Use hardware SPI?

[Peque]

See: https://github.com/libopencm3/libopencm3/issues/876#issuecomment-485852573

The MPU PCB is supposed to already have the 10 k pull-up, so this should be possible?

Maybe something like:

static void
spi_setup(void) {
    rcc_periph_clock_enable(RCC_SPI1);
    gpio_set_mode(
        GPIOA,
        GPIO_MODE_OUTPUT_50_MHZ,
        GPIO_CNF_OUTPUT_ALTFN_PUSHPULL,
        GPIO4|GPIO5|GPIO7 // NSS=PA4,SCK=PA5,MOSI=PA7
    );
    gpio_set_mode(
        GPIOA,
        GPIO_MODE_INPUT,
        GPIO_CNF_INPUT_FLOAT,
        GPIO6 // MISO=PA6
    );
    spi_reset(SPI1);
    spi_init_master(
        SPI1,
        SPI_CR1_BAUDRATE_FPCLK_DIV_256,
        SPI_CR1_CPOL_CLK_TO_0_WHEN_IDLE,
        SPI_CR1_CPHA_CLK_TRANSITION_1,
        SPI_CR1_DFF_8BIT,
        SPI_CR1_MSBFIRST
    );
    spi_disable_software_slave_management(SPI1);
    spi_enable_ss_output(SPI1);
}

See Warren Gay's "Beginning STM32" book.