Timing delay on my PIC

Question

Prepare for a nooby question.

I'm writing some ultra-simple code for this new PIC which I've just got. All I'm trying to do is to flash an LED. Below are two code samples - the first works but the second doesn't. Why?? I can't see any problem with the second one.

WORKS:

        while(1)
{
    i=99999;
    while(i--) {
        LATAbits.LATA0 = 0;  // set RA0 to logic 1
    }
    i=99999;
    while(i--) {
    LATAbits.LATA0 = 1;  // set RA0 to logic 0
    }
}    

DOESN'T WORK:

        while(1)
{
    LATAbits.LATA0 = 1;  // set RA0 to logic 1
    for(i=0;i<99999;i++) {}
    LATAbits.LATA0 = 0;  // set RA0 to logic 0
    for(i=0;i<99999;i++) {}
}

Thanks in advance for the help!

Answer 1

Try this:

while(1)
{
    for(i=0;i<99999;i++) 
    {
        LATAbits.LATA0 = 1;  // set RA0 to logic 1
    } 
    for(i=0;i<99999;i++) 
    {
        LATAbits.LATA0 = 0;  // set RA0 to logic 0
    }
}

Maybe your compiler is optimizing and ignoring the for statements as they have no code to execute inside. What I did was put the RA0 assignment inside these statements, forcing the compiler to keep the delay loops.

You can also pass the argument -S to your compiler to see the assembly code generated and confirm that the for statements were removed. The -S option generates an intermediate file with a ".S" extension.