PIC microcontroller

PIC microcontrollers in DIP and QFN packages
16-bit 28-pin PDIP PIC24 microcontroller next to a metric ruler
Die of a PIC12C508 8-bit, fully static, EEPROM/EPROM/ROM-based CMOS microcontroller manufactured by Microchip Technology using a 1200 nanometre process.
Die of a PIC16C505 CMOS ROM-based 8-bit microcontroller manufactured by Microchip Technology using a 1200 nanometre process.

PIC is a family of modified Harvard architecture microcontrollers made by Microchip Technology, derived from the PIC1650[1][2][3] originally developed by General Instrument's Microelectronics Division. The name PIC initially referred to Peripheral Interface Controller.[4][5] The first parts of the family were available in 1976; by 2013 the company had shipped more than twelve billion individual parts, used in a wide variety of embedded systems.

Early models of PIC had read-only memory (ROM) or field-programmable EPROM for program storage, some with provision for erasing memory. All current models use Flash memory for program storage, and newer models allow the PIC to reprogram itself. Program memory and data memory are separated. Data memory is 8-bit, 16-bit and in latest models, 32-bit wide. Program instructions vary in bit-count by family of PIC, and may be 12, 14, 16, or 24 bits long. The instruction set also varies by model, with more powerful chips adding instructions for digital signal processing functions.

The hardware capabilities of PIC devices range from 8-pin DIP chips up to 100-pin SMD chips, with discrete I/O pins, ADC and DAC modules, and communications ports such as UART, I2C, CAN, and even USB. Low-power and high-speed variations exist for many types.

The manufacturer supplies computer software for development known as MPLAB, assemblers and C/C++ compilers, and programmer/debugger hardware under the MPLAB and PICKit series. Third party and some open-source tools are also available. Some parts have in-circuit programming capability; low-cost development programmers are available as well has high-production programmers.

PIC devices are popular with both industrial developers and hobbyists due to their low cost, wide availability, large user base, extensive collection of application notes, availability of low cost or free development tools, serial programming, and re-programmable Flash-memory capability.

History

Various older (EPROM) PIC microcontrollers

The original PIC was built to be used with General Instrument's new CP1600 16-bit Central processing unit (CPU). 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 was not used at the time, it shares some common features with RISC designs.

In 1985, General Instrument 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 an internal EPROM to produce a programmable channel controller. 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 256 words to 64k words and more (a "word" is one assembly language instruction, varying in length from 8 to 16 bits, depending on the specific PIC micro family).

PIC and PICmicro are registered trademarks of Microchip Technology. It is generally thought that PIC stands for Peripheral Interface Controller, although General Instruments' original acronym for the initial PIC1640 and PIC1650 devices was "Programmable Interface Controller".[4] The acronym was quickly replaced with "Programmable Intelligent Computer".[5]

The Microchip 16C84 (PIC16x84), introduced in 1993, was the first Microchip CPU with on-chip EEPROM memory.

By 2013, Microchip was shipping over one billion PIC microcontrollers every year.[6]

Core architecture

The PIC architecture is characterized by its multiple attributes:

There is no distinction between memory space and register space because the RAM serves the job of both memory and registers, and the RAM is usually just referred to as the register file or simply as the registers.

Data space (RAM)

PICs have a set of registers that function as general-purpose RAM. Special-purpose control registers for on-chip hardware resources are also mapped into the data space. The addressability of memory varies depending on device series, and all PIC devices have some banking mechanism to extend addressing to additional memory. Later series of devices feature move instructions, which can cover the whole addressable space, independent of the selected bank. In earlier devices, any register move had to be achieved through the accumulator.

To implement indirect addressing, a "file select register" (FSR) and "indirect register" (INDF) are used. A register number is written to the FSR, after which reads from or writes to INDF will actually be to or from the register pointed to by FSR. Later devices extended this concept with post- and pre- increment/decrement for greater efficiency in accessing sequentially stored data. This also allows FSR to be treated almost like a stack pointer (SP).

External data memory is not directly addressable except in some PIC18 devices with high pin count.

Code space

The code space is generally implemented as ROM, EPROM or flash RAM. In general, external code memory is not directly addressable due to the lack of an external memory interface. The exceptions are PIC17 and select high pin count PIC18 devices.[8]

Word size

All PICs handle (and address) data in 8-bit chunks. However, the unit of addressability of the code space is not generally the same as the data space. For example, PICs in the baseline (PIC12) and mid-range (PIC16) families have program memory addressable in the same wordsize as the instruction width, i.e. 12 or 14 bits respectively. In contrast, in the PIC18 series, the program memory is addressed in 8-bit increments (bytes), which differs from the instruction width of 16 bits.

In order to be clear, the program memory capacity is usually stated in number of (single-word) instructions, rather than in bytes.

Stacks

PICs have a hardware call stack, which is used to save return addresses. The hardware stack is not software-accessible on earlier devices, but this changed with the 18 series devices.

Hardware support for a general-purpose parameter stack was lacking in early series, but this greatly improved in the 18 series, making the 18 series architecture more friendly to high-level language compilers.

Instruction set

PIC's instructions vary from about 35 instructions for the low-end PICs to over 80 instructions for the high-end PICs. The instruction set includes instructions to perform a variety of operations on registers directly, the accumulator and a literal constant or the accumulator and a register, as well as for conditional execution, and program branching.

Some operations, such as bit setting and testing, can be performed on any numbered register, but bi-operand arithmetic operations always involve W (the accumulator), writing the result back to either W or the other operand register. To load a constant, it is necessary to load it into W before it can be moved into another register. On the older cores, all register moves needed to pass through W, but this changed on the "high-end" cores.

PIC cores have skip instructions, which are used for conditional execution and branching. The skip instructions are "skip if bit set" and "skip if bit not set". Because cores before PIC18 had only unconditional branch instructions, conditional jumps are implemented by a conditional skip (with the opposite condition) followed by an unconditional branch. Skips are also of utility for conditional execution of any immediate single following instruction. It is possible to skip skip instructions. For example, the instruction sequence "skip if A; skip if B; C" will execute C if A is true or if B is false.

The 18 series implemented shadow, registers which save several important registers during an interrupt, providing hardware support for automatically saving processor state when servicing interrupts.

In general, PIC instructions fall into 5 classes:

  1. Operation on working register (WREG) with 8-bit immediate ("literal") operand. E.g. movlw (move literal to WREG), andlw (AND literal with WREG). One instruction peculiar to the PIC is retlw, load immediate into WREG and return, which is used with computed branches to produce lookup tables.
  2. Operation with WREG and indexed register. The result can be written to either the Working register (e.g. addwf reg,w). or the selected register (e.g. addwf reg,f).
  3. Bit operations. These take a register number and a bit number, and perform one of 4 actions: set or clear a bit, and test and skip on set/clear. The latter are used to perform conditional branches. The usual ALU status flags are available in a numbered register so operations such as "branch on carry clear" are possible.
  4. Control transfers. Other than the skip instructions previously mentioned, there are only two: goto and call.
  5. A few miscellaneous zero-operand instructions, such as return from subroutine, and sleep to enter low-power mode.

Performance

The architectural decisions are directed at the maximization of speed-to-cost ratio. The PIC architecture was among the first scalar CPU designs and is still among the simplest and cheapest. The Harvard architecture, in which instructions and data come from separate sources, simplifies timing and microcircuit design greatly, and this benefits clock speed, price, and power consumption.

The PIC instruction set is suited to implementation of fast lookup tables in the program space. Such lookups take one instruction and two instruction cycles. Many functions can be modeled in this way. Optimization is facilitated by the relatively large program space of the PIC (e.g. 4096 × 14-bit words on the 16F690) and by the design of the instruction set, which allows embedded constants. For example, a branch instruction's target may be indexed by W, and execute a "RETLW", which does as it is named – return with literal in W.

Interrupt latency is constant at three instruction cycles. External interrupts have to be synchronized with the four-clock instruction cycle, otherwise there can be a one instruction cycle jitter. Internal interrupts are already synchronized. The constant interrupt latency allows PICs to achieve interrupt-driven low-jitter timing sequences. An example of this is a video sync pulse generator. This is no longer true in the newest PIC models, because they have a synchronous interrupt latency of three or four cycles.

Advantages

Limitations

The following stack limitations have been addressed in the PIC18 series, but still apply to earlier cores:

With paged program memory, there are two page sizes to worry about: one for CALL and GOTO and another for computed GOTO (typically used for table lookups). For example, on PIC16, CALL and GOTO have 11 bits of addressing, so the page size is 2048 instruction words. For computed GOTOs, where you add to PCL, the page size is 256 instruction words. In both cases, the upper address bits are provided by the PCLATH register. This register must be changed every time control transfers between pages. PCLATH must also be preserved by any interrupt handler.[10]

Compiler development

While several commercial compilers are available, in 2008, Microchip released their own C compilers, C18 and C30, for the line of 18F 24F and 30/33F processors.

As of 2013, Microchip offers their XC series of compilers, for use with MPLAB X. Microchip will eventually phase out its older compilers, such as C18, and recommends using their XC series compilers for new designs.[11]

The easy to learn RISC instruction set of the PIC assembly language code can make the overall flow difficult to comprehend. Judicious use of simple macros can increase the readability of PIC assembly language. For example, the original Parallax PIC assembler ("SPASM") has macros, which hide W and make the PIC look like a two-address machine. It has macro instructions like mov b, a (move the data from address a to address b) and add b, a (add data from address a to data in address b). It also hides the skip instructions by providing three-operand branch macro instructions, such as cjne a, b, dest (compare a with b and jump to dest if they are not equal).

Device families

PICmicro chips are designed with a Harvard architecture, and are offered in various device families. The baseline and mid-range families use 8-bit wide data memory, and the high-end families use 16-bit data memory. The latest series, PIC32MX is a 32-bit MIPS-based microcontroller. Instruction words are in sizes of 12-bit (PIC10 and PIC12), 14-bit (PIC16) and 24-bit (PIC24 and dsPIC). The binary representations of the machine instructions vary by family and are shown in PIC instruction listings.

PIC10 and PIC12

For more details on this family of microcontrollers, see PIC instruction listings § Baseline core devices (12 bit).

These devices feature a 12-bit wide code memory, a 32-byte register file, and a tiny two level deep call stack. They are represented by the PIC10 series, as well as by some PIC12 and PIC16 devices. Baseline devices are available in 6-pin to 40-pin packages.

Generally the first 7 to 9 bytes of the register file are special-purpose registers, and the remaining bytes are general purpose RAM. Pointers are implemented using a register pair: after writing an address to the FSR (file select register), the INDF (indirect f) register becomes an alias for the addressed register. If banked RAM is implemented, the bank number is selected by the high 3 bits of the FSR. This affects register numbers 16–31; registers 0–15 are global and not affected by the bank select bits.

Because of the very limited register space (5 bits), 4 rarely read registers were not assigned addresses, but written by special instructions (OPTION and TRIS).

The ROM address space is 512 words (12 bits each), which may be extended to 2048 words by banking. CALL and GOTO instructions specify the low 9 bits of the new code location; additional high-order bits are taken from the status register. Note that a CALL instruction only includes 8 bits of address, and may only specify addresses in the first half of each 512-word page.

Lookup tables are implemented using a computed GOTO (assignment to PCL register) into a table of RETLW instructions. § Baseline core devices (12 bit).

PIC16

These devices feature a 14-bit wide code memory, and an improved 8 level deep call stack. The instruction set differs very little from the baseline devices, but the 2 additional opcode bits allow 128 registers and 2048 words of code to be directly addressed. There are a few additional miscellaneous instructions, and two additional 8-bit literal instructions, add and subtract. The mid-range core is available in the majority of devices labeled PIC12 and PIC16.

The first 32 bytes of the register space are allocated to special-purpose registers; the remaining 96 bytes are used for general-purpose RAM. If banked RAM is used, the high 16 registers (0x70–0x7F) are global, as are a few of the most important special-purpose registers, including the STATUS register which holds the RAM bank select bits. (The other global registers are FSR and INDF, the low 8 bits of the program counter PCL, the PC high preload register PCLATH, and the master interrupt control register INTCON.)

The PCLATH register supplies high-order instruction address bits when the 8 bits supplied by a write to the PCL register, or the 11 bits supplied by a GOTO or CALL instruction, is not sufficient to address the available ROM space.

PIC17

The 17 series never became popular and has been superseded by the PIC18 architecture. It is not recommended for new designs, and availability may be limited.

Improvements over earlier cores are 16-bit wide opcodes (allowing many new instructions), and a 16 level deep call stack. PIC17 devices were produced in packages from 40 to 68 pins.

The 17 series introduced a number of important new features:

PIC18

For more details on this family of microcontrollers, see PIC instruction listings § PIC18 high end core devices (8 bit).

In 2000, Microchip introduced the PIC18 architecture. Unlike the 17 series, it has proven to be very popular, with a large number of device variants presently in manufacture. In contrast to earlier devices, which were more often than not programmed in assembly, C has become the predominant development language.[12]

The 18 series inherits most of the features and instructions of the 17 series, while adding a number of important new features:

The RAM space is 12 bits, addressed using a 4-bit bank select register and an 8-bit offset in each instruction. An additional "access" bit in each instruction selects between bank 0 (a=0) and the bank selected by the BSR (a=1).

A 1-level stack is also available for the STATUS, WREG and BSR registers. They are saved on every interrupt, and may be restored on return. If interrupts are disabled, they may also be used on subroutine call/return by setting the s bit (appending ", FAST" to the instruction).

The auto increment/decrement feature was improved by removing the control bits and adding four new indirect registers per FSR. Depending on which indirect file register is being accessed it is possible to postdecrement, postincrement, or preincrement FSR; or form the effective address by adding W to FSR.

In more advanced PIC18 devices, an "extended mode" is available which makes the addressing even more favorable to compiled code:

These changes were primarily aimed at improving the efficiency of a data stack implementation. If FSR2 is used either as the stack pointer or frame pointer, stack items may be easily indexed—allowing more efficient re-entrant code. Microchip's MPLAB C18 C compiler chooses to use FSR2 as a frame pointer.

PIC24 and dsPIC

For more details on these families of microcontrollers, see PIC instruction listings § PIC24 and dsPIC 16-bit microcontrollers.

In 2001, Microchip introduced the dsPIC series of chips,[13] which entered mass production in late 2004. They are Microchip's first inherently 16-bit microcontrollers. PIC24 devices are designed as general purpose microcontrollers. dsPIC devices include digital signal processing capabilities in addition.

Although still similar to earlier PIC architectures, there are significant enhancements:[14]

(But operations on f operands always reference W0.)

Some features are:

dsPICs can be programmed in C using Microchip's XC16 compiler (formerly called C30) which is a variant of GCC.

Instruction ROM is 24 bits wide. Software can access ROM in 16-bit words, where even words hold the least significant 16 bits of each instruction, and odd words hold the most significant 8 bits. The high half of odd words reads as zero. The program counter is 23 bits wide, but the least significant bit is always 0, so there are 22 modifiable bits.

Instructions come in 2 main varieties. One is like the classic PIC instructions, with an operation between W0 and a value in a specified f register (i.e. the first 8K of RAM), and a destination select bit selecting which is updated with the result. The W registers are memory-mapped. so the f operand may be any W register,

PIC32MX

In November 2007, Microchip introduced the new PIC32MX family of 32-bit microcontrollers. The initial device line-up is based on the industry standard MIPS32 M4K Core.[15] The device can be programmed using the Microchip MPLAB C Compiler for PIC32 MCUs, a variant of the GCC compiler. The first 18 models currently in production (PIC32MX3xx and PIC32MX4xx) are pin to pin compatible and share the same peripherals set with the PIC24FxxGA0xx family of (16-bit) devices allowing the use of common libraries, software and hardware tools. Today starting at 28 pin in small QFN packages up to high performance devices with Ethernet, CAN and USB OTG, full family range of mid-range 32-bit microcontrollers are available.

The PIC32 architecture brings a number of new features to Microchip portfolio, including:

PIC32MZ

In November 2013, Microchip introduced the PIC32MZ series of microcontrollers, based on the MIPS M14K core. The PIC32MZ series include:[17]

Clones

ELAN Microelectronics

For more details on this family of microcontrollers, see PIC instruction listings § ELAN Microelectronics clones (13 bit).

ELAN Microelectronics Corp. produce a series of PICmicro-like microcontrollers with a 13-bit instruction word.[18] The instructions are mostly compatible with the mid-range 14-bit instruction set, but limited to a 6-bit register address (16 special-purpose registers and 48 bytes of RAM) and a 10-bit (1024 word) program space.

The 10-bit program counter is accessible as R2. Reads access only the low bits, and writes clear the high bits. An exception is the TBL instruction, which modifies the low byte while preserving bits 8 and 9.

The 7 accumulator-immediate instructions are renumbered relative to the 14-bit PICmicro, to fit into 3 opcode bits rather than 4, but they are all there, as well as an additional software interrupt instruction.

There are a few additional miscellaneous instructions, and there are some changes to the terminology (the PICmicro OPTION register is called the CONTrol register; the PICmicro TRIS registers 1–3 are called I/O control registers 5–7), but the equivalents are obvious. Some models support multiple ROM or RAM banks, in a manner similar to other PIC microcontrollers.

Holtek

Holtek produce a large number of PICMicro-like microcontrollers,[19] in the HT37, HT4x, HT56, HT6x, HT82 and HT95 families. They are most similar to the 14-bit mid-range PICmicro processors, but not exact clones. The STATUS register contains an arithmetic overflow flag, they include several additional instructions, and many include two pointer/indirect registers.

They come in three instruction widths:

Instructions not in the basic 14-bit PIC instruction set are:

Notable differences from the original PICmicro:

Parallax

Main article: Parallax SX

Parallax produced a series of PICMicro-like microcontrollers known as the Parallax SX. It is currently discontinued. Designed to be architecturally similar to the PIC microcontrollers used in the original versions of the BASIC Stamp, SX microcontrollers replaced the PIC in several subsequent versions of that product.

Parallax's SX are 8-bit RISC microcontrollers, using a 12-bit instruction word, which run fast at 75 MHz (75 MIPS). They include up to 4096 12-bit words of Flash memory and up to 262 bytes of random access memory, an eight bit counter and other support logic. There are software library modules to emulate I2C and SPI interfaces, UARTs, frequency generators, measurement counters and PWM and sigma-delta A/D converters. Other interfaces are relatively easy to write, and existing modules can be modified to get new features.

Hardware features

PIC devices generally feature:

Variants

Within a series, there are still many device variants depending on what hardware resources the chip features:

Trends

The first generation of PICs with EPROM storage are almost completely 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) and windowed (UV-erasable) versions of some of its EPROM based PICs for legacy support or volume orders. The Microchip website lists PICs that are not electrically erasable as OTP. UV erasable windowed versions of these chips can be ordered.

Part number

The F in a PICMicro part number generally indicates the PICmicro uses flash memory and can be erased electronically. Conversely, a C generally means it can only be erased by exposing the die to ultraviolet light (which is only possible if a windowed package style is used). An exception to this rule is the PIC16C84 which uses EEPROM and is therefore electrically erasable.

An L in the name indicates the part will run at a lower voltage, often with frequency limits imposed.[20] Parts designed specifically for low voltage operation, within a strict range of 3 - 3.6 volts, are marked with a J in the part number. These parts are also uniquely I/O tolerant as they will accept up to 5 V as inputs.[20]

Development tools

Main article: MPLAB

Microchip provides a freeware IDE package called MPLAB, which includes an assembler, linker, software simulator, and debugger. They also sell C compilers for the PIC18, PIC24, PIC32 and dsPIC, which integrate cleanly with MPLAB. Free student versions of the C compilers are also available with all features. But for the free versions, optimizations will be disabled after 60 days.[21] The cheapest compiler for the most common PIC18 serie and commercial use starts at around $500.[22]

Several third parties develop C language compilers for PICs, many of which integrate to MPLAB and/or feature their own IDE. A fully featured compiler for the PICBASIC language to program PIC microcontrollers is available from meLabs, Inc. Mikroelektronika offers PIC compilers in C, Basic and Pascal programming languages.

A graphical programming language, Flowcode, exists capable of programming 8- and 16-bit PIC devices and generating PIC-compatible C code. It exists in numerous versions from a free demonstration to a more complete professional edition.

Device programmers

Main article: PICkit

Devices called "programmers" are traditionally used to get program code into the target PIC. Most PICs that Microchip currently sell feature 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.

Microchip offers programmers/debuggers under the MPLAB and PICKit series. MPLAB ICD and MPLAB REAL ICE are the current programmers and debuggers for professional engineering, while PICKit is a low-cost programmer-only line for hobbyists and students.

Bootloading

Many of the higher end flash based PICs can also self-program (write to their own program memory), a process known as bootloading. Demo boards are available with a small bootloader factory programmed that can be used to load user programs over an interface such as RS-232 or USB, thus obviating the need for a programmer device.

Alternatively there is bootloader firmware available that the user can load onto the PIC using ICSP. After programming the bootloader onto the PIC, the user can then reprogram the device using RS232 or USB, in conjunction with specialized computer software.

The advantages of a bootloader over ICSP is faster programming speeds, immediate program execution following programming, and the ability to both debug and program using the same cable.

Third party

There are many programmers for PIC microcontrollers, ranging from the extremely simple designs which rely on ICSP to allow direct download of code from a host computer, to intelligent programmers that can verify the device at several supply voltages. Many of these complex programmers use a pre-programmed PIC themselves to send the programming commands to the PIC that is to be programmed. The intelligent type of programmer is needed to program earlier PIC models (mostly EPROM type) which do not support in-circuit programming.

Third party programmers range from plans to build your own, to self-assembly kits and 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 often built using PICs themselves for control).

Debugging

Main article: MPLAB devices

In-circuit debugging

All newer PIC devices feature an ICD (in-circuit debugging) interface, built into the CPU core, that allows for interactive debugging of the program in conjunction with MPLAB IDE. MPLAB ICD and MPLAB REAL ICE debuggers can communicate with this interface using the ICSP interface.

This debugging system comes at a price however, namely limited breakpoint count (1 on older devices, 3 on newer devices), loss of some I/O (with the exception of some surface mount 44-pin PICs which have dedicated lines for debugging) and loss of some on-chip features.

In-circuit emulators

Microchip offers three full in-circuit emulators: the MPLAB ICE2000 (parallel interface, a USB converter is available); the newer MPLAB ICE4000 (USB 2.0 connection); and most recently, the REAL ICE (USB 2.0 connection). All such tools are typically used in conjunction with MPLAB IDE for source-level interactive debugging of code running on the target.

Operating systems

PIC projects may utilize Real time operating systems such as FreeRTOS, AVIX RTOS, uRTOS, Salvo RTOS or other similar libraries for task scheduling and prioritization.

An open source project by Serge Vakulenko adapts 2.11BSD to the PIC32 architecture, under the name RetroBSD. This brings a familiar Unix-like operating system, including an on board development environment, to the microcontroller, within the constraints of the onboard hardware.[23]

See also

References

External links

Wikibooks has a book on the topic of: Embedded Systems/PIC Microcontroller
Wikibooks has a book on the topic of: A Guide To PIC Microcontroller Documentation
Wikimedia Commons has media related to PIC microcontrollers.
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