Stepper Motor Control with Raspberry Pico PIO and DRV8711 driver- Part 5: a more autonomous PIO

RE: Stepper Motor Control with Raspberry Pico PIO and DRV8711 driver- Part 5: a more autonomous PIO

Jan Cumps — Sat, 19 Apr 2025 20:22:16 GMT

The PIO program that I wrote works, but it violates a requirement from the stepper motor driver I use, a TI DRV8711

There should be a 200 ns time between setting the DIR pin, and taking a step. My code violated that blatantly. It had 4 PIO clock ticks between the two events.

I don't have a perfect solution yet, that works for all PIO clock speeds. Wat I've done now is configure the highest speed that works for my project.
Then stretch the clocks used to set the DIR pin:

.define public DIR_DELAY 4
# ...
out pins, 1 side 0 [DIR_DELAY] ; shift direction out of OSR and output to dir. 200 ns before next step

The [DIR_DELAY] stretches the clock ticks used by the out command from one tick to 5. (the command, and 4 delay ticks)
It's not perfect. For slower PIO clock speeds (something you set, there will be a (nanosecond order of magnitude) delay. But I bet that you won't notice it.
This is the scope capture of the time between the events, with the adapted code:

RE: Stepper Motor Control with Raspberry Pico PIO and DRV8711 driver- Part 5: a more autonomous PIO

Jan Cumps — Mon, 07 Apr 2025 19:36:29 GMT

Time taken by the ARM core to load an instruction into the PIO FIFO. I used a GPIO to measure the time, so you 'd have to discount two gpio_put() calls from the 360 ns it takes to pass the instruction from ARM to PIO.

An instruction can have many steps (millions, if you need - anything that fits in 31 bits. For any process that needs a motor to step for milliseconds or seconds (or longer), this is definitely a resource gain. (although you can't put the Pico to sleep during that time, because then the PIO would stop too)

RE: Stepper Motor Control with Raspberry Pico PIO and DRV8711 driver- Part 5: a more autonomous PIO

Jan Cumps — Mon, 07 Apr 2025 19:10:36 GMT

In this video, a LED shows how much time it takes to send 6 instructions to PIO. In total 1600 steps, including direction changes in between.
The motor is fast too [emoticon:c4563cd7d5574777a71c318021cbbcc8] - but that is because I uptimised its speed and torque. Even if each of these 6 instructions would generate 100.000s of steps, the setup time in the ARM controller would be the same.

[View:https://youtu.be/9sVK7T45DL8:640:360]

In a classic design, the microcontroller would have to spend clock ticks controlling the motor. Ticks it can't spend on other activities.

RE: Stepper Motor Control with Raspberry Pico PIO and DRV8711 driver- Part 5: a more autonomous PIO

Jan Cumps — Mon, 07 Apr 2025 16:40:19 GMT

Using an old school counter to validate the stepper motor firmware:

[View:https://youtu.be/OoEPBERgOKc:640:360]

My oscilloscope can do that too, as long as I can fit the signal on a single screen. A counter doesn't have that limitation.

RE: Stepper Motor Control with Raspberry Pico PIO and DRV8711 driver- Part 5: a more autonomous PIO

Jan Cumps — Mon, 07 Apr 2025 16:12:38 GMT

Project now available on GitHub: https://github.com/jancumps/pio_drv8711_stepper

RE: Stepper Motor Control with Raspberry Pico PIO and DRV8711 driver- Part 5: a more autonomous PIO

Jan Cumps — Sun, 06 Apr 2025 17:42:49 GMT

A test to see if the PIO actually generates the amount of steps that I request. In this case: 350.

I've used my oscilloscope's pulse counter to do the work for me. It's a match.

RE: Stepper Motor Control with Raspberry Pico PIO and DRV8711 driver- Part 5: a more autonomous PIO

Jan Cumps — Sun, 06 Apr 2025 16:25:08 GMT

Here's an example stepper motor program:

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