Add example USART driver

Makefile

@@ -5,7 +5,7 @@ CPU = -mcpu=cortex-m4
 THUMB = -mthumb
 
 LDFLAGS = -T linker.ld
-CFLAGS = $(CPU) $(THUMB) -I libc/
+CFLAGS = $(CPU) $(THUMB) -I libc/ -I ./
 
 QEMU = qemu-system-arm
 BOARD = netduino2
@@ -13,7 +13,7 @@ BOARD = netduino2
 # Output file
 TARGET = main.elf
 # Source files
-SRCS = os/main.c os/process.c os/scheduler.c os/alloc.c libc/library.c tasks/tasks.c
+SRCS = os/main.c os/process.c os/scheduler.c os/alloc.c os/delay.c os/driver/usart.c libc/library.c tasks/tasks.c
 # Object files
 OBJS = $(SRCS:.c=.o)
 

libc/library.c

@@ -1,10 +1,6 @@
 #include "library.h"
-
+#include "os/driver/usart.h"
-volatile unsigned int *const USART1_PTR = (unsigned int *)0x40011004;
 
 void puts(const char *s) {
-    while(*s != '\0') { /* Loop until end of string */
+    usart_tx_write_string(s);
-        *USART1_PTR= (unsigned int)(*s); /* Transmit char */
-        s++; /* Next char */
-    }
 }

os/delay.c

@@ -0,0 +1,18 @@
+#define CLOCK_FREQUENCY 120000
+
+unsigned int ms_to_ticks(unsigned int ms) {
+    return ms * (CLOCK_FREQUENCY/3);
+}
+
+void delay_routine(unsigned int delay_counter) {
+    asm("mov r1, %[input]\n"
+    "loop_2:\n"
+    "subs r1, #1\n"
+    "cmp r1, #0\n"
+    "bne loop_2\n"
+    : [input] "=r" (delay_counter));
+}
+
+void delay_ms(unsigned int seconds) {
+    delay_routine(ms_to_ticks(seconds));
+}

os/delay.h

@@ -0,0 +1,2 @@
+void delay_routine(unsigned int delay_counter);
+void delay_ms(unsigned int ms);

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;
+}

os/driver/usart.h

@@ -0,0 +1,12 @@
+#include <stdint.h>
+#define TIMEOUT 1000
+
+enum UartErrorCode {
+    USART_TX_ERROR,
+    USART_TX_BUSY,
+    USART_TX_COMPLETE
+};
+
+static uint32_t usart_tx_ready(void);
+uint32_t usart_tx_write(const uint8_t *data_bytes, uint32_t n_bytes);
+uint32_t usart_tx_write_string(const uint8_t *data_bytes);