Lpc2000

At 10:44 AM 12/3/03 +0000, you wrote:
>I'm using the following code for software direct digital waveform synthesis:

Ah, it just showed up.

>   adder = 2000;               //set up adder
>   acc = 0;                    //clear accumulator
>   while (1)
>   {
>     acc = acc + adder;      //update phase accumulator
>     ptr = (acc &= 0xFF);    //pointer into table
>     IOSET = square[ptr] << 8;//output byte to port
>     IOCLR = ~(square[ptr] << 8);
>   }
>   return 0;
>}
>
>but I can't get an equal mark-space ratio out of it for a square wave, and
>sine waves are completely screwed up.
>
>The following code for an AVR works very well:
>
>; main loop
>;
>; r28,r29,r30 is the phase accumulator
>;   r24,r25,r26 is the adder value determining frequency
>;
>;  add value to accumulator
>; load byte from current table in ROM
>; output byte to port
>; repeat
>;
>LOOP1:
>   add  r28,r24   ; 1
>   adc  r29,r25   ; 1
>   adc  r30,r26   ; 1
>   lpm      ; 3
>   out  PORTB,r0  ; 1
>   rjmp LOOP1   ; 2 => 9 cycles
>
>I'm using the same AD557 8-bit DAC and the same look-up tables. I'm
>obviously doing something silly, but I can't see what it is. Can anyone
>help?

OK, this makes things a little more clear.

Q: Is your distortion coming from loop to loop delay or from the delay 
between set and clear?  (I suspect the latter).

Robert