A Programmer’s view of the PIC

What is a programmer?

In the world of embedded systems, a programmer can be a person who writes software for a microcontroller or a device for downloading programs onto a microcontroller.

Useful documents

• PIC24HJ32GP202/204 and PIC24HJ16GP304 data sheet
• PIC24HJ32GP302/304, PIC24HJ64GPX02/X04 and PIC24HJ128GPX02/X04 data sheet
• 16-bit MCU and DSC Programmer’s Reference Manual
• PIC24H Family Reference Manual
• MPLAB^® XC16 Assembler, Linker and Utilities User’s Guide

Harvard architecture

Just the number

• Program memory (chip data sheet, pp. 25)
  • 2²⁴ 16-bit words   32 MB
  • But ignoring the ”phantom“ byte, 2²³ 24-bit words   24MB
• Data memory (chip data sheet, pp. 28)
  • 2¹⁶ 8-bit words   64 kB

A little detail

• Program memory (chip data sheet, pp. 25)
  • User memory
    • address of interrupts and traps
    • flash memory for program (a few thousand instructions)
  • Configuration memory
    • Device configuration
    • DEVID — Device identification
• Data memory (chip data sheet, pp. 28)
  • SFR: Special Function Registers
  • RAM
  • PSV: Program Space Visibility

Special Function Registers are similar to the memory-mapped I/O of larger processors. Program Space Visibility is similar to virtual memory.

Registers

The Special Function Registers are similar to two types of registers found in larger computers.

• Processor registers — a small collection of memory, separate from addressable memory, used for arithmetic, flow control, and memory access operators
• Device registers —

The PIC processor also contains shadow registers (see the programmer view on page 19).

PIC SFR’s are described in great detail in the processor data sheet (starting at page 29).

• working registers
• stack pointer limit
• program counter
• status register
• port registers (page 105)

Assembling language programming

You can write the following on the PIC.

     MOV   X,W9
     ADD   W9,#4,W10
     MOV   W10,Y
     BRA   NZ,FACT

But every one of the following instructions are illegal on the PIC processor used in class.

     MOV   X,Z
     ADD   W7,#57,W3
     MUL   W9,#4,W10
     BRA   W3,NZ,FACT

More more thing about integers

On the PIC processor stores integers in 16 bits or 2 addressable bytes. If 2013, also known as 0x7DD, is stored in two bytes starting at memory location 0xA00; what is stored in byte 0xA00 and what is stored in byte 0xA01.

In a little-endian computer, such as the PIC or Intel x86-64, byte 0xA00 contains value 0xDD and byte 0xA01 contains value 0x07. It’s the other way around on a big-endian computer.

The Network byte order of the Internet Protocol is big-endian. Programmers writing low-level networking code have to call routines such as htons to avoid misunderstandings.