SPI master (spim) stops working after `.free()`

[eflukx]

Hi I'm building a battery-powered sensing device with Rust, based on the nRF52833.

For conserving power, I need to disable the SPI peripheral when not in use. Currently a nice solution for enabling/disabling peripherals is not provided in the nrf-hal (tracking #279), so for the moment I will be juggling the SPI in a wrapping enum encapsulating the enabled and disabled types/modes.

I ran into an unexpected problem though:

• When initializing the SPI peripheral for the first time all works well!
• After free()-ing the peripheral instance, the power usage drops as expected (side note: for anyone debugging inexplicable SPI power use, please be aware of: Erratum [195] SPIM: SPIM3 continues to draw current after disable)
• The free'd parts (spim and spins) can be made into a new spi instance, also good...
• But, this new resulting SPI instance does not seem to work at all/correctly. write() functions return without error, but nothing happens...

It seems something is not working well when creating the new spim instance from the parts returned by spidev.free()

Freeing and re-creating the SPI instance:

    let (spim, pins) = spidev.free();
    let mut spidev = spim::Spim::new(spim, pins, spim::Frequency::M16, spim::MODE_0, 0);

for completeness, initial initialization is done like this:

    let epd_mosi = port1.p1_06.into_push_pull_output(Level::Low).degrade();
    let epd_sck = port1.p1_07.into_push_pull_output(Level::Low).degrade();

    let mut epd_spi = spim::Spim::new(
        periphs.SPIM2,
        spim::Pins {
            sck: epd_sck,
            miso: None,
            mosi: Some(epd_mosi),
        },
        spim::Frequency::M16,
        spim::MODE_0,
        0,
    );

[jonas-schievink]

Looks like we never reset the enable register in free, that might cause this.