pwmgen — software PWM/PDM generation


loadrt pwmgen output_type=type0[,type1…​]


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.


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.


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 ro
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 rw
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 rw
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 rw
pwmgen.N.offset float rw
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 rw
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 rw
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 20KHz rate, and pwm-freq is 2KHz, 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.