3f05545ef1
Also update comments in pwmgen.c to reflect that HAL connections are all via pins instead of params. No code changes.
120 lines
4.9 KiB
Groff
120 lines
4.9 KiB
Groff
.TH PWMGEN "9" "2007-01-16" "LinuxCNC Documentation" "HAL Component"
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.de TQ
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.br
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.ns
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.TP \\$1
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..
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.SH NAME
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pwmgen \- software PWM/PDM generation
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.SH SYNOPSIS
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\fBloadrt pwmgen output_type=\fItype0\fR[,\fItype1\fR...]
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.SH DESCRIPTION
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\fBpwmgen\fR is used to generate PWM (pulse width modulation) or
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PDM (pulse density modulation) signals. The maximum PWM frequency
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and the resolution is quite limited compared to hardware-based
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approaches, but in many cases software PWM can be very useful.
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If better performance is needed, a hardware PWM generator is a
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better choice.
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.P
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\fBpwmgen\fR supports a maximum of eight channels. The number of
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channels actually loaded depends on the number of \fItype\fR values
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given. The value of each \fItype\fR determines the outputs for that
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channel.
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.P
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.TP
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type 0: single output
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A single output pin, \fBpwm\fR, whose duty cycle is determined by the
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input value for positive inputs, and which is off (or at \fBmin\-dc\fR)
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for negative inputs. Suitable for single ended circuits.
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.TP
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type 1: pwm/direction
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Two output pins, \fBpwm\fR and \fBdir\fR. The duty cycle on \fBpwm\fR
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varies as a function of the input value. \fBdir\fR is low for positive
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inputs and high for negative inputs.
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.TP
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type 2: up/down
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Two output pins, \fBup\fR and \fBdown\fR. For positive inputs, the
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PWM/PDM waveform appears on \fBup\fR, while \fBdown\fR is low. For
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negative inputs, the waveform appears on \fBdown\fR, while \fBup\fR
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is low. Suitable for driving the two sides of an H-bridge to generate
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a bipolar output.
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.SH FUNCTIONS
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.TP
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\fBpwmgen.make\-pulses \fR(no floating-point)
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Generates the actual PWM waveforms, using information computed by
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\fBupdate\fR. Must be called as frequently as possible, to maximize
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the attainable PWM frequency and resolution, and minimize jitter.
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Operates on all channels at once.
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.TP
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\fBpwmgen.update \fR(uses floating point)
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Accepts an input value, performs scaling and limit checks, and converts
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it into a form usable by \fBmake\-pulses\fR for PWM/PDM generation. Can
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(and should) be called less frequently than \fBmake\-pulses\fR. Operates
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on all channels at once.
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.SH PINS
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.TP
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\fBpwmgen.\fIN\fB.enable\fR bit in
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Enables PWM generator \fIN\fR - when false, all \fBpwmgen.\fIN\fR output
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pins are low.
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.TP
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\fBpwmgen.\fIN\fB.value\fR float in
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Commanded value. When \fBvalue\fR = 0.0, duty cycle is 0%, and when
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\fBvalue\fR = \(+-\fBscale\fR, duty cycle is \(+- 100%. (Subject to
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\fBmin\-dc\fR and \fBmax\-dc\fR limitations.)
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.TP
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\fBpwmgen.\fIN\fB.pwm\fR bit out (output types 0 and 1 only)
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PWM/PDM waveform.
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.TP
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\fBpwmgen.\fIN\fB.dir\fR bit out (output type 1 only)
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Direction output: low for forward, high for reverse.
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.TP
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\fBpwmgen.\fIN\fB.up\fR bit out (output type 2 only)
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PWM/PDM waveform for positive input values, low for negative inputs.
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.TP
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\fBpwmgen.\fIN\fB.down\fR bit out (output type 2 only)
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PWM/PDM waveform for negative input values, low for positive inputs.
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.TP
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\fBpwmgen.\fIN\fB.curr\-dc\fR float out
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The current duty cycle, after all scaling and limits have been applied.
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Range is from \-1.0 to +1.0.
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.TP
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\fBpwmgen.\fIN\fB.max\-dc\fR float in/out
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The maximum duty cycle. A value of 1.0 corresponds to 100%. This can
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be useful when using transistor drivers with bootstrapped power supplies,
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since the supply requires some low time to recharge.
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.TP
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\fBpwmgen.\fIN\fB.min\-dc\fR float in/out
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The minimum duty cycle. A value of 1.0 corresponds to 100%. Note that
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when the pwm generator is disabled, the outputs are constantly low,
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regardless of the setting of \fBmin\-dc\fR.
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.TP
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\fBpwmgen.\fIN\fB.scale\fR float in/out
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.TQ
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\fBpwmgen.\fIN\fB.offset\fR float in/out
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These parameters provide a scale and offset from the \fBvalue\fR pin to the
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actual duty cycle. The duty cycle is calculated according to \fIdc =
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(value/scale) + offset\fR, with 1.0 meaning 100%.
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.TP
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\fBpwmgen.\fIN\fB.pwm\-freq\fR float in/out
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PWM frequency in Hz. The upper limit is half of the frequency at which
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\fBmake\-pulses\fR is invoked, and values above that limit will be changed
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to the limit. If \fBdither\-pwm\fR is false, the value will be changed to
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the nearest integer submultiple of the \fBmake\-pulses\fR frequency. A
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value of zero produces Pulse Density Modulation instead of Pulse Width
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Modulation.
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.TP
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\fBpwmgen.\fIN\fB.dither\-pwm\fR bit in/out
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Because software-generated PWM uses a fairly slow timebase (several to many
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microseconds), it has limited resolution. For example, if \fBmake\-pulses\fR
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is called at a 20KHz rate, and \fBpwm\-freq\fR is 2KHz, there are only 10
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possible duty cycles. If \fBdither\-pwm\fR is false, the commanded duty cycle
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will be rounded to the nearest of those values. Assuming \fBvalue\fR remains
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constant, the same output will repeat every PWM cycle. If \fBdither\-pwm\fR is
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true, the output duty cycle will be dithered between the two closest values,
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so that the long-term average is closer to the desired level. \fBdither\-pwm\fR
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has no effect if \fBpwm\-freq\fR is zero (PDM mode), since PDM is an inherently
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dithered process.
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