Using PIO OUT instruction for parallel interfaces

[rafl]

I wrote my first few PIO programs today and had success using set and side-set mappings to have PIO programs change GPIO pins. I now want to move on to interfacing with a device that needs more pins than set or side-set mappings can provide, so I want to use an out mapping instead.

I started with

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@rp2.asm_pio(out_init=(rp2.PIO.OUT_HIGH,)*6, out_shiftdir=rp2.PIO.SHIFT_RIGHT)
def prog0():
    pull() # just hang

rp2.StateMachine(0, prog0, freq=2000, out_base=machine.Pin(4)).active(1)

This seems to work as expected, driving GPIO pins 4 through 9 high.

I then moved on to

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@rp2.asm_pio(out_init=(rp2.PIO.OUT_HIGH,)*6, out_shiftdir=rp2.PIO.SHIFT_RIGHT)
def prog1():
    mov(osr, 0xffffffff) # fill osr with all 1s
    out(pins, 6) # output the 6 least-significant bits of osr to our 6 pins
    pull()

rp2.StateMachine(0, prog1, freq=2000, out_base=machine.Pin(4)).active(1)

In this example I would've expected all six pins to remain high, but instead I observe all of them being driven low.

Why is that? Did I misunderstand how the OUT instruction works or how it interacts with the output shift register or out IO mapping?

Thanks!

[Roberthh]

The statement:

mov(osr, 0xffffffff) # fill osr with all 1s

is wrong. there is no way to use an arbitrary constant as source. You have to use:

mov(osr, invert(null)) # fill osr with all 1s

[rafl]

Thank you for clearing up my misunderstanding, Robert. There isn't even space in the instruction to fit a 32 bit value !

With that insight, and the work you and others put into https://github.com/micropython/micropython/issues/6933, I am now able drive the 10 GPIOs I need for this particular application.

[Roberthh]

The bug mentioned here https://github.com/micropython/micropython/issues/6933 is fixed in the actual daily builds.

[rafl]

Thanks - I didn't realise daily builds were a thing and just built from source after seeing the fix was merged.