Software pwm

[peterverkaik]

Hi,

I am trying to create a software pwm using 1 or 2 timers.

One timer:

Code: Select all

sub procedure Timer3Int org $22
  'When TMR3 matches PR3 this interrupt is executed and TMR3 is cleared
  T3CON.15 = 0 'T3 stopped
  if LATD.8 = 1 then
    'If softscale_duty = softscale_period then LATD.8 remains high for another softscale_period
    if softscale_duty <> softscale_period then
      LATD.8 = 0
      PR3 = softscale_period - softscale_duty 'low period
    end if
  else
    'If softscale_duty = 0 then LATD.8 remains low for another softscale_period
    if softscale_duty <> 0 then
      LATD.8 = 1
      PR3 = softscale_duty 'high period
    end if
  end if
  IFS0.7 = 0   'Clear T3IF
  TMR3 = 0
  T3CON.15 = 1 'T3 started
end sub

Two timers:

Code: Select all

sub procedure Timer4Int org $3E
  'When TMR4 matches PR4 this interrupt is executed and TMR4 is cleared.
  'LATD.8 is high when entering this routine.
  'If softscale_duty = softscale_period then LATD.8 remains high for another softscale_period
  if softscale_duty <> softscale_period then
    LATD.8 = 0
    T4CON.15 = 0 'T4 stopped
    PR5 = softscale_period - softscale_duty 'low period
    TMR5 = 0
    T5CON.15 = 1 'T5 started
  end if
  IFS1.5 = 0 'Clear T4IF
end sub

sub procedure Timer5Int org $40
  'When TMR5 matches PR5 this interrupt is executed and TMR5 is cleared.
  'LATD.8 is low when entering this routine.
  'If softscale_duty = 0 then LATD.8 remains low for another softscale_period
  if softscale_duty <> 0 then
    LATD.8 = 1
    T5CON.15 = 0 'T5 stopped
    PR4 = softscale_duty 'high period
    TMR4 = 0
    T4CON.15 = 1 'T4 started
  end if
  IFS1.6 = 0 'Clear T5IF
end sub

softscale period is a constant of 999.
There are of course lost clockcycles due to interrupt latency
and interrupt code.

My goal is to achieve that LATD.8 low period + high period
equals softscale_period+1

Which approach would give the best result?
What is the number of lost cycles (so I can compensate
the PR values to achieve my goal) ?
Any better approach (without using output compare)?

regards peter

[Skyline]

Hi,

What is the number of lost cycles (so I can compensate the PR values to achieve my goal) ?

Interrupt latency is 21 cycles for current version.

Any better approach (without using output compare)?

Another way is to use a single timer, and do not start and stop the clock. Set the PR value on the fly inside the timer interrupt. Do most of your logic, eg if softscale_duty = 0 or softscale_period then ... in the main program instead.