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Код: Выделить всё

static int txrdy = 1;

void usart3_setup(){
    RCC->IOPENR = RCC_IOPENR_GPIODEN; // enable PD
    // set PD8 and PD9 as AF
    GPIOD->MODER = (GPIOD->MODER & ~(GPIO_MODER_MODE8 | GPIO_MODER_MODE9))
            | GPIO_MODER_MODER8_AF | GPIO_MODER_MODER9_AF;
    // AF0 for USART3 @ PD8/PD9
    GPIOD->AFR[1] = GPIOD->AFR[1] & ~(GPIO_AFRH_AFSEL8 | GPIO_AFRH_AFSEL9);
    // enable USART3 clocking
    RCC->APBENR1 |= RCC_APBENR1_USART3EN;
    // baudrate 115200
    USART3->BRR = 640000 / 1152;
    // charmatch interrupt, enable transmitter and receiver, enable usart
    USART3->CR1 = USART_CR1_CMIE | USART_CR1_TE | USART_CR1_RE | USART_CR1_UE;
    //
}

// return amount of bytes sents
int usart3_send_blocking(const char *str, int len){
    if(!txrdy) return 0;
    for(int i = 0; i < len; ++i){
        USART3->TDR = *str++;
        while(!(USART3->ISR & USART_ISR_TXE_TXFNF));
    }
    return len;
}

Код: Выделить всё

RCC->IOPENR = RCC_IOPENR_GPIODEN; // enable PD