If you only need to generate the fixed frequency of 625Hz, you could easily
do it with a 12C509A by using a single bit highly oversampled output.

Use a 4MHz crystal to get 1,000,000 instructions per second. This
corresponds to 1600 instructions for each cycle of the 625Hz output.

You would compute the ideal oversampled output waveform as a long string of
zeros and ones using a program on a PC. (Anybody have any pointers to how to
do this part?).

Then, given the sequence of zeros and ones you would (manually, or via a
program you write on the PC) generate ASM code for the body of the frequency
generating loop on the PIC:

You would like to just generate:

    BCF    GPIO,0    for each zero, and
    BSF    GPIO,0    for each one

But this won't work because you only have a total of 1024 instructions
available and it takes 1600 to generate a full cycle.
However, you can take advantage of sequences of three or more consecutive
zeros or ones like this:

Save one instruction for a sequence of 3 zeros(ones) in a row:

    BCF(BSF)    GPIO,0
    goto    $+1

Also, save one instruction for a sequence of 4 zeros(ones) in a row:

    BCF(BSF)    GPIO,0
    nop
    goto    $+1

For N or more zeros(ones) in a row, where N >= 5 you can save N-2
instructions using:

    BCF(BSF)    GPIO,0
    call      DelayN

Where off to the side in another part of the source you have:

Delay10:    nop
Delay9:      nop
Delay8:      nop
Delay7:      nop
Delay6:      nop
Delay5:      retlw    0

You will, of course, have to have a general structure of:

loop:
    BCF(BSF)    GPIO,0
    .
    .
    goto loop

and will have to make sure the entire loop takes exactly 1600 instructions,
including the final GOTO. Also, the final goto will have to occur at a time
when you are sending several identical bits.

Here is a puzzle/challenge:

How can you reduce the program memory requirements by a factor of
approximately 2?

Bob Ammerman
RAm Systems

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu