Lpc2000

I'm using the following code for software direct digital waveform synthesis:

int main(void)
{
  unsigned int i;
  unsigned long int acc, adder;
  unsigned int ptr, temp;

  initPLL();
  MAMCR = 0;
  MAMTIM = 5;
  MAMCR = 2;
  VPBDIV = 0x01;
  portInit();
  //main loop
  //add value to accumulator
  //load byte from current table
  //output byte to port
  //repeat

  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?

Leon
--
Leon Heller, G1HSM
Email: aqzf13@...
http://www.geocities.com/leon_heller