Registers and Peripherals

// UART transmit without HAL — STM32F1
#include "stm32f1xx.h"

void uart1_init(uint32_t baud) {
    // 1. Clock enable: USART1 on APB2, GPIOA on APB2
    RCC->APB2ENR |= RCC_APB2ENR_USART1EN | RCC_APB2ENR_IOPAEN;

    // 2. Configure PA9 (TX) as alternate function push-pull
    GPIOA->CRH &= ~(0xF << 4);
    GPIOA->CRH |=  (0xB << 4);  // AF output push-pull, 50MHz

    // 3. Configure PA10 (RX) as input floating
    GPIOA->CRH &= ~(0xF << 8);
    GPIOA->CRH |=  (0x4 << 8);  // input floating

    // 4. Set baud rate (PCLK2 = 72MHz assumed)
    USART1->BRR = 72000000UL / baud;

    // 5. Enable USART, TX, RX
    USART1->CR1 = USART_CR1_UE | USART_CR1_TE | USART_CR1_RE;
}

void uart1_putchar(char c) {
    while (!(USART1->SR & USART_SR_TXE));  // wait for TX buffer empty
    USART1->DR = c;
}

void uart1_puts(const char *s) {
    while (*s) uart1_putchar(*s++);
    // Wait for last byte to finish transmitting
    while (!(USART1->SR & USART_SR_TC));
}

int main(void) {
    uart1_init(9600);
    uart1_puts("Hello from bare-metal UART!
");

    char buf[32];
    uint32_t count = 0;
    while (1) {
        // Simple integer to string (no sprintf without libc)
        int i = 0, n = count;
        if (n == 0) { buf[i++] = '0'; }
        while (n > 0) { buf[i++] = '0' + (n % 10); n /= 10; }
        buf[i++] = '
'; buf[i++] = '
'; buf[i] = '';
        // Reverse
        for (int a=0, b=i-3; a<b; a++,b--) {
            char t=buf[a]; buf[a]=buf[b]; buf[b]=t;
        }
        uart1_puts(buf);
        count++;
        for (volatile int d=0; d<1000000; d++);
    }
}