PIO stepper motor pulse engine for Raspberry Pi pico
Two adjacent GPIOs are driven as step and direction, output is a 50% duty cycle with the rising edge in the center. One command type efficiently supports fixed velocity and allows frequent updates for more complex profiles. Multiple SMs on the same pico will stay in synch for coordinated motion control. An external step clock can be used to keep SMs across multiple picos in synch.
Project status is "Work In Progress" - PIO program timings are verified, wrapper API is incomplete and unstable.
Each command is a 32 bit word written to the TxFIFO:
31:30 second half step+dir
29:28 first half step+dir
27: 8 delay count
7: 0 repeat count
Note that step+dir order is application defined - the value in the lower numbered bit will be output to the lower numbered of the GPIO pins.
Pulse period is delay+2 step clocks.
The pulse is output repeat+1 times.
When idle time must be accurate to maintain synchronization, supply commands where the step bit does not change between halves.
For very long pulses, use two or more commands with the step bit changing between commands.
Two implementations are provided - one using the
internal clock and cycle counting, one synched to
an external clock source via wait irq.
Both are designed for a nominal
3.125 MHz step clock
but count at 6.25 MHz - the same delay count twice
and producing a 50% duty cycle.
The internal clock variation requires the SM to be clocked at 8x the step clock. Up to four SMs in the same PIO can be used, but there will be no synchronization between picos.
For an external step clock, the SM clock should be at least 10x faster than the step clock. One SM is needed to convert the GPIO edges to 6.25 MHz IRQs, leaving three SMs available to control steppers. This configuration can be expanded to synchronize many picos.
Note that while shared clocks ensure all SMs run at the same rate, additional care may be needed to start them all at the same time within tolerances needed by the application.
Minimum pulse period is less than a microsecond, maximum period is over 335 milliseconds. Two commands are sufficient to construct a pulse with a period over 1 minute, or four commands if rounding the period is not allowed.
Design goals were to support command rates from 10 Hz to 1 kHz, and step pulse rates up to 32 kHz (ie, 300 rpm assuming 32 microsteps and 200 steps/rev). The implementation far exceeds the goals as well as the useful limits for stepper motor control. Refer to the source code for details.
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