PIC MICROCONTROLLER

Manufactured by Microchip, the PIC ("Programmable Intelligent Computer" or "Peripheral Interface Controller" ) microcontroller is popular among engineers and hobbyists alike. PIC microcontrollers come in a variety of "flavors", each with different components and capabilities.

Many types of electronic projects can be constructed easily with the PIC family of microprocessors, among them clocks, very simple video games, robots, servo controllers, and many more. The PIC is a very general purpose microcontroller that can come with many different options, for very reasonable prices.

General Instruments produced a chip called the PIC1650, described as a Programmable Intelligent Computer.

Recommended "first PIC" 

At one time, the PIC16F84 was far and away the best PIC for hobbyists. But Microchip, Parallax, and Holtek are now manufacturing many chips that are even better and often even cheaper, because of the higher level of production.

PIC18F4620: it has 13 analog inputs --recommends that hobbyists use the largest and most capable chip available.

PIC16F877A , this is probably the most popular PIC used by the hobbyist community that is still under production. This is the best PIC of its family and used to be "the PIC" for bigger hobbyist projects, along with the PIC16F84 for smaller ones. Features 14KB of program memory, 368 bytes of RAM, a 40 pin package, 2 CPP modules, 8 ADC channels capable of 10-bit each. It also counts with the UART and MSSP, which is a SSP capable of being master, controlling any devices connected to the I2c and SPI busses. The lack of internal oscillator, as opposed to the other PICs mentioned until now, is something to be aware of. Also, this PIC is relatively expensive for the features included. This may be caused by Microchip to force the migration to better chips. 

PIC16F88, this is enhanced version of the PIC16F628A. It has all the features of the 16F628, plus twice the program memory, 7KB; seven 10-bit ADCs, a SSP (Synchronous Serial Port), capable of receiving messages sent over I2C and SPI busses. It also supports self-programming, a feature used by some development boards to avoid the need of using a programmer

PIC16F628A, this is a good starter PIC because of its compatibility with what used to be one of the hobbyist's favorite PICs: the PIC16F84. This way, the beginner can select from a vast catalog of projects and programs, specially when created in low level languages like the PIC Assembler. It features a 18 pin package, 3.5KB of Flash Memory, can execute up to 5 million instructions per second (MIPS) using a 20MHZ crystal. The lack of an Analog-Digital Converter (ADC) is something to point out. As opposed to the PIC16F84A it has an UART, which is capable of generating and receiving RS-232 signals, which is very useful for debugging. Some people use to find ironic that this chip is cheaper than the less-featured PIC16F84A. 

PIC16F1936, a powerful mid-range PIC, comes with an 11 channel, 10-bit ADC; two indirect pointer registers; XLP (extreme low power) for low power consumption on battery powered devices. -- recommended by some people on the PIClist as a faster, better, cheaper replacement for the 16F877. -
PIC12F683, a small 8-pin microcontroller. It is a good microcontroller for small applications due to its small size and relatively high power and diverse features, like 4 ADC channels and internal 4MHZ oscillator.

More selection tips

• The "F" Suffix implies that the chip has Re-programmable Flash memory.
• The "C" suffix implies that the chip uses EPROM memory. A few of these chips used to be erased with a very expensive Ultra-Violet eraser. This method was primarily used by companies. But most of these chips are specifically made so that once you write it you can't change it: it's OTP (one-time programmable). People used to check their programs minutely before programming them into such chips. Recently, this chips are becoming less used as the cost of Flash memory decreases, but some of them are still used because of their reliability or reduced costs.
• Each family has one "full" member with all the goodies and a subset of variant members that lack one thing or another. For example, on the 16F84 family, the 16F84 was the fully featured PIC, with Flash memory and twice the program space of the 16F83. The family was also composed by the 16C84 and 16C83, one of the few reprogrammable C suffix PICs. For prototyping, we generally use the "full" version to make sure we can get the prototype working at all. During prototyping we want to tweak code, reprogram, and test, over and over until it works. So we use one of the above "Flash" families, not the "OTP" families, unless required. For short production, the C parts are recommended. For very long production lines some PICs with mask-programmed ROMs where used. Now in-factory preprogramming is available from Microchip.

PIC 16x

The PIC 16 family is considered to be a good, general purpose family of PICs. PIC 16s generally have 3 output ports to work with. 

Flash-based chips such as the PIC16F88 are far more convenient to develop on, and can run code written for the above chips with little or no changes.

PIC 12x

The PIC12x series is the smallest series with 8 pins and up to 6 available I/O pins. These are used when space and/or cost is a factor.

PIC 18x

The PIC 18x series are available in a 28 and 40-pin DIP package. They have more ports, more ADC, etc... PIC 18s are generally considered to be very high-end microcontrollers, and are even sometimes called full-fledged CPUs.

Microchip is currently (as of 2007) producing 6 Flash microcontrollers with a USB interface. All are in the PIC18Fx family. 

The PIC Stack

The PIC stack is a dedicated bank of registers (separate from programmer-accessible registers) that can only be used to store return addresses during a function call (or interrupt).

• 12 bit: A PIC microcontroller with a 12 bit core (the first generation of PIC microcontrollers) ( including most PIC10, some PIC12, a few PIC16 ) only has 2 registers in its hardware stack. Subroutines in a 12-bit PIC program may only be nested 2 deep, before the stack overflows, and data is lost. People who program 12 bit PICs spend a lot of effort working around this limitation. 

• 14 bit: A PIC microcontroller with a 14 bit core (most PIC16) has 8 registers in the hardware stack. This makes function calls much easier to use. 

• 16 bit: A PIC microcontroller with a 16 bit core (all PIC18) has a "31-level deep" hardware stack depth. This is more than deep enough for most programs people write.

Many algorithms involving pushing data to, then later pulling data from, some sort of stack. People who program such algorithms on the PIC must use a separate software stack for data 

Call-tree analysis can be used to find the deepest possible subroutine nesting used by a program. (Unless the program uses w:recursion). As long as the deepest possible nesting of the "main" program, plus the deepest possible nesting of the interrupt routines, give a total sum less than the size of the stack of the microcontroller it runs on, then everything works fine. Some compilers automatically do such call-tree analysis, and if the hardware stack is insufficient, the compiler automatically switches over to using a "software stack". Assembly-language programmers are forced to do such analysis by hand.

Power Supply

The most important part of any electronic circuit is the power supply. The PIC programmer requires a +5 volt and a +13 volt regulated power supply. The need for two power supplies is due to the different programming algorithms:

• High Power Programming Mode - In this mode, we enter the programming mode of the PIC by driving the RB7(Data) and RB6(CLOCK) pins of the PIC low while driving the MCLR pin from 0 to VCC(+13v).
• Low Power Programming Mode - This alogrithm requires only +5v for the programming operation. In this algorithm, we drive RB3(PGM) from VDD to GND to enter the programming mode and then set MCLR to VDD(+5v).

Oscillator Circuits

Not all the PIC microcontrollers have built-in RC oscillator circuits available, although they are slow, and have high granularity. External oscillator circuits may be applied as well, up to a maximum frequency of 20MHz. PIC instructions require 4 clock cycles for each machine instruction cycle, and therefore can run at a maximum effective rate of 5MHz. However, certain PICs have a PLL (phase locked loop) multiplier built in. The user can enable the Times 4 multiplier, thus yielding a virtual oscillator frequency of 4 X External Oscillator. For example, with a maximum allowable oscillator of 16MHz, the virtual oscillator runs at 64MHz. Thus, the PIC will perform 64 / 4 = 16 MIPS (million instructions per second). Certain pics also have built-in oscillators, usually 4Mhz for precisely 1MIPS, or a low-power imprecise 48kHz. This frees up to two I/O pins for other purposes. The pins can also be used to produce a frequency if you want to synchronize other hardware to the same clock as one PIC's internal one.