esp32c3 how to change SPI bus clock speed

[jeancf]

Some time ago I wrote an SD Card driver for the Longan Nano to provide filesystem access through the fatfs crate. I am now porting it to the esp32c3.

I am running into an issue when I want to change the clock speed of the SPI bus. For the initialization of the card, the SPI bus clock rate must be set between 100 kHz and 400 kHz. Then the SCLK frequency should be changed to 20-25 MHz to maximize the read/write performance.

My driver Card struct receives ownership of the SPI peripheral, the pins, the SPIMode variant, &Clocks and &mut SystemClockControl from the user through this call:

pub fn new(
        p_spi: pac::SPI2,
        cs: Gpio14<Output<PushPull>>,
        sck: Gpio15<Alternate<PushPull>>,
        miso: Gpio17<Input<Floating>>,
        mosi: Gpio16<Alternate<PushPull>>,
        clocks: &Clocks,
        pcc: &mut PeripheralClockControl,
    ) -> Self

and then creates the esp32c3_hal::spi::Spi struct with a bus frequency of 400 KHz by calling Spi::new_no_cs(). So far so good.

Now, to increase the bus speed to 20 MHz I tried to Spi::free() the Spi and recreate a new one by calling Spi::new_internal() BUT the driver struct I no longer has the &mut PeripheralClockControl that is required for that call. How can I solve this issue?

I see 2 options:

• The easiest from my perspective would be to have a method in esp32c3_hal::spi::Spi to change the bus clock speed. That is what the gd32vf103xx_hal does.I think it would need to call the spi::Instance::setup() method in hal-common.
• Spi::free() could also return &mut PeripheralClockControl so that it can be re-injected in a following call to Spi::new_internal(). This approach has the disadvantage that I have to keep the &Clocks around in the Card struct, which contaminates the crate with lifetimes everywhere.

[bjoernQ]

Thanks for bringing this up! I think having this feature would be quite useful

Option 1 seems to be the right thing but it also needs the &Clocks or at least the SPI driver needs to hold a copy of apb_clock

Option 2 is a problem since the driver doesn't hold &mut PeripheralClockControl (since that would prevent anyone else from using PeripheralClockControl)

[jeancf]

OK. I am not sure if I can implement it but I will give it a try.

[bjoernQ]

Sounds great! In case you get stuck or have questions - we are happy to help

[bjoernQ]

Fix by #235