Add example USART driver

os/driver/usart.c

@@ -0,0 +1,57 @@
+#include "usart.h"
+#include "os/delay.h"
+#include <stddef.h>
+#include <stdint.h>
+#define USART_SR_TXE (1 << 7)
+
+typedef struct uart_s {
+    uint32_t USART_SR;
+    uint32_t USART_DR;
+    uint32_t USART_BRR;
+    uint32_t USART_CR1;
+    uint32_t USART_CR2;
+    uint32_t USART_CR3;
+    uint32_t USART_GTPR;
+} uart_c;
+
+volatile uart_c *const USART1 = (uart_c *)0x40011000;
+
+static uint32_t usart_tx_ready(void) {
+    return (USART1->USART_SR & USART_SR_TXE) >> 7;
+}
+
+uint32_t usart_tx_write(const uint8_t *data_bytes, uint32_t n_bytes) {
+    if (data_bytes == NULL) return USART_TX_ERROR;
+
+    for (int i = 0; i < n_bytes; i++) {
+        unsigned int timeout = TIMEOUT;
+        while (!usart_tx_ready()) {
+            delay_ms(1);
+            timeout--;
+            if (timeout == 0) return USART_TX_BUSY;
+        }
+
+        USART1->USART_DR = *data_bytes;
+
+        data_bytes++;
+    }
+    return USART_TX_COMPLETE;
+}
+
+uint32_t usart_tx_write_string(const uint8_t *data_bytes) {
+    if (data_bytes == NULL) return USART_TX_ERROR;
+
+    while (*data_bytes != '\0') {
+        unsigned int timeout = TIMEOUT;
+        while (!usart_tx_ready()) {
+            delay_ms(1);
+            timeout--;
+            if (timeout == 0) return USART_TX_BUSY;
+        }
+
+        USART1->USART_DR = *data_bytes;
+
+        data_bytes++;
+    }
+    return USART_TX_COMPLETE;
+}