NAME

       pwmgen - software PWM/PDM generation

SYNOPSIS

       loadrt pwmgen output_type=type0[,type1...]

DESCRIPTION

       pwmgen  is  used to generate PWM (pulse width modulation) or PDM (pulse density modulation) signals.  The
       maximum PWM frequency and the resolution is quite limited compared to hardware-based approaches,  but  in
       many cases software PWM can be very useful.  If better performance is needed, a hardware PWM generator is
       a better choice.

       pwmgen  supports  a  maximum  of  eight  channels.  The number of channels actually loaded depends on the
       number of type values given.  The value of each type determines the outputs for that channel.

       type 0: single output A single output pin, pwm, whose duty cycle is determined by the input value for
       positive inputs, and which is off (or at min-dc) for negative inputs.
              Suitable for single ended circuits.

       type 1: pwm/direction
              Two output pins, pwm and dir.  The duty cycle on pwm varies as a function of the input value.  dir
              is low for positive inputs and high for negative inputs.

       type 2: up/down
              Two output pins, up and down.  For positive inputs, the PWM/PDM waveform appears on up, while down
              is low.  For negative inputs, the waveform appears on down, while up is low.  Suitable for driving
              the two sides of an H-bridge to generate a bipolar output.

FUNCTIONS

       pwmgen.make-pulses (no floating-point)
              Generates the actual PWM waveforms, using information computed  by  update.   Must  be  called  as
              frequently  as  possible,  to  maximize  the attainable PWM frequency and resolution, and minimize
              jitter.  Operates on all channels at once.

       pwmgen.update (uses floating point)
              Accepts an input value, performs scaling and limit checks, and converts it into a form  usable  by
              make-pulses  for PWM/PDM generation.  Can (and should) be called less frequently than make-pulses.
              Operates on all channels at once.

PINS

       pwmgen.N.enable bit in
              Enables PWM generator N - when false, all pwmgen.N output pins are low.

       pwmgen.N.value float in
              Commanded value.  When value = 0.0, duty cycle is 0%, and when value = ±scale,  duty  cycle  is  ±
              100% (subject to min-dc and max-dc limitations).

       pwmgen.N.pwm bit out (output types 0 and 1 only)
              PWM/PDM waveform.

       pwmgen.N.dir bit out (output type 1 only)
              Direction output: low for forward, high for reverse.

       pwmgen.N.up bit out (output type 2 only)
              PWM/PDM waveform for positive input values, low for negative inputs.

       pwmgen.N.down bit out (output type 2 only)
              PWM/PDM waveform for negative input values, low for positive inputs.

       pwmgen.N.curr-dc float out
              The  current  duty  cycle,  after all scaling and limits have been applied.  Range is from -1.0 to
              +1.0.

       pwmgen.N.max-dc float in/out
              The maximum duty cycle.  A value of 1.0 corresponds to  100%.   This  can  be  useful  when  using
              transistor  drivers  with  bootstrapped power supplies, since the supply requires some low time to
              recharge.

       pwmgen.N.min-dc float in/out
              The minimum duty cycle.  A value of 1.0 corresponds to 100%.  Note that when the pwm generator  is
              disabled, the outputs are constantly low, regardless of the setting of min-dc.

       pwmgen.N.scale float in/out
       pwmgen.N.offset float in/out
              These parameters provide a scale and offset from the value pin to the actual duty cycle.  The duty
              cycle is calculated according to dc = (value/scale) + offset, with 1.0 meaning 100%.

       pwmgen.N.pwm-freq float in/out
              PWM  frequency  in  Hz.  The upper limit is half of the frequency at which make-pulses is invoked,
              and values above that limit will be changed to the limit.  If dither-pwm is false, the value  will
              be  changed  to  the  nearest  integer  submultiple of the make-pulses frequency.  A value of zero
              produces Pulse Density Modulation instead of Pulse Width Modulation.

       pwmgen.N.dither-pwm bit in/out
              Because software-generated PWM uses a fairly slow timebase (several to many microseconds), it  has
              limited  resolution.   For  example,  if make-pulses is called at a 20 kHz rate, and pwm-freq is 2
              kHz, there are only 10 possible duty cycles.  If dither-pwm is false,  the  commanded  duty  cycle
              will  be rounded to the nearest of those values.  Assuming value remains constant, the same output
              will repeat every PWM cycle.  If dither-pwm is true,  the  output  duty  cycle  will  be  dithered
              between  the  two  closest  values,  so that the long-term average is closer to the desired level.
              dither-pwm has no effect if pwm-freq is zero (PDM mode),  since  PDM  is  an  inherently  dithered
              process.

LinuxCNC Documentation                             2007-01-16                                          PWMGEN(9)