel Servo controller.
3. Temperature controller.
4. Inductance meter.
5. Touch lamp.
6. Courtesy light time delay.
Note: To get data out of the device you can implement a serial RS232 transmit interface to your PC.
Why use it? One reason is that because of its size its easy to put into restricted spaces e.g. for a model aircraft or model trains and it's cheaper than the larger devices.
Note: The 12F629 is the same device without the ADC - so it's even more cost effective.
So it's useful in designs that you would not normally think of using a microcontroller for instance you could make a touch lamp dimmer - Note using the microcontroller means you can make far better functionality than using discrete hardware (and even change its programming later on).
With a lamp dimmer you could have an auto off delay function e.g. if no activity for an hour then turn off.
State machines You could also implement a state machine for more complex control of the functionality e.g. pressing once moves to the next dimming level in the current direction while press and hold changes the dimming direction.
Using a state machine while not trivial lets you control complex operation which you could not achieve (without a great deal of effort) using discrete hardware - and the advantage of using the microcontroller is that if you get it wrong you just re code your software and test it again.
Note: The 12F675 and 12F629 use Flash programming memory i.e. they are re-programmable - you can change their functionality instantaneously with NO re-wiring.
The only problems are:
1. You need to program the device.
2. You need a programming language.
Programming the Device Surprisingly you can program the device using the standard 4 pin PIC serial interface - ICSP (In Circuit Serial Programming) and with careful design you can even connect your programmer to the same pins that your circuit uses.
Programming language The programming language normally recommended is assembler and there are good reasons for using assembler - e.g. very fast code and smaller final code size but I would recommend using a high level language such as Basic or C
This is because for assembler you need to work at such a low level that you spend a lot of effort to do trivial tasks and this is better left to the high level language.
For the example mentioned setting up and maintaining a state machine would be extremely difficult in assembler but much easier in C.
Some of the 12F675 projects are available here:12F675 Projects
John Main's website http://www.best-microcontroller-projects.com provides microcontroller resources and free projects which are fully documented including schematics and source code.
Serial communication is a technique of transmitting data between two pieces of hardware. The smallest piece of data that is transmitted is the byte. A byte is made up of 8 bits. When data is transmitted between two pieces of hardware, the bits are sent one at a time. The hardware sends these bits by sending varying voltages across the wires connecting the devices. The sender and the receiver agree on how often a bit (or voltage level) will be sent.
How often a bit will be sent is referred to as baud rate or bits per second (bps). Then with the help of very precise clocks they can send a series of voltage levels between each other and then reassmble these voltage levels into bytes.
The Problem Communicating Between a PC and a Microcontroller
The RS232 serial port on a PC uses +3 to +25 volts to signify a logic level of 0, and -3 to -25 volts to signify a logic level of 1.
Most microcontrollers use TTL / CMOS logic levels which use 0 to some threshold voltage to signify a logic level of 0, and some threshold voltage to 5 volts to signify a logic level of 1.
In order for the PC and a microcontroller to successfully communicate some logic level translation is necessary.
Using a TTL / CMOS To RS232 Converter
There are many companies that manufacture modules that take care of the required logic level translation. The TTL / CMOS To RS232 Converter is one such module. Typically, these modules connect to the RS232 port on the PC and connect to the microcontroller's transmit and receive pins. Supply 5 volts and ground and you are ready to communicate.
Applications Ideas
Once you have your microcontroller communicating with your PC it opens up all kinds of possible applications ranging from data loggers, pc based oscilloscopes, controlling your pc from your microcontroller, controlling your microcontroller from your pc, etc.
Gone are the days when there were less than a handful of micro controller's to choose from. The fact is, choosing a controller for a new product is quite a daunting task and is way beyond the scope of this article, but will be covered in more detail in a future article.
However choosing a microcontroller to 'play with' for your first project, is a little easier and the tools (both hardware and software?) these days, are extremely cheap, if not, free!....
In the past, emulators, programmers etc use to cost thousands of dollars (what were they thinking?). Now it is possible to get very low cost development tools.
10 years back I chose to go with Microchip. Why?, well the fact that they provided not only a free development environment (MPLAB IDE), but also tons of support through the forums and application notes on their website.
At that time my simple project was essentially an electronic switch. So starting withsuch a simple task, almost any micro would do the job.
I picked one of their low end PIC12xxx series of micro's. These little babies had around 512 bytes of OTP (One Time Programmable) ROM and around 25 bytes of RAM! (can it run WinXP?...I don't think so!!!). This was plenty for what I had in mind though.
Today there are many more vendors (TI, Atmel etc) which provide similar devices and excellent support too.
One of the relatively new 'players' that stand out from the crowd, is Cypress' mixed signal PSoC series of micro controllers. These devices are extremely flexible from the point of view of re configurable hardware. The other big plus with these devices is the capability, using their free tool (PSoC Designer now incorporating PSoC Express) to program their micro controllers without writing a single piece of code!...more on these in another article.
So if you are just getting started in this fascinating area, keep it simple and start with a lower end micro. Most of the vendors provide samples for free from their websites listed below.
TI - Texas Instruments
Microchip Inc
Cypress
There are many many more, but these have been the main players I have been interested in.
If you have any good micro controller recommendations or related, please leave a comment.
Cheers.
Dale is a design expert in the field of electronics and imparts over 20 years of knowledge gained from 'hands on' experience in the industry. Find articles like this and more at his respectful site online at http://www.allthingsembedded.com
Microcontrollers give you a fantastic way of creating projects.
A PIC microcontroller is a processor with built in memory and RAM and you can use it to control your projects (or build projects around it). So it saves you building a circuit that has separate external RAM, ROM and peripheral chips. What this really means for you is that you have a very powerful device that has many useful built in modules e.g.
EEPROM.
Timers.
Analogue comparators.
UART.
Even with just these four modules (note these are just example modules - there are more) you can make up many projects e.g.:
* Frequency counter - using the internal timers and reporting through UART (RS232) or output to LCD.
* Capacitance meter - analogue comparator oscillator.
* Event timer - using internal timers.
* Event data logger -capturing analogue data using an internal ADC and using the internal EEPROM for storing data (using an external I2C for high data storage capacity.
* Servo controller (Control through UART) - using the internal PWM module or using a software created PWM.
The PIC Micro is one of the most popular microcontrollers and in case you were wondering the difference between a microprocessor and a microcontroller is that a microcontroller has an internal bus with in built memory and peripherals.
In fact the 8 pin (DIL) version of the 12F675 has an amazing number of internal peripherals. These are:
Two timers.
One 10bit ADC with 4 selectable inputs.
An internal oscillator (or you can use an external crystal).
An analogue comparator.
1024 words of program memory.
64 Bytes of RAM.
128 Bytes of EEPROM memory.
External interrupt (as well as interrupts from internal peripherals).
External crystal can go up to 20MHz.
ICSP : PIC standard programming interface.
And all of these work from within an 8 pin DIL package!
In the mid-range devices the memory space ranges from 1k to 8k (18F parts have more) - this does not sound like a lot but the processor has an efficient instruction set and you can make useful projects even with 1k e.g. LM35temperature sensing project that reports data to the serial port easily fits within 1k.
Features
In fact a PIC microcontroller is an amazingly powerful fully featured processor with internal RAM, EEROM FLASH memory and peripherals. One of the smallest ones occupies the space of a 555 timer but has a 10bit ADC, 1k of memory, 2 timers, high current I/O ports a comparator a watch dog timer... I could go on as there is more!
Programming
One of the most useful features of a PIC microcontroller is that you can re-program them as they use flash memory (if you choose a part with an F in the part number e.g. 12F675 not 12C509). You can also use the ICSP serial interface built into each PIC Microcontroller for programming and even do programming while it's still plugged into the circuit!
You can either program a PIC microcontroller using assembler or a high level language and I recommend using a high level language such as C as it is much easier to use (after an initial learning curve). Once you have learned the high level language you are not forced to use the same processor e.g. you could go to an AVR or Dallas microcontroller and still use the same high level language.
Input / Output - I/O
A PIC Microcontroller can control outputs and react to inputs e.g. you could drive a relay or read input buttons.
With the larger devices it's possible to drive LCDs or seven segment displays with very few control lines as all the work is done inside the PIC Micro.
Comparing a frequency counter to discrete web designs you'll find two or three chips for the microcontroller design and ten or more for a discrete design. So using them saves prototype design effort as you can use built in peripherals to take care of lots of the circuit operation.
Many now have a built in ADC so you can read analogue signal levels so you don't need to add an external devices e.g. you can read an LM35 temperature sensor directly with no interface logic.
Peripherals
The PIC microcontroller has many built in peripherals and this can make using them quite daunting at first which is why I have made this introductory page with a summary of each major peripheral block.
The best way to start is to learn about the main features of a chip and then begin to use each peripheral in a project. I think learning by doing is the best way.
Simple programming using In Circuit Serial Programming.
Note:these are some of the main features (some chips have all of these and some don't).
Flash memory
This is the program storage area and gives you the most important benefit for using a PIC microcontroller - You program the device many times. Since when does anyone get a program right first time ?
Devices used in projects on this site can be re-programmed up to 100,000 times (probably more) as they use Flash memory - these have the letter F in the part name. You can get cheaper (OTP) devices but these are One-Time-Programmable; once programmed you can't program it again!
ICSP
In Circuit Serial Programming (ICSP) is the next most important benefit. Instead of transferring your chip from the programmer to the development board you just leave it in the board. By arranging the programming connections to your circuit correctly you won't need to remove the chip!
You can re-program the device while it's still in the circuit so once your programmer is setup you can leave it on the bench and test your programs without moving the chip around and it makes the whole process much easier.
I/O Ports
Input / Output ports let you communicate with the outside world so you can control leds, LCDs or just about anything with the right interface. You can also set them as inputs to gather information.
Pin direction
Most PIC microcontroller pins can be set as an input or and output and this can be done on the fly e.g. for a dallas 1 wire system a pin can be written to generate data and read at a later stage. The TRIS register controls the I/O direction and setting a bit in this register to zero sets the pin as output while setting it as one sets the pin as input.
This allows you to use a pin for multiple operations e.g. the Real Time clock project uses RA0, the first pin of PORTA, to output data to a seven segment display and at a later point in the program read the analogue value as an input.
Current
The PIC I/O ports are high current ports capable of directly driving LEDs (up to 25ma output current) - the total current allowed usually ~200mA this is often for the whole chip (or specified for several ports combined together).
Timer / Counters
Each PIC microcontroller has up to three timers that you can either use as a timer or a counter (Timer 1 & 2) or a baud clock (Timer 2).
Timer 0
The original timer: Timer 0 was the first timer developed and you can find it in all the earliest devices e.g. 16F84 up to the most current e,g, 16F877A.
It is an 8 bit timer with an 8 bit prescaler that can be driven from an internal (Fosc/4) or external clock. It generates an interrupt on overflow when the count goes from 255 to zero.
Timer 0 always synchronizes the input clock (when using external clock).
Note: You can read and write timer 0 but you can not read the prescaler.
Note: The prescaler changes its effect depending on whether it is a timer prescaler or a watch dog prescaler - so the same prescaler setting may prescale by 2 or by 1 depending on its use!
Timer 1
This is a 16 bit timer that generates an overflow interrupt when it goes from 65535 to zero. It has an 8 bit programmable prescaler and you can drive it from the internal clock (Fosc/4) or an external pin.
To eliminate false triggering it also has an optional input synchronizer for external pin input.
This timer can be used in sleep mode and will generate a wakeup interrupt on overflow.
Timer 1 is also read by the CCP module to capture an event time.
Note: Using this timer in sleep mode will use more current.
In addition it can be used to drive a low power watch crystal. This is something that sounds good but I don't recommend you do it as watch crystals are extremely difficult to drive correctly. You should only use it if you are going to make a pcb and follow all the guidelines in making it noise free. I used a DS1307 in the Real Time clock project which drives the crystal directly but even this is difficult to get operating accurately.
Timer 2
This is an 8 bit timer with an 8 bit prescaler and an 8 bit postscaler. It takes its input only from the internal oscillator (Fosc/4).
This timer is used for the timebase of a PWM when PWM is active and it can be software selected by the SSP module as a baud clock.
It also has a period register that allows easy control of the period. When timer 2 reaches the PR2 register value then it resets. This saves having to check the timer value in software and then reset the timer and since it is done in hardware the operation is much faster - so you can generate fast clocks with periods that are multiples of the main clock.
USART
The USART is a useful module and saves having to code up a software version so it saves valuable program memory. You can find more information on RS232 here and how to make it work. Look here for pin outs.
All you need to interface it to a PC serial port is a MAX232 chip (or equivalent).
Note: An equivalent MAX232 chip is the SP202ECP that has the same pinout as the MAX232 but lets you use 100nF capacitors - so you don't need the large 1uF caps.
Baud Rates
You have to be careful using the baud rates as they depend on the main clock in use and normal oscillator values in general do not fit very well with 'real' baud rates.
There is a table of baud rates in microchip data sheet DS33023A which indicates the expected percentage error for a specific clock rate and in general the higher the main clock the lower the error.
You sometimes have to play around with the register settings to get a better fit with your clock rate and the baud rate you want. An example is for an 8MHz clock - if you use BRGH=1 and an 8MHz clock (see the 16F88 datasheet) you get accurate baud rates up to 38.4kbaud. You have to force this to work e.g. in mikroC the built in USART routines use BRGH=0 so at 8MHz the baud rate is only accurate to 9.6kbaud.
If you want a super-accurate baud rate the best way is to use a clock crystal that ends up giving you that baud rate i.e. work back through the baud rate equations to find the crystal you need.
CCP
The Capture/Compare/PWM module has three modes of operation:
Capture - Capture the time of an event.
Compare - Generate an output when Timer 1 reaches a value.
PWM - Pulse Width Modulation.
Capture
Capture mode is used to capture the value of Timer 1 when a signal at the CCP pin goes high (or low depending on how the CCP is set up). The CCP can accurately capture the arrival time of a signal at the CCP pin so it can be used for pulse time measurement.
Compare
Compare mode is used to generate an output when Timer 1 reaches a value you put into CCPR1. One special event trigger mode lets you start the ADC when the compare mode triggers.
PWM
PWM gives you one Pulse Width Modulation output with 10 bit resolution and with no software overhead - once started it operates all by itself unless you want to change the duty cycle.
It uses Timer 2 to define its operation using Timer 2 period register to define the frequency of the PWM.
Note: The duty cycle is not a percentage it is the number of periods of the PWM clock that the output is high!
SSP
The Synchronous Serial Port lets you communicate with devices that use either the SPI (Serial Peripheral Interface) or I2C (Inter IC communication) protocols. Note that for full Master mode I2C operation you need to choose a PIC device that has the MSSP device (Master Synchronous Serial Port).
SPI and I2C are shared so you can only use one at a time (or you could use the I2C bit banged routines in the Real Time Clock project to have both at the same time).
You can find a project that uses I2C here and you can find more information on I2C here.
Comparator and comparator voltage reference
The comparator is module that has two analogue comparators which can be set up in one of 8 different ways. Either digital or analogue inputs can be compared to reference voltages.
In one mode an internally generated voltage reference is used as an input to both comparators and in the same mode multiplexing lets you monitor up to four different input pins.
You can even send the output of the comparator to a pin so that it is used independently from the microcontroller e.g. in a circuit where you need a comparator you don't need an extra chip!
The analogue level must be between Vdd and Vss as protection diodes won't allow anything else.
The module will generate an interrupt if the comparator output changes.
You can use it in sleep mode and the interrupt will wake it up.
The source impedance of the analogue signal must be smaller than 10k.
ADC
The single 10 bit Analogue to Digital Converter can have up to 8 inputs for a device multiplexed from input pins.
The ADC can be used during sleep but you have to use the RC clock mode. One benefit of this is that there will be no digital switching noise so you will get better conversion accuracy.
For the 16F877A you can not just choose to use an analogue input if you feel the need as there are only a specific and limited number of ways that the analogue input pins can be enabled. It is best to start with AN0 and add more as necessary - see the datasheet for which analogue inputs can be enabled e.g. if you started a design using only AN5 you would find that you may have to enable a few more analogue inputs as well!
The 16F675 can measure 4 analogue input pins!
PSP
The Parallel Slave Port lets you to connect the PIC microcontroller directly into a microprocessor system. It provides an 8 bit read/write data bus and RD (read) WR (write) and CS (chip select) inputs - all active low.
This will let you add a PIC microcontroller to a system so that the PIC microcontroller can be treated as a memory mapped peripheral. It will let the microcontroller behave just as though it was another microprocessor building block e.g. some memory or ram but in this case you have full control over exactly what the building block is i.e. you can re-program the PIC microcontroller to do just about anything.
This provides an easy route to adding a PIC microcontroller to an 8 bit system that already exists.
LCD
The LCD interface lets you directly interface to an LCD saving you having to use an LCD module such as the HD44780. I have not used this feature as it is another commercial requirement where removing a chip (HD44780) saves money in a production run. I think it is capable of driving a graphic LCD.
This detects if the power supply dips slightly and resets the device if so.
POR
Power on reset
This starts microcontroller initialization.
PWRT
PoWeR up Time
A time delay to let Vdd rise.
OST
Oscillator start up timer
Wait for 1024 cycles after PWRT.
SLEEP
PIC microcontroller sleep mode
Enter low power mode.
WDT
If your software goes haywire then this timer resets the processor. To stop the reset the well behaved software must periodically issue the CLRWDT instruction to stop a resert. The WDT runs using its own oscillator. It runs during sleep and shares Timer 0 prescaler.
POR
Power On Reset starts PIC microcontroller initialization when it detects a rising edge on MCLR.
PWRT
If you enable this then 72ms after a POR the PIC microcontroller is started.
OST
Oscillator Startup Timer delays for 1024 oscillator cycles after PWRT (if PWRT is enabled) ensuring that the oscillator has started and is stable. It is automatic and only used for crystal oscillator modes and is active after POR or wake from sleep.
SLEEP
Sleep mode (or low power consumption mode) is entered by executing the 'SLEEP' command. The device can wake from sleep caused by an external reset, Watch Dog Timer timeout, INT pin RB port change or peripheral interrupt.
Project device overview
This site mainly uses three PIC devices out of the hundreds of different chips that microchip produces. This does not sound like a lot but you can use the devices in almost any project and they have so many built in peripherals that you can make hundreds of projects with them.
The other microchip devices are all useful in different situations - perhaps they have more memory or different peripherals - this is useful if you want to tailor your designs to the system you build - but probably more useful in a commercial environment where every cent counts in a production run.
All three devices are extremely powerful and the main difference is that they have different numbers of pins and memory size.
Note: There are differences in using the devices i.e. there are some registers that are different but in the generally you can interchange them - this is made easier using a high level language.
The devices used in this site are:
PIC microcontroller Device
PIC microcontroller No. Pins
PIC microcontroller Flash memory WORDS
12F675
8
1k
16F88
18
4k
16F877A
40
8k
Note : When looking at the microchip site the memory size is kwords - ignore kbytes - you need the kword size as this is what each instruction occupies - the kbyte size is for comparison to other types of micros (probably). But the microcontroller data bus is 8 bits wide so it is an 8 bit microcontroller (different program memory and data memory due to using Harvard architecture).
(Note: that all of them have the letter F in - this means it is a Flash re-programmable part - don't go and buy a part with O in as its OTP - programmable only once! - only do that if you are really really sure it's the final design).
PIC Microcontroller Flash Memory size
You may think that 1k or even 8k is so tiny that it won't be useful but each PIC microcontroller uses RISC (Reduced Instruction Set Computing) which simply means that it has a cleverly arranged instruction set that only has a few instructions. The mid range parts have 35 instructions.
If you use the high level language as recommended in this site then you won't need to be too aware of the instruction set it just means you can do a lot with a small amount of memory. Most of the projects on this site although they are fully working projects fit within 2k words!
Note: If you need more memory you can always move to the 18F series of PIC microcontrollers. Another option is to add an I2C serial eprom.
PIC microcontroller RAM and EEPROM size
The PIC microcontroller RAM size is also important as it stores all your variables and intermediate data.
Note: You can usually alter the program to useless RAM by choosing the right variable sizes or changing how your program works
For example don't use floating point alter it to use a different variable type e.g. you can use long integers with fixed point operation to avoid floating point.
PIC microcontroller EEROM : Electrically Erasable ROM is used to store data that must be saved between power up and power down.
This area is readable and writable and has a much longer life than the main program store i.e. it has been designed for more frequent use.
Building a Digital Thermometer
Now that you understand a little bit about your Stamp and the LCD, we can add another component and create a digital thermometer. To create a thermometer, we will use a chip called the DS1620. This chip contains:
A temperature-sensing device
An analog-to-digital (A/D) converter for the temperature-sensing device
A shift register to read the data out of the A/D converter
A little EEPROM (electrically erasable programmable read-only memory) to remember settings
The DS1620 has two modes: In one mode, it acts as a stand-alone thermostat chip, and in the other mode you hook it up to a computer and use it as a thermometer. The EEPROM remembers the current mode as well as the set temperatures for the thermostat.
Hooking up the DS1620 to the Stamp is very easy. The DS1620 comes in an 8-pin chip. Supply +5 volts from the Stamp to pin 8 of the DS1620. Supply ground to pin 4 of the DS1620. You then use three I/O pins from the Stamp to drive three pins on the DS1620:
Pin 1 on the DS1620 is the data pin. You read and write data bits on this pin.
Pin 2 on the DS1620 is the clock pin. You clock data in and out of the shift register with this pin.
Pin 3 on the DS1620 is the reset/select pin. You set pin 3 high to select the chip and communicate with it.
For this example code, it is assumed that:
The data pin goes to I/O pin 2 on the Stamp.
The clock pin goes to I/O pin 1 on the Stamp.
The reset/select pin goes to I/O pin 0 on the Stamp.
The completed wiring looks like this:
You can get a DS1620 either from Jameco (part number 146456) or Parallax (part number 27917) in an "application kit" that includes the chip, the capacitor, some good documentation and sample code. Or you can buy the chip on its own from Jameco (part number 114382). I would suggest getting the application kit the first time you try using the DS1620 because the documentation is very useful.
You can assemble the DS1620 in the prototype area of the Stamp carrier board or on a separate breadboard. Once you have assembled it, hook your LCD display up to I/O pin 3 of the Stamp, and then load and run the following program:
symbol RST = 0 ' select/reset line on 1620 symbol CLK = 1 ' clock line for shift registers on 1620 symbol DQ = 2 ' data line on 1620 symbol DQ_PIN = pin2 ' pin representation for DQ symbol LCD = 3 ' data line for LCD
begin: low RST ' deselect the 1620 unless talking to it high CLK ' clock pin on 1620 should default high pause 1000 ' wait for the thermometer and LCD to boot
setup: high RST ' select the 1620 b0 = $0C ' $0c is the 1620 command byte ' saying "Write Config" gosub shift_out ' send it to the 1620 b0 = %10 ' %10 is the 1620 command byte ' to set thermometer mode gosub shift_out ' send it to the 1620 low RST ' deselect the 1620 pause 50 ' delay 50ms for EEPROM
start_convert: b0 = $EE ' $EE is the 1620 command byte ' to start conversions high RST ' select the 1620 gosub shift_out ' send it to the 1620 low RST ' deselect the 1620
' This is the main loop ' - reads and displays temperature every second main_loop: high RST ' select the 1620 b0 = $AA ' $AA is the 1620 command byte ' for reading temperature gosub shift_out ' send it to the 1620 gosub shift_in ' read the temperature ' from the 1620 low RST ' deselect the DS1620. gosub display ' display the temp in degrees C pause 1000 ' wait a second goto main_loop
' The shift_out subroutine sends whatever is in ' the b0 byte to the 1620 shift_out: output DQ ' set the DQ pin to ' output mode for b2 = 1 to 8 low CLK ' prepare to clock the bit ' into 1620 DQ_PIN = bit0 ' Send the data bit high CLK ' latch data bit into 1620 b0 = b0/2 ' shift all bits right ' toward bit 0 next return
' The shift_in subroutine gets a 9-bit ' temperature from the 1620 shift_in: input DQ ' set the DQ pin to ' input mode w0 = 0 ' clear w0 for b5 = 1 to 9 w0 = w0/2 ' shift input right. low CLK ' ask 1620 for next bit bit8 = DQ_PIN ' read the bit high CLK ' toggle clock pin next return
' Displays the temperature in degrees C display: if bit8 = 0 then pos ' if bit8=1 ' then temp is negative b0 = b0 &/ b0 ' invert b0 by NANDing it ' with itself b0 = b0 + 1 pos: serout LCD, n2400, (254, 1) ' clear the LCD serout LCD, n2400, ("Temp = ") ' display "Temp=" ' on the display bit9 = bit0 ' save the half degree b0 = b0 / 2 ' convert to degrees if bit8 = 1 then neg ' see if temp is negative serout LCD, n2400, (#b0) ' display positive temp goto half neg: serout LCD, n2400, ("-", #b0)' display negative temp half: if bit9 = 0 then even serout LCD, n2400, (".5 C") ' display the half degree goto done even: serout LCD, n2400, (".0 C") ' display the half degree done: return
If you run this program, you will find that it displays the centigrade temperature with an accuracy of one-half degree.
The DS1620 measures temperatures in centigrade half-degrees. It returns the temperature in a 9-bit 2s-complement number with a range of -110 to 250 F (-55 to 125 C). You divide the number you receive by 2 to get the actual temperature. 2s-complement binary numbers are a convenient way to represent negative values. The following list shows the values for a 4-bit 2s-complement number:
You can see that instead of the 4 bits representing values from 0 to 15, the 4 bits in a 2s-complement number represent the values -8 to 7. You can look at the left-most bit to determine if the number is negative or positive. If the number is negative, you can invert the bits and add 1 to get the positive representation of the number.
Here's what goes on with the digital thermometer program shown here:
It uses the symbol keyword to set up several constants that make the program slightly easier to read (and also make it easy for you to move the chip to different I/O pins on the Stamp).
It sets the CLK and RST pins on the DS1620 to their expected values.
It writes a command byte to the EEPROM on the DS1620 to tell the chip to operate in "thermometer mode." Because the mode is stored in EEPROM, you only have to do it once, so you could technically take this section of the code out of the program after you run the program once (to save program space).
The program sends the command $EE ("$" means "hexadecimal number" -- $EE is 238 in decimal) to tell the thermometer to start up its conversion process.
The program then enters a loop. Every second, it sends a command to the DS1620 telling the DS1620 to return the current temperature, and then it reads the 9-bit value that the DS1620 returns into the w0 variable. The Stamp sends and receives data 1 bit at a time by toggling the CLK line on the DS1620. Remember that the w0 (16-bit) variable overlays the b0/b1 (8-bit) variables, which overlay the bit0/bit1/.../bit15 (1-bit) variables, so when you insert a bit from the DS1620 into bit 8 and divide w0 by 2, what you are doing is shifting each bit to the right to store the 9-bit temperature from the DS1620 into w0. Once the temperature has been saved in w0, the display subroutine determines whether the number is positive or negative and displays it appropriately on the LCD as a centigrade temperature. The conversion from degrees C to degrees F is:
dF = dC * 9/5 + 32
At this point, we have succeeded in creating an extremely expensive thermometer. What might you do with it? Here's one idea. Let's say you work for a drug company and you are shipping expensive drugs across the country that MUST remain at a certain temperature the entire way or the drugs will spoil. What you can do with a Stamp is create a data logging thermometer. Both Jameco (part number 143811) and Parallax (part number 27960) sell a device called the "RAM Pack module." It contains a low-power 8-kilobyte (or optionally 32-kilobyte) RAM chip with a serial interface. You could add this component (or something similar) to your Stamp and write code that saves temperature readings to the RAM every minute. You could then slip your Stamp into the drug shipment, and at the other end of the trip retrieve the Stamp. The RAM module would contain the temperature history of the entire trip and you would know whether or not the drugs ever thawed out.
There are all kinds of neat, useful devices like this that you can build with a Stamp now that you know how microcontrollers work!
For more information on microcontrollers and related topics, check out the links on the next page.
Creating a Really Expensive Digital Clock
Spending $79 to flash an LED may seem extravagant to you. What you would probably like to do is create something useful with your BASIC stamp. By spending about $100 more you can create a really nice digital clock! This may seem extremely extravagant, until you realize that the parts are reusable in a variety of other projects that you may want to build later.
Let's say that we would like to use the I/O pins on the BASIC Stamp to display numeric values. In the digital clock article, we saw how to interface to a 7-segment LED display using a 7447 chip. 7447s would work just as well with the BASIC Stamp. You could wire four of the I/O pins straight into a 7447 and easily display a number between 0 and 9. Since the BS-1 Stamp has eight I/O pins, it is easy to drive two 7447s directly like this.
For a clock, we need a minimum of four digits. To drive four 7447s with eight I/O pins, we have to be slightly more creative. The following diagram shows you one approach:
In this diagram, the eight I/O lines from the Stamp enter from the left. This approach uses four lines that run to all four 7447s. Then the other four lines from the Stamp activate the 7447s in sequence ("E" on the chips means "Enable" -- on a 7447, that would be the blanking input on pin 5). To make this arrangement work, the BASIC program in the Stamp would output the first digit on the four data lines and activate the first 7447 by toggling its E pin with the first control line. Then it would send out the value for the second digit and activate the second 7447, sequencing through all four of the 7447s like this repeatedly. By wiring things slightly differently, you could actually do this with only one 7447. By using a 74154 demultiplexer chip and some drivers, you could drive up to 16 digits using this approach.
This is, in fact, a standard way to control LED displays. For example, if you have an old LED calculator, turn it on and shake it while watching the display. You will actually be able to see that only one digit is ever illuminated at once. The approach is called multiplexing the display.
While this approach works fine for clocks and calculators, it has two important problems:
LEDs consume a lot of power.
7-segment LEDs can only display numeric values.
An alternative approach is to use an LCD screen. As it turns out, LCDs are widely available and can be easily hooked to a Stamp. For example, the two-line by 16-character alphanumeric display shown below is available from both Jameco (part number 150990) and Parallax (part number 27910). A typical display is shown here, mounted on a breadboard for easier interfacing:
This sort of LCD has several advantages:
The display can be driven by a single I/O pin. The display contains logic that lets a Stamp communicate with it serially, so only one I/O pin is needed. In addition, the SEROUT command in Stamp BASIC handles serial communication easily, so talking to the display is simple.
The LCD can display alphanumeric text: letters, numbers and even custom characters.
The LCD consumes very little power -- only 3 milliamps.
The only problem is that one of these displays costs $59. Obviously, you would not embed one of these in a toaster oven. If you were designing a toaster oven, however, you would likely prototype with one of these displays and then create custom chips and software to drive much cheaper LCDs in the final product.
To drive a display like this, you simply supply it with +5 volts and ground (the Stamp supplies both from the 9-volt battery) and then hook one of the I/O pins from the Stamp to the display's input line. The easiest way I have found to connect the Stamp's I/O pins to a device like an LCD is to use a wire-wrap tool (Jameco part number 34577) and 30-gauge wire wrap wire (Jameco part number 22541 is typical). That way, no soldering is involved and the connections are compact and reliable.
The following BASIC program will cause a BASIC Stamp to behave like a clock and output the time on the LCD (assuming the LCD is connected to I/O pin 0 on the Stamp):
pause 1000 'wait for LCD display to boot serout 0, n2400, (254, 1) 'clear the display serout 0, n2400, ("time:") 'Paint "time:" on the display 'preset before loading program b0 = 0 'seconds b1 = 27 'minutes b2 = 6 'hours
again: b0 = b0 + 1 'increment seconds if b0 < 60 then minutes b0 = 0 'if seconds=60 b1 = b1 + 1 ' then increment minutes minutes: if b1 < 60 then hours b1 = 0 'if minutes=60 b2 = b2 + 1 ' then increment hours hours: if b2 < 13 then show b2 = 1 'if hours=13 reset to 1
In this program, the SEROUT commands send data to the LCD. The sequence (254, 1) clears the LCD (254 is the escape character and 1 is the command to clear the screen). The sequence (254, 135) positions the cursor. The other two SEROUT commands simply send text strings to the display.
This approach will create a reasonably accurate clock. By tweaking the PAUSE statement you can get the accuracy to within a few seconds a day. Obviously, in a real clock you would like to wire up a push-button or two to make setting it easier -- in this program, you preset the time before you download the program to the Stamp.
While this approach is simple and works, it is not incredibly accurate. If you want better accuracy, one good approach would be to wire a real-time clock chip up to your Stamp. Then, every second or so, you can read the time from the chip and display it. A real-time clock chip uses a quartz crystal to give it excellent accuracy. Clock chips also usually contain date information and handle leap year correction automatically.
One easy way to interface a real-time clock to a stamp is to use a component called the Pocket Watch B.
Pocket Watch B Module
The Pocket Watch B is available from both Jameco (part number 145630) and Parallax (part number 27962). This part is about as big as a quarter and contains the clock chip, crystal and a serial interface so that only one I/O pin is necessary to communicate with it. This component costs about $30 -- again, not something you want to embed in a toaster oven, but easy to play with when constructing prototypes.
Playing with a BASIC Stamp
If you would like to play with a BASIC Stamp, it's very easy to get started. What you need is a desktop computer and a BASIC Stamp starter kit. The starter kit includes the Stamp, a programming cable and an application that you run on your desktop computer to download BASIC programs into the Stamp.
You can get a starter kit either from Parallax (the manufacturer) or from a supplier like Jameco (who should be familiar to you from the electronic gates and digital clock articles). From Parallax, you can order the BASIC Stamp D Starter Kit (part number 27202), or from Jameco you can order part number 140089. You will receive the Stamp (pictured below), a programming cable, software and instructions. The kit is $79 from both suppliers. Occasionally, Parallax runs a special called "We've Bagged the Basics" that also includes Scott Edward's Programming and Customizing the BASIC Stamp Computer.
Hooking up the Stamp is easy. You connect it into the parallel port of your PC. Then you run a DOS application to edit your BASIC program and download it to the Stamp. Here is a screenshot of a typical editor (in this case, the one from Scott Edward's book):
To run the program in this editor, you hit ALT-R. The editor application checks the BASIC program and then sends it down the wire to the EEPROM on the Stamp. The Stamp then executes the program. In this case, the program produces a square wave on I/O pin 3. If you hook up a logic probe or LED to pin 3 (see the electronic gates article for details), you will see the LED flash on and off twice per second (it changes state every 250 milliseconds because of the PAUSE commands). This program would run for several weeks off of a 9-volt battery. You could save power by shortening the time that the LED is on (perhaps it is on for 50 milliseconds and off for 450 milliseconds), and also by using the NAP instruction instead of PAUSE.
Programming the BASIC Stamp
You program a BASIC Stamp using the BASIC programming language. If you already know BASIC, then you will find that the BASIC used in a Stamp is straightforward but a little stripped-down. If you don't know BASIC, but you do know another language like C, Pascal or Java, then picking up BASIC will be trivial. If you have never programmed before, you probably want to go learn programming on a desktop machine first. Here is a quick rundown on the instructions available in Stamp BASIC. (For complete documentation, go to Parallax: BASIC Stamp Documentation.)
Standard BASIC instructions:
for...next - normal looping statement
gosub - go to a subroutine
goto - goto a label in the program (e.g. - "label:")
if...then - normal if/then decision
let - assignment (optional)
return - return from a subroutine
end - end the program and sleep
Instructions having to do with I/O pins:
button - read a button on an input pin, with debounce and auto-repeat
high - set an I/O pin high
input - set the direction of an I/O pin to input
low - set an I/O pin low
output - set the direction of an I/O pin to output
pot - read a potentiometer on an I/O pin
pulsin - read the duration of a pulse coming in on an input pin
pulsout - send a pulse of a specific duration out on an output pin
pwm - perform pulse width modulation on an output pin
reverse - reverse the direction of an I/O pin
serin - read serial data on an input pin
serout - write serial data on an output pin
sound - send a sound of a specific frequency to an output pin
toggle - toggle the bit on an output pin
Instructions specific to the BASIC Stamp:
branch - read a branching table
debug - send a debugging string to the console on the desktop computer
eeprom - download a program to EEPROM
lookdown - return the index of a value in a list
lookup - array lookup using an index
nap - sleep for a short time
pause - delay for the specified time
random - pick a random number
read - read a value from EEPROM
sleep - power down for the specified time
write - write data to EEPROM
Operations:
+ - addition
- - subtraction
* - multiplication (low-word)
** - multiplication (high-word)
/ - division
// - mod
max - return maximum of 2 values
min - return minimum of 2 values
& - AND
| - OR
^ - XOR
&/ - NAND
|/ - NOR
^/ - XNOR
If statement logic:
=
<>
<
<=
>
>=
AND
OR
Variables
All variables in the BS-1 have pre-defined names (which you can substitute with names of your own). Remember that there are only 14 bytes of RAM available, so variables are precious. Here are the standard names:
w0, w1, w2...w6 - 16-bit word variables
b0, b1, b2...b13 - 8-bit byte variables
bit0, bit1, bit2...bit15 - 1-bit bit variables
Because there are only 14 bytes of memory, w0 and b0/b1 are the same locations in RAM, and w1 and b2/b3 are the same, and so on. Also, bit0 through bit15 reside in w0 (and therefore b0/b1 as well).
I/O pins You can see that 14 of the instructions in the BS-1 have to do with the I/O pins. The reason for this emphasis is the fact that the I/O pins are the only way for the BASIC Stamp to talk to the world. There are eight pins on the BS-1 (numbered 0 to 7) and 16 pins on the BS-2 (numbered 0 to 15).
The pins are bi-directional, meaning that you can read input values on them or send output values to them. The easiest way to send a value to a pin is to use the HIGH or LOW functions. The statement high 3 sends a 1 (+5 volts) out on pin 3. LOW sends a 0 (Ground). Pin 3 was chosen arbitrarily here -- you can send bits out on any pin from 0 to 7.
There are a number of interesting I/O pin instructions. For example, POT reads the setting on a potentiometer (variable resistor) if you wire it up with a capacitor as the POT instruction expects. The PWM instruction sends out pulse-width modulated signals. Instructions like these can make it a lot easier to attach controls and motors to the Stamp. See the documentation for the language for details. Also, a book like Scott Edward's Programming and Customizing the BASIC Stamp Computer can be extremely helpful because of the example projects it contains.
Using Microcontrollers
In How Electronic Gates Work, you learned about 7400-series TTL devices, as well as where to buy them and how to assemble them. What you found is that it can often take many gates to implement simple devices. For example, in the digital clock article, the clock we designed might contain 15 or 20 chips. One of the big advantages of a microcontroller is that software -- a small program you write and execute on the controller -- can take the place of many gates. In this article, therefore, we will use a microcontroller to create a digital clock. This is going to be a rather expensive digital clock (almost $200!), but in the process you will accumulate everything you need to play with microcontrollers for years to come. Even if you don't actually create this digital clock, you will learn a great deal by reading about it.
The microcontroller we will use here is a special-purpose device designed to make life as simple as possible. The device is called a "BASIC Stamp" and is created by a company called Parallax. A BASIC Stamp is a PIC microcontroller that has been customized to understand the BASIC programming language. The use of the BASIC language makes it extremely easy to create software for the controller. The microcontroller chip can be purchased on a small carrier board that accepts a 9-volt battery, and you can program it by plugging it into one of the ports on your desktop computer. It is unlikely that any manufacturer would use a BASIC Stamp in an actual production device -- Stamps are expensive and slow (relatively speaking). However, it is quite common to use Stamps for prototyping or for one-off demo products because they are so incredibly easy to set up and use.
They are called "Stamps," by the way, because they are about as big as a postage stamp.
Parallax makes two versions of the BASIC Stamp: the BS-1 and the BS-2. Here are some of the differences between the two models:
Spec
BS-1
BS-2
RAM
14 bytes
26 bytes
EEPROM
256 bytes
2 kilobytes
Max program length
about 75 instructions
about 600 instructions
Execution speed
2,000 lines/sec
4,000 lines/sec
I/O pins
8
16
The specific BASIC Stamp we will be using in this article is called the "BASIC Stamp Revision D" (pictured below).
The BASIC Stamp Revision D is a BS-1 mounted on carrier board with a 9-volt battery holder, a power regulator, a connection for a programming cable, header pins for the I/O lines and a small prototyping area. You could buy a BS-1 chip and wire the other components in on a breadboard. The Revision D simply makes life easier.
You can see from the previous table that you aren't going to be doing anything exotic with a BASIC stamp. The 75-line limit (the 256 bytes of EEPROM can hold a BASIC program about 75 lines long) for the BS-1 is fairly constraining. However, you can create some pretty neat stuff, and the fact that the Stamp is so small and battery operated means that it can go almost anywhere.
What is a Microcontroller?
A microcontroller is a computer. All computers -- whether we are talking about a personal desktop computer or a large mainframe computer or a microcontroller -- have several things in common:
All computers have a CPU (central processing unit) that executes programs. If you are sitting at a desktop computer right now reading this article, the CPU in that machine is executing a program that implements the Web browser that is displaying this page.
The CPU loads the program from somewhere. On your desktop machine, the browser program is loaded from the hard disk.
The computer has some RAM (random-access memory) where it can store "variables."
And the computer has some input and output devices so it can talk to people. On your desktop machine, the keyboard and mouse are input devices and the monitor and printer are output devices. A hard disk is an I/O device -- it handles both input and output.
The desktop computer you are using is a "general purpose computer" that can run any of thousands of programs. Microcontrollers are "special purpose computers." Microcontrollers do one thing well. There are a number of other common characteristics that define microcontrollers. If a computer matches a majority of these characteristics, then you can call it a "microcontroller":
Microcontrollers are "embedded" inside some other device (often a consumer product) so that they can control the features or actions of the product. Another name for a microcontroller, therefore, is "embedded controller."
Microcontrollers are dedicated to one task and run one specific program. The program is stored in ROM (read-only memory) and generally does not change.
Microcontrollers are often low-power devices. A desktop computer is almost always plugged into a wall socket and might consume 50 watts of electricity. A battery-operated microcontroller might consume 50 milliwatts.
A microcontroller has a dedicated input device and often (but not always) has a small LED or LCD display for output. A microcontroller also takes input from the device it is controlling and controls the device by sending signals to different components in the device.
For example, the microcontroller inside a TV takes input from the remote control and displays output on the TV screen. The controller controls the channel selector, the speaker system and certain adjustments on the picture tube electronics such as tint and brightness. The engine controller in a car takes input from sensors such as the oxygen and knock sensors and controls things like fuel mix and spark plug timing. A microwave oven controller takes input from a keypad, displays output on an LCD display and controls a relay that turns the microwave generator on and off.
A microcontroller is often small and low cost. The components are chosen to minimize size and to be as inexpensive as possible.
A microcontroller is often, but not always, ruggedized in some way.
The microcontroller controlling a car's engine, for example, has to work in temperature extremes that a normal computer generally cannot handle. A car's microcontroller in Alaska has to work fine in -30 degree F (-34 C) weather, while the same microcontroller in Nevada might be operating at 120 degrees F (49 C). When you add the heat naturally generated by the engine, the temperature can go as high as 150 or 180 degrees F (65-80 C) in the engine compartment.
On the other hand, a microcontroller embedded inside a VCR hasn't been ruggedized at all.
The actual processor used to implement a microcontroller can vary widely. For example, the cell phone shown on Inside a Digital Cell Phone contains a Z-80 processor. The Z-80 is an 8-bit microprocessor developed in the 1970s and originally used in home computers of the time. The Garmin GPS shown in How GPS Receivers Work contains a low-power version of the Intel 80386, I am told. The 80386 was originally used in desktop computers.
In many products, such as microwave ovens, the demand on the CPU is fairly low and price is an important consideration. In these cases, manufacturers turn to dedicated microcontroller chips -- chips that were originally designed to be low-cost, small, low-power, embedded CPUs. The Motorola 6811 and Intel 8051 are both good examples of such chips. There is also a line of popular controllers called "PIC microcontrollers" created by a company called Microchip. By today's standards, these CPUs are incredibly minimalistic; but they are extremely inexpensive when purchased in large quantities and can often meet the needs of a device's designer with just one chip.
A typical low-end microcontroller chip might have 1,000 bytes of ROM and 20 bytes of RAM on the chip, along with eight I/0 pins. In large quantities, the cost of these chips can sometimes be just pennies. You certainly are never going to run Microsoft Word on such a chip -- Microsoft Word requires perhaps 30 megabytes of RAM and a processor that can run millions of instructions per second. But then, you don't need Microsoft Word to control a microwave oven, either. With a microcontroller, you have one specific task you are trying to accomplish, and low-cost, low-power performance is what is important.
