Electronic – PIC microcontroller memory limitations

compilerflashmicrocontrollerpicram

I am trying to program a PIC10f202 using the XC8 compiler to compare the value of a timer to a variable which will be a function of a look-up table. Here is a sample code :

#include <xc.h>

#pragma config MCLRE = OFF
#pragma config CP = OFF
#pragma config WDTE = OFF

Init(void){
    TRIS=0;
    GPIO=0;
    TMR0 = 0;
    OPTION = 0b00000011;
}

const unsigned char LUT[250] = {
0b00000000, 0b00000001, 0b00000001, 0b00000001, 0b00000001,
0b00000001, 0b00000001, 0b00000001, 0b00000001, 0b00000001,
0b00000001, 0b00000001, 0b00000001, 0b00000001, 0b00000001,
0b00000001, 0b00000001, 0b00000001, 0b00000001, 0b00000001,
0b00000001, 0b00000001, 0b00000001, 0b00000010, 0b00000010,
0b00000010, 0b00000010, 0b00000010, 0b00000010, 0b00000011,
0b00000011, 0b00000011, 0b00000011, 0b00000011, 0b00000100,
0b00000100, 0b00000100, 0b00000101, 0b00000101, 0b00000101,
0b00000110, 0b00000110, 0b00000110, 0b00000111, 0b00000111,
0b00001000, 0b00001000, 0b00001001, 0b00001001, 0b00001010,
0b00001010, 0b00001011, 0b00001100, 0b00001100, 0b00001101,
0b00001101, 0b00001110, 0b00001111, 0b00010000, 0b00010000,
0b00010001, 0b00010010, 0b00010011, 0b00010011, 0b00010100,
0b00010101, 0b00010110, 0b00010111, 0b00011000, 0b00011001,
0b00011010, 0b00011011, 0b00011100, 0b00011101, 0b00011110,
0b00011111, 0b00100000, 0b00100001, 0b00100010, 0b00100100,
0b00100101, 0b00100110, 0b00100111, 0b00101001, 0b00101010,
0b00101011, 0b00101100, 0b00101110, 0b00101111, 0b00110000,
0b00110010, 0b00110011, 0b00110101, 0b00110110, 0b00111000,
0b00111001, 0b00111011, 0b00111100, 0b00111110, 0b00111111,
0b01000001, 0b01000010, 0b01000100, 0b01000110, 0b01000111,
0b01001001, 0b01001011, 0b01001100, 0b01001110, 0b01010000,
0b01010001, 0b01010011, 0b01010101, 0b01010111, 0b01011000,
0b01011010, 0b01011100, 0b01011110, 0b01100000, 0b01100001,
0b01100011, 0b01100101, 0b01100111, 0b01101001, 0b01101010,
0b01101100, 0b01101110, 0b01110000, 0b01110010, 0b01110100,
0b01110110, 0b01111000, 0b01111001, 0b01111011, 0b01111101,
0b01111111, 0b10000001, 0b10000011, 0b10000101, 0b10000111,
0b10001000, 0b10001010, 0b10001100, 0b10001110, 0b10010000,
0b10010010, 0b10010100, 0b10010110, 0b10010111, 0b10011001,
0b10011011, 0b10011101, 0b10011111, 0b10100000, 0b10100010,
0b10100100, 0b10100110, 0b10101000, 0b10101001, 0b10101011,
0b10101101, 0b10101111, 0b10110000, 0b10110010, 0b10110100,
0b10110101, 0b10110111, 0b10111001, 0b10111010, 0b10111100,
0b10111110, 0b10111111, 0b11000001, 0b11000010, 0b11000100,
0b11000101, 0b11000111, 0b11001000, 0b11001010, 0b11001011,
0b11001101, 0b11001110, 0b11010000, 0b11010001, 0b11010010,
0b11010100, 0b11010101, 0b11010110, 0b11010111, 0b11011001,
0b11011010, 0b11011011, 0b11011100, 0b11011110, 0b11011111,
0b11100000, 0b11100001, 0b11100010, 0b11100011, 0b11100100,
0b11100101, 0b11100110, 0b11100111, 0b11101000, 0b11101001,
0b11101010, 0b11101011, 0b11101100, 0b11101101, 0b11101101,
0b11101110, 0b11101111, 0b11110000, 0b11110000, 0b11110001,
0b11110010, 0b11110011, 0b11110011, 0b11110100, 0b11110100,
0b11110101, 0b11110110, 0b11110110, 0b11110111, 0b11110111,
0b11111000, 0b11111000, 0b11111001, 0b11111001, 0b11111010,
0b11111010, 0b11111010, 0b11111011, 0b11111011, 0b11111011,
0b11111100, 0b11111100, 0b11111100, 0b11111101, 0b11111101,
0b11111101, 0b11111101, 0b11111101, 0b11111110, 0b11111110,
0b11111110, 0b11111110, 0b11111110, 0b11111110, 0b11111111};

unsigned char cnt1=0;

void main(){
        Init();
        for(;;){
            cnt1 = LUT[100];
            while(TMR0 > 100){
            GPIO=0b00000001;
            }
            GPIO=0b00000000;
        }
}

The look-up table is 250 characters because this is the maximum size that was allowed by the compiler before it gave me an error "Can't find x0FF words (x0ff withtotal) for psect "stringtext1" in class "Entry"". The code compiles fine, with MPLAB X stating 55% flash memory usage and 0% RAM usage. If I change the "while" condition to this :

while(TMR0 > cnt1)

The compiler will again give me a similar error : "Can't find x0FA words (x0fa withtotal) for psect "stringtext1" in class "Entry"". I understand the compiler can't find enough memory to write the code, but I don't see how 45% free space is not enough for this task. Also, why am I not able to create an array with 255 elements? The PIC10f202 has 512 words of memory, it should be possible.

Why is the compiler giving those errors?

Best Answer

This is a perfect example of how blindly using a compliler on a small resource-limited system without actually understanding the limitations of the machine will get you into trouble.

First, look at how a table of constants in program memory would need to be implemented. Go read the datasheet. No, really, go read it. How would this need too be done? Do you see any way of reading program memory, like a table read mechanism that some other PICs have? The only way to get constants out of program memory is with instructions that contain literals. How do you implement a table with that? No, actually think about it before reading on. Take a look at the list of instructions. The only way to implement a lookup table is with the RETLW instruction. That means you basically do a subroutine call to the table entry, which returns the value for that entry in W. Now look at the subroutine call mechanism. After you've done the reading you should have before writing the first line of code, it should be obvious why you can't possibly have more than 256 table entries with some of those possible table entries needing to be used for code.

Show that you've done some reading of the datasheet and I can go into more details.

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