Use hardware SPI?

[Peque]

See: libopencm3/libopencm3#876 (comment)

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.