P1 software PASM DAC

Comments

• Christof Eb. Posts: 1,436

  2024-06-24 04:45

  Hi,
  Pwm plus a RC Filter does the trick.
• evanh Posts: 16,842

  2024-06-24 07:15 edited 2024-06-24 07:21

  Filtered PDM (duty-cycle counter mode) is far better than filtered PWM. PWM filters noisily in the mid-level range, worst when set at 50%, therefore it requires an oversized capacitor to compensate. This of course makes the filter response much slower. Whereas PDM has consistence noise at all set levels, allowing for an optimised filter.
• evanh Posts: 16,842

  2024-06-24 10:41 edited 2024-06-24 10:44

  Here's an example using counterB. It's configured for 0 to 10000. If you prefer 16-bit resolution then just delete the scaling line from dacset().

  PUB  tester | val
  
      dacinit( 16 )    ' pin #16 has R-C filter attached
  
      repeat
          repeat val from 0 to 10000
              dacset( val )
          waitms( 500 )
  
          repeat val from 10000 to 0
              dacset( val )
          waitms( 500 )
  
  
  PUB  dacinit( pinnum )
  
      ctrb := %00110 << 26 | pinnum    ' DUTY single-ended
      dira := 1 << pinnum    ' enable pin drive
  
  
  PUB  dacset( percent )    ' PDM percentage in hundreths of percent
  { duty = 4G * percent / 10000
  }
      percent := (||percent << 16 + 5000) / 10000 <# $ffff    ' scaled and bounded
      frqb := percent << 16 | percent    ' 16bit -> 32bit
  
  
  PUB  waitms( duration ) | timer, msec
  
      timer := cnt
      msec := clkfreq >> 10    ' one millsecond
      repeat duration
          timer += msec
          waitcnt( timer )
  
• evanh Posts: 16,842

  2024-06-24 12:53 edited 2024-06-24 13:00

  Oops, small bug - DIRA should be OR'd with rather than overwritten.

  PUB  dacinit( pinnum )
  
      ctrb := %00110 << 26 | pinnum    ' DUTY single-ended
      dira |= 1 << pinnum    ' enable pin drive
  

  or clearer still

  PUB  dacinit( pinnum )
  
      ctrb := %00110 << 26 | pinnum    ' DUTY single-ended
      dira[pinnum] := true    ' enable pin drive