shared-module/fourwire/FourWire.c: make the chip_select pin optional

[Fabien-Chouteau]

If there's only one device on the bus, the chip_select pin of the peripheral can be fixed in hardware, therefore lowering the number of pins required on the microcontroller side.

This patch allows this by making the chip_select pin optional.

[dhalbert]

This doesn't look finished. There are a number of places in shared-module/fourwire/FourWire.c where self.chip_select is referenced and assumed not to be None, e.g. line 83, and a bunch of others.

[Fabien-Chouteau]

I'm sorry @dhalbert, I am not familiar with the codebase and I should've spend more time on this.

I added checks for the chip_select pin.

[dhalbert]

Thanks for the update. One more place to test if chip_select pin is set or not:

void common_hal_fourwire_fourwire_deinit(fourwire_fourwire_obj_t *self) {
    if (self->bus == &self->inline_bus) {
        common_hal_busio_spi_deinit(self->bus);
    }

    common_hal_reset_pin(self->command.pin);
    common_hal_reset_pin(self->chip_select.pin);     //// *************
    common_hal_reset_pin(self->reset.pin);
}

[Fabien-Chouteau]

Hi @dhalbert

Thanks for the update. One more place to test if chip_select pin is set or not:

Since the command pin can also be None and is not checked here, my understanding is that the common_hal_reset_pin will check if the pin is None:

void common_hal_reset_pin(const mcu_pin_obj_t *pin) {
    if (pin == NULL) {
        return;
    }
    reset_pin_number(pin->port, pin->number);
}

But maybe there is an oversight on the command pin as well.

[dhalbert]

Since the command pin can also be None and is not checked here, my understanding is that the common_hal_reset_pin will check if the pin is None:

This depends on self->chip_select being initialized to zero, so self->chip_select.pin will be NULL (zero) at first. I was worried about that at first, but I cannot think of a situation where the pin might be set to a non-NULL value and then later the chip_select type was set to NoneType, so I think it is ok.

[dhalbert]

Thanks for this!