8 Bit Micro Controllers

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I have an interest in microcontrollers, both in my job, and as a hobby. 8 bit microcontrollers (those having a data bus width of 8 bits) have the advantage of being inexpensive but are also powerful enough to perform complicated tasks. The cheapest devices I have seen are 12C508 which cost £1.27 in quantity.

A microcontroller is effectively a computer on a chip. All computers need a central processing unit (CPU), memory holding the program to be run, memory for data to be stored and, if it's to do anything meaningful, some peripherals to speak to the outside world.

Quite often, computers, e.g. the PC, use the same memory for both program and data storage. This is usually read/write memory (RAM) because the program itself is frequently changed.

Microcontrollers are generally used for embedded processes where the program, once written, does not change e.g. car ignition systems, burglar alarms. In fact they are so cheap and easy to use, they are more and more being used to replace devices that traditionally used discrete electronic components e.g. the ubiquitous "dark room timer", a standard electronics project since time began.

Typically a microcontroller will contain ..

• an 8 bit central processing unit
• 512 - 8192 program memory locations (may be wider than 8 bit)
• 24 -8192 bytes of RAM
• various peripherals, for example
  • 5 - 40 digital IO lines
  • 1 - 3 counter/timers
  • built-in serial devices e.g. RS232 ports, I2C buses.
  • 1 - 8 channels of analogue to digital converters (ADC)
  • 1 - 2 pulse width modulated outputs (PWM)
  • a watch dog time
• they frequently have some form of power-down or sleep mode to reduce current consumption to a minimum during periods of inactivity.

Because the program is intended to be embedded, a method has to be found to create the program, which can change frequently during the development period.

This can be done in several ways ..

• use a version of the device that can access external program memory. This memory can be either EPROM, EEPROM or RAM, into which your program has been loaded.
• some devices have EPROM or EEPROM devices integrated into the chip. This can be programmed using a special programmer.
• more commonly now (Jul 2005) devices have FLASH (eeprom) memory on-chip to hold the program. They can be prgrammed directly from a PC using the serial or parallel (printer) ports (and a bit of hardware e.g. RS232 to logic level shifter)

Generally the program would be written on a development system, e.g. a PC with text editor and cross compiler or cross assembler, and then down loaded into the microcontroller's program memory from where it is run. Errors (God forbid) can be easily corrected on the PC and the modified program downloaded again (and again).

I have experience of several families of devices.

• Atmel's AVR 8 bit RISC family (specifically ATMEGA16L)
• The 8051 family from Intel, Phillips, Atmel and Dallas
• The PIC family from Microchip (the 16c5xx, 16C6xx devices)

Here are some links. If you are downloading data sheets, get up early and do it before the WWW locks solid.

Documentation

• Chip directory - find a chip.
• Russ Hersch's EXCELLENT micrcontroller FAQ
• Russ Hersch's EXCELLENT 8051 FAQ
• A PIC microcontroller FAQ
• A good I2C FAQ.

Manufacturers.

• Philips Semiconductors Home Page 8051 derivatives.
• ATMEL Home Page 8051 derivatives with EEPROM.
• Dallas Semiconductor Corp: Home Page fast 8051 derivatives.
• Microchip Home Page the PIC family.
• Xicor, Inc. 8051 (and other) support chips

Tools/Programmesr

• Forest Electronic Developments Home Page
• C51 Primer
• 8051 Development Tools
• 8051 Tools, Code andProjects

Misc

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Pages written by Stuart Allen. Last updated 5 Jul 2005