PIC microcontroller

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Image:PIC1655A GI.jpg PIC is a family of RISC microcontrollers made by Microchip Technology, derived from the PIC1650 originally developed by General Instrument's Microelectronics Division.

Microchip Technology does not use PIC as an acronym; in fact the brand name is PICmicro. It is generally regarded that PIC stands for Peripheral Interface Controller, although General Instruments' original acronym for the PIC1650 was "Programmable Intelligent Computer". The original PIC was built to be used with GI's new 16-bit CPU, the CP1600. While generally a good CPU, the CP1600 had poor I/O performance, and the 8-bit PIC was developed in 1975 to improve performance of the overall system by offloading I/O tasks from the CPU. The PIC used simple microcode stored in ROM to perform its tasks, and although the term wasn't used at the time, it is a RISC design that runs one instruction per cycle (4 oscillator cycles).

In 1985 GI spun off their microelectronics division, and the new ownership cancelled almost everything — which by this time was mostly out-of-date. The PIC, however, was upgraded with EPROM to produce a programmable channel controller, and today a huge variety of PICs are available with various on-board peripherals (serial communication modules, UARTs, motor control kernels, etc.) and program memory from 512 words to 32k words and more (a "word" is one assembly language instruction, varying from 12, 14 or 16 bits depending on the specific PICmicro family).

Image:PIC16CxxxWIN.JPG

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Coding for PICs

PICs use a RISC instruction set, which varies in length from about 35 instructions for the low-end PICs to about 70 instructions for the high-end PICs. The instruction set includes instructions to perform a variety of operations on the accumulator and a constant or the accumulator and a memory location, as well as for conditionally executing code and jumping/calling other parts of the program and returning from them, and specific hardware features like interrupts and one low-power mode called sleep. Microchip provides a freeware IDE package called MPLAB, that also includes a software simulator as well as an assembler.

Third parties make C and BASIC language compilers for PICs. Microchip also sells compilers for the high-end PICs ("C18" for the 18F series and "C30" for the dsPICs). They also make available for download a "student edition/demo" version of C18 or C30 which disables some optimiser features after a timeout period. Open-source compilers for the C, Pascal, JAL, and for the Forth programming language, PicForth, have also been released.

GPUTILS is an Open Source collection of tools, distributed under the GNU General Public License. GPUTILS includes an assembler and linker and works on Linux, Mac OS X, OS/2 and Microsoft Windows. GPSIM is an Open Source simulator for the PIC microcontrollers featuring hardware modules that simulate specific devices that might be connected to them, like LCDs.

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Programming PICs

Devices called "programmers" are traditionally used to get program code into the target PIC. Most PICs that Microchip sells nowadays have ICSP (In Circuit Serial Programming) and/or LVP (Low Voltage Programming) capabilities, allowing the PIC to be programmed while it is sitting in the target circuit. ICSP programming is performed using the RB6 and RB7 pins for clock and data, while a high voltage (12V) is present on the Vpp/MCLR pin. Low voltage programming allows for the elimination of the extra voltage rail in the programmer but comes at the cost of an IO pin and can therefore be disabled (once disabled it can only be re-enabled using high voltage programming). There are many programmers for PIC microcontrollers, ranging from the extremely simple designs that rely on the communications software for taking care of all the communication details to complex designs that can verify the device at several supply voltages and can do much of the work in the hardware. Many of these complex programmers use a pre-programmed PIC themselves to send the programming commands to the PIC that is to be programmed.

Many of the higher end flash based PICs can also write to their own program memory. Demo boards are available with a small bootloader factory programmed that can be used to load user programs over an interface such as RS232 or USB.

There are many programmers/debuggers available directly from Microchip:

Microchip Programmers (as of 10/2005)

  • PICStart Plus (RS232 serial interface)
  • Promate II (RS232 serial interface)
  • MPLAB PM3 (RS232 serial and USB interface)
  • ICD2 (RS232 serial and USB interface)
  • PICKit 1 (USB interface)
  • PICKit 2 (USB interface)

Development tools:

Microchip home page

PICBASIC Popular BASIC Compiler, comprehensive and well supported by its users

KTechlab (open-source development environment for electronic circuits)

There are also programmers available from other sources, ranging from plans as to how to build your own, to kits to put together yourself, to fully tested ready-to-go units. Some are simple designs which require a PC to do the low-level programming signalling (these typically connect to the serial or parallel port and consist of a few simple components), while others have the programming logic built into them (these typically use a serial or USB connection, are usually faster, and are many times built using PICs themselves for control). These are some common programmers:

  • JDM programmer (simple serial-port programmer)
  • Feng's RCD programmer (another serial-port programmer)
  • Tait-style programmer (simple parallel port programmer)
  • Wisp628 (intelligent serial port in-circuit programmer built around a PIC16F628. Kit available.)
  • EasyProg (intelligent serial port in-circuit and standalone programmer. Kit and ready-to-go version available)

An exhaustive list of these is not practical, but a query put out on the piclist (piclist@mit.edu -- subscription required) would quickly provide many answers from the knowledgeable people who frequent that list and are happy to help.

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Debugging

MPLAB (which is a free download) includes a software emulator for PICs. However software emulation of a microcontroller will always suffer from limited simulation of the devices interactions with its target circuit.

The MPLAB ICD2 (RS232 serial and USB interface). Offers fairly cheap (about £100) in circuit debugging. A programming/debug connector (the ICD2 has a RJ12 socket and comes with a cross wired RJ12 lead but there is nothing stopping user of custom cables to other connector types) is designed into the target circuit. This cheap and simple debugging system comes at a price however, namely limited breakpoint count (1 on older pics 3 on newer pics), loss of some IO (with the exception of some surface mount 44 pin pics which have dedicated lines for debugging) and loss of some features of the chip. For small pics where the loss of IO caused by this method would be unacceptable special headers are made which are fitted with pics that have extra pins specifically for debugging. Some third party debugging solutions also exist using the on chip facilities provided for the ICD2.

Finally microchip offer two full in circuit emulators. The MPLAB ICE2000 (parallel interface, a USB converter is available) and the newer MPLAB ICE4000 (USB interface). Microchip is also planning to release a new debugging system called realICE. This appears to be an improved version of the ICD2 which is functionally similar to the ICE2000 and 4000 emulators. All PICs will be supported including the newer dsPIC 30/33 and PIC 24 series. Release is planned for later this year and will cost around £300.

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Word Size

The word size of PICs is a source of much confusion. All PICs (except dsPICs and PIC24s) handle data in 8-bit chunks, so they should be called 8-bit microcontrollers. But unlike most CPUs, PICs use a Harvard architecture, so the size of an instruction can be different from the size of the data. In fact, different PIC families use different instruction sizes, which makes it a challenge to compare the code size of PICs to other microcontrollers. For example, say a microcontroller has 6144 bytes of program memory --- for a 12-bit PIC, this works out to 4096 words (or assembly instructions); for a 16-bit PIC, this is 3072 words.

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Modern PICs

The old PROM and EPROM PICs are now gradually replaced by chips with Flash memory. Likewise the original 12-bit instruction set of the PIC1650 and its direct descendants has been superseded by 14-bit and 16-bit instruction sets. Microchip still sells OTP (one-time-programmable, or PROM) and UV-eraseable (EPROM) versions of most of its PICs for legacy support, or volume orders. It should be noted that the Microchip website lists PICs that are not electrically erasable as OTP despite the fact that UV erasable windowed versions of these chips can be ordered.

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Features

Current PICs offer a wide range of built-in hardware, such as:

  • 8/16 bit Modified Harvard Architecture CPU cores
  • Flash and ROM Memory options in 256 byte to 256 kilobyte array sizes
  • I/O Ports (0 to 5.5 volts Typical)
  • 8/16 Bit Timers
  • Nanowatt Technology for Power Moding
  • Synchronous/Asynchronous Serial Peripherals USART, AUSART, EUSARTs
  • Analog-to-digital converters, 10/12 bit
  • Voltage Comparators
  • Capture/Compare/PWM modules
  • LCD Drivers
  • MSSP Peripheral for I2C, SPI, and I2S Communications
  • Internal EEPROM Memory - up to 1M erase/write cycles durability
  • Motor Control Peripherals
  • USB interfacing support
  • Ethernet controller support
  • CAN controller support
  • LIN controller support
  • IRDA controller support
  • Integrated analog RF front ends (PIC16F639, and rfPIC)
  • KEELOQ Rolling code encryption peripheral (encode/decode)
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PICs on the Internet

PICs are well-documented on the Internet, mainly for two reasons. First, PICs were commonly used to defeat the security system of popular consumer products (pay-TV, PlayStation), which attracted the attention of crackers. Second, the now obsolete PIC16F84 was the first widely available microcontroller that could easily be re-programmed by hobbyists. There are many forums and lists concerning PIC microcontrollers where the user can post questions and receive free answers.

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PIC clones

Every now and then there are companies that offer cheap and/or enhanced PIC versions. Most seem to disappear quite soon. Ubicom (formerly Scenix) seems to avoid this fate and produces PIC clones that run much faster than the originals. OpenCores has a pic16f84 core written in Verilog.

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Wireless PICs

The rfPIC microcontroller devices integrate the power of Microchip´s PICmicro devices with UHF wireless communication capabilities for low power RF applications. The devices offer small package outline and low external component count to fit the most space-constrained applications.

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dsPICs (Digital Signal PICs)

dsPICs are Microchip's newest family, which entered mass production in late 2004. They are designed as a PIC with digital signal processing capabilities. These are Microchip's first inherent 16-bit (data) microcontrollers. They build on the PICs existing strengths by offering hardware MAC (multiply-accumulate), barrel shifting, bit reversal, (16x16)-bit multiplication and other digital signal processing operations.

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8/16-bit PIC microcontroller product families

These links take you to product selection matrices at the manufacturer's site.

8-bit Microcontrollers

16-bit Microcontrollers

16-bit Digital Signal Controllers

The F in a name generally indicates the PICmicro is flash programmable, i.e. it can be reprogrammed a large number of times. A C generally means it can be programmed only once.

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Commonly used PICs

  • PIC12C508/509 (Small 8-pin package, internal oscillator, popular in small designs such as the iPod remote)
  • PIC16F84 (generally considered obsolete, but still popular)
  • PIC16F84A (upgrade to above, some versions do 20 MHz, 1:1 compatible with PIC16F84)
  • PIC16F88 (an excellent all-round 18-pin PICmicro)
  • PIC12F629/675/683
  • PIC16F628 (replaces the PIC16F84. The 16F648A has more program memory and fully compatible)
  • The PIC16F87X family (The PIC16F84's "big brothers", with lots of built in hardware similar to the 16F88. Quite common in hobby projects.)
  • PIC18F452
  • PIC18F4550 and relatives (Full-speed USB support, all sorts of inbuilt hardware. Very powerful and versatile)
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See also

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Forums & Discussion Groups

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Other Links

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