PWM Problem

John Abshier

I thought that this code, a modification to the PWM example in the counter application note would give me a 20kHz period.· The measured values (Parallax oscilloscope) are approximately 50Hz (actual value 48Hz).· I'm off by a factor of 400.· I did a simple Spin test to toggle a pin at 1kHz and it tested out OK on the oscilloscope.

''Demonstration of PWM version of NCO/PWM counter mode
CON _clkmode = xtal1 + pll8x
    _xinfreq = 10_000_000
VAR long parameter
PUB go | x
  cognew(@entry, @parameter)
  repeat
    parameter := 1000                    'a constant here locks to value x percent high
    waitcnt(1_000_000 + cnt)          'wait a little while before next update
DAT
'assembly cog which updates the PWM cycle on APIN
'for audio PWM, fundamental freq which must be out of auditory range (period < 50µS)
        org
entry   mov dira, diraval              'set APIN to output
        mov ctra, ctraval              'establish counter A mode and APIN
        mov frqa, #1                   'set counter to increment 1 each cycle
        mov time, cnt                  'record current time
        add time, period               'establish next period
:loop   rdlong value, par              'get an up to date pulse width
        waitcnt time, period           'wait until next period
        neg phsa, value                'back up phsa so that it
        jmp #:loop                     'loop for next cycle
diraval long 3                      'APIN=0 BPIN=0
ctraval long %00101 << 26 + (%000001 << 9)+ 0          'NCO-D/PWM APIN=0 BPIN = 1
period  long 4000                      '20kHz (50µS period) (_clkfreq / period)                      
time    res 1
value   res 1

John Abshier

Never mind. I guess I need to go reread the oscilloscope manual. The horizontal time division control was set to 5mS. When I changed it to a smaller value I get 20kHz as expected. A good way to kill the morning though.