OpenCM3 USB clock configuration

_stm32f070-libopencm3-usb-clock.md

USB with libopenCM3 on the STM32F070F6

This specific part doesn't work out of the box with the OpenCM3 STM32 F0 USB example/testcase for two reasons:

• The USB pins (PA11/PA12) are hidden behind a mux on pins PA9/PA10 and need to be switched in. This physically connects the USB peripheral to the pins.
• The STM32F070 must use an external oscillator to drive the 48MHz USB clock. My design used a 12MHz HSE, which libopencm3 didn't have a built-in clock configuration function for.

stm32f070f6-libopencm3-usb-clock.c

#include <libopencm3/stm32/flash.h>
#include <libopencm3/stm32/rcc.h>
#include <libopencm3/stm32/syscfg.h>
#include <libopencm3/usb/usbd.h>

/**
 * Configure a 12MHz HSE to give a 48MHz PLL clock for the USB peripheral to function
 *
 * This is based on a clock configuration calculated in STM32 Cube. Note that
 * the STM32F070 requires an external oscillator for USB as its HSI is not
 * precise enough per the datasheet:
 *
 * > [USB] requires a precise 48 MHz clock which can be generated from the
 * > internal main PLL (the clock source must use an HSE crystal oscillator).
 */
static void clock_setup_12mhz_hse_out_48mhz(void)
{
    rcc_osc_on(RCC_HSE);
    rcc_wait_for_osc_ready(RCC_HSE);
    rcc_set_sysclk_source(RCC_HSE);

    rcc_set_hpre(RCC_CFGR_HPRE_NODIV);  // AHB Prescaler
    rcc_set_ppre(RCC_CFGR_PPRE_NODIV); // APB1 Prescaler

    flash_prefetch_enable();
    flash_set_ws(FLASH_ACR_LATENCY_024_048MHZ);

    // PLL for USB: (12MHz * 8) / 2 = 48MHz
    rcc_set_prediv(RCC_CFGR2_PREDIV_DIV2);
    rcc_set_pll_multiplication_factor(RCC_CFGR_PLLMUL_MUL8);
    rcc_set_pll_source(RCC_CFGR_PLLSRC_HSE_CLK);
    rcc_set_pllxtpre(RCC_CFGR_PLLXTPRE_HSE_CLK_DIV2);

    rcc_osc_on(RCC_PLL);
    rcc_wait_for_osc_ready(RCC_PLL);
    rcc_set_sysclk_source(RCC_PLL);

    rcc_set_usbclk_source(RCC_PLL);

    rcc_apb1_frequency = 48000000;
    rcc_ahb_frequency = 48000000;
}

int main(void)
{
    // Turn on the SYSCFG module and switch out PA9/PA10 for PA11/PA12
    // The USB peripheral is not phsyically connected without this and won't be enumerated
    RCC_APB2ENR |= RCC_APB2ENR_SYSCFGCOMPEN;
    SYSCFG_CFGR1 |= SYSCFG_CFGR1_PA11_PA12_RMP;

    clock_setup_12mhz_hse_out_48mhz();
    
    // ...
}