PIC – P eripheral I nterface C ontroller

1. PIC – Peripheral Interface Controller

2. Introduction • PIC is a family of microcontroller • PIC has  Harvard architecture – separate program memory and data memory. •  Made by Microchip Technology. • Low - end PIC Architectures : • Microchip PIC microcontrollers -various types. • PIC microcontroller was first available from General Instruments in early 1980's - consisted of a simple processor executing 12-bit wide instructions with basic I/O functions,known as low-end architectures. • They have limited program memory and are meant for applications requiring simple interface functions and small program & data memories. Some of the low-end device numbers are 12C5XX, 16C5X, 16C505

3. Mid range PIC Architectures • Built by upgrading low-end architectures with more number of peripherals, more number of registers and more data/program memory. Some of the mid-range devices are • 16C6X 16C7X 16F87X • Program memory type is indicated by an alphabet. C = EPROM F = Flash RC = Mask ROM

4. Core Architecture • The PIC architecture is characterized by • Separate code and data spaces (Harvard architecture) for devices other than PIC32, which has a Von Neumann architecture. • A small number of fixed length instructions • Most instructions are single cycle execution (2 clock cycles, or 4 clock cycles in 8-bit models), with one delay cycle on branches . • One accumulator, the use of which (as source operand) is implied (i.e. is not encoded in the opcode) • All RAM locations function as registers as both source and/or destination of math and other functions.

5. A hardware stack for storing return addresses • A fairly small amount of addressable data space (typically 256 bytes), extended through banking • Data space mapped CPU, port, and peripheral registers • The program counter . • There is no distinction between memory space and register space because the RAM serves the job of both memory and registers. • RAM is usually referred to as the register file or simply as the registers

6. 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. • All PIC devices have some banking mechanism to extend addressing to additional memory. • To implement indirect addressing, a "file select register" (FSR) and "indirect register" (INDF) are used. • External data memory is not directly addressable except in some high pin count PIC18 devices.

7. Code Space • The code space is generally implemented as • ROM • EPROM • Flash ROM. • External code memory is not directly addressable due to the lack of an external memory interface.

8. Word Size • All PICs handle data and address in 8-bit chunks. • 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 word size as the instruction width. • In the PIC18 series, the program memory is addressed in 8-bit increments (bytes), which differs from the instruction width of 16 bits. • The program memory capacity is usually stated in number of (single word) instructions, rather than in bytes.

9. Stack • PICs have a hardware called 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.

10. CPU Architecture of PIC

11. Instruction Set • A 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 constant or the accumulator and a register. • 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. • Control transfers - only two: GOTO and CALL. • A few miscellaneous zero-operand instructions, such as return from subroutine, and sleep to enter low-power mode.

12. In general, PIC instructions fall into 5 classes: • Operation on working register (WREG) with 8-bit immediate ("literal") operand. • Operation with WREG and indexed register. The result can be written to either the Working register or the selected register . • 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.

13. Advantages • The PIC architectures have these advantages: • Small instruction set • RISC architecture • Built in oscillator with selectable speeds • Easy entry level, in circuit programming plus in circuit debugging . • Inexpensive microcontrollers • Wide range of interfaces including I²C, SPI, USB, USART, A/D, programmable comparators, PWM, LIN, CAN, PSP, and Ethernet

14. Limitations • One accumulator • Register-bank switching is required to access the entire RAM of many devices • Operations and registers are not orthogonal; some instructions can address RAM and/or immediate constants, while others can only use the accumulator • The hardware call stack is not addressable, so preemptive task switching cannot be implemented • Software-implemented stacks are not efficient so • With paged program memory, there are two page sizes.

15. Popularity of the PIC microcontrollers is due to the following factors. • Speed: Harvard Architecture, RISC architecture, 1 instruction cycle = 4 clock cycles. • Instruction set simplicity: The instruction set consists of just 35 instructions (as opposed to 111 instructions for 8051). • Power-on-reset and brown-out reset. Brown-out-reset means when the power supply goes below a specified voltage (say 4V), it causes PIC to reset; hence malfunction is avoided. A watch dog timer (user programmable) resets the processor if the software/program ever malfunctions and deviates  from its normal operation. • PIC microcontroller has four optional clock sources. •   Low power crystal •   Mid range crystal •   High range crystal •   RC oscillator (low cost). • Programmable timers and on-chip ADC. • Up to 12 independent interrupt sources. • Powerful output pin control (25 mA (max.) current sourcing capability per pin.) • EPROM/OTP/ROM/Flash memory option. • I/O port expansion capability.

16. PIC 16F877 Architecture • Most common microcontroller • Does not have USB or built in network interfaces • Main program memory is flash ROM – stores a list of 14 bits instruction. • These are fed to execution unit and used to modify RAM file registers. • Includes special control registers , port registers and general purpose registers. • A separate working register (W) is used with ALU to process data

17. 512 RAM file registers(000H – 1FFH), organized in 4 banks. • Each bank contains 128 addresses. • Default bank – bank 0 (0 – 7F), bank 1 (80 – FFH) • These contain SFRs and GPRs • Deducting SFRs from the total number of RAM locations , we get 368 bytes of GPRs • 35 instructions • RISC processor • 5 ports – A to E • A – 5pins, B – 8 pins, C – 8pins, D – 8pins and E – 3pins giving total of 32 I/O pins • Most of them have more than one function.

18. Pin diagram

19. PIC 16F877 Architecture