Storing constant data in program memory

Original Assignment

Your PIC has a relatively large amount of program memory (32 Kbytes) but relatively little data memory (1536 bytes). For some applications, we want to have lookup tables or calibration data stored on our PIC, but we don't want it hogging data memory. This project is to demonstrate how to write a program that stores constant data in program memory. Test your ability to do this by creating a look-up table for the sin function. This is a 1-d array with integer index corresponding to an angle (e.g., numbers 0 to 89 if the index is in degrees; angles in other quadrants can be determined by simple transformations). Depending on your choice of int8, int16, or float to represent the number, what angle resolution can you use for the index before you reach the limits of program memory?

See p. 43 of the PIC MCU C Compiler book.

Finally, give sample code for storing data in the EEPROM, too.

Overview

Your PIC has a relatively large amount of program memory (32 Kbytes) but relatively little data memory (1536 bytes). For some applications, we want to have lookup tables or calibration data stored on our PIC, but we don't want it hogging data memory. The CCS C Compiler provides a few different ways to use program memory for data. They are discussed below.

Constant Data

The CONST qualifier will place variables into program memory. If the keyword CONST is used before the identifier, the identifier is treated as a constant. These constants need to be initialized and cannot be changed at run-time.

The ROM qualifier puts data in program memory with 3 bytes per instruction space. The address used for ROM data is a true byte address not a physical address. The & operator can be used on ROM variables even though the address is logical not physical.

The syntax is: const type id[cexpr] = {value}

#ROM Directive

Another method that can be used to assign data to program memory is the #rom.

Built-in Functions

The compiler provides built in functions to place data in program memory.

write_program_eeprom(address,data);

• Writes data to program memory

write_program_memory(address, dataptr, count);

• Writes count bytes of data from dataptr to address in program memory.
• Every fourth byte of data will not be written, this needs to be filled with 0x00.

Code

Constant Data Example

Placing data into ROM

• const int table[16]={0,1,2...15}

Placing a string into ROM

• const char cstring[6]={"hello"}

Creating pointers to constants

• const char *cptr;cptr = string;