Add usart.{h, c}

src/gpio.h

@@ -17,6 +17,10 @@ struct gpio {
   volatile uint32_t AFRH;     // Alternative function high register
 };
 
+// AFRH, AFRL registers
+#define GPIO_AF_USART2_RX (0b0111) // Alternative function 7 (AF7)
+#define GPIO_AF_USART2_TX (0b0111) // Alternative function 7 (AF7)
+
 #define GPIO_BASE_ADDR (0x40020000U)
 #define GPIO_PORT_OFFSET (0x400U)
 #define GPIO(port) ((struct gpio*)(uintptr_t)(GPIO_BASE_ADDR + (GPIO_PORT_OFFSET * port)))

src/rcc.h

@@ -118,6 +118,9 @@ struct rcc {
 #define RCC_APB1ENR_PWREN_BIT 28
 #define RCC_APB1ENR_PWREN_CLOCK_ENABLE (1 << RCC_APB1ENR_PWREN_BIT)
 
+#define RCC_APB1ENR_USART2EN_BIT 17
+#define RCC_APB1ENR_USART2EN_ENABLE (1 << RCC_APB1ENR_USART2EN_BIT)
+
 #define RCC_APB1ENR_TIM4_BIT 2
 #define RCC_APB1ENR_TIM4_ENABLE (1 << RCC_APB1ENR_TIM4_BIT)
 

src/usart.c

@@ -0,0 +1,66 @@
+#include "rcc.h"
+#include "gpio.h"
+#include "usart.h"
+
+void usart2_init(void) {
+  // Enable clock for GPIOA as USART2 is on PORT A pins
+  RCC->AHB1ENR |= (1 << PORT('A'));
+
+  // Configure PA2 and PA3 (USART2 pins) to use alternative functions
+  uint16_t txPin = PIN('A', 2);
+  gpio_set_mode(txPin, GPIO_MODE_AF);
+  gpio_set_af(txPin, GPIO_AF_USART2_TX);
+
+  uint16_t rxPin = PIN('A', 3);
+  gpio_set_mode(rxPin, GPIO_MODE_AF);
+  gpio_set_af(rxPin, GPIO_AF_USART2_RX);
+
+
+  // Enable USART
+  RCC->APB1ENR |= RCC_APB1ENR_USART2EN_ENABLE;
+
+  // Clear control registers
+  USART2->CR1 = 0;
+  USART2->CR2 = 0;
+  USART2->CR3 = 0;
+
+  // Calculate Baud rate:
+  // baud = f_clck / (8 * (2 - OVER8) * USARTDIV) =>
+  // (8 * (2 - OVER8) * USARTDIV) = f_clock / baud =>
+  // baud * (8 * (2 - OVER8) * USARTDIV) = f_clock =>
+  // USARTDIV = (f_clock / (baud * (8 * (2 - OVER8)))
+
+  // Target Baud rate = 115200, f_clock = 48MHz, OVER8 = 0
+  // USARTDIV = 48E6 / (115200 * 8 * 2) = 26.0416666
+  // mantissa = 26 = 0x1A
+  // fraction = 0.041666 * 16 = 0.666656 ~= 1
+
+  // baud = 48E6 / (8 * 2 * 26) = 115384.61538461539
+  // error of 0.16% (115384.61538461539 / 115200 ) = 1.001602564102564
+  //
+  // skipping fractional part as error rate is good.
+
+  USART2->BRR &= ~(USART_BRR_MANTISSA_MASK << USART_BRR_MANTISSA_BIT);
+  USART2->BRR |= (0x1A << USART_BRR_MANTISSA_BIT);
+  USART2->BRR &= ~(USART_BRR_FRACTION_MASK << USART_BRR_FRACTION_BIT);
+  USART2->BRR |= (0x0 << USART_BRR_MANTISSA_BIT);
+
+  // Enable transmitter and receiver
+  USART2->CR1 |= USART_CR1_TE_ENABLE;
+  USART2->CR1 |= USART_CR1_RE_ENABLE;
+}
+
+void usart2_start(void) {
+  USART2->CR1 |= USART_CR1_UE_ENABLE;
+}
+
+void usart2_write_byte(uint8_t c) {
+  USART2->DR = c;
+
+  // Wait indefinitely for transmission to be ready for data
+  while (!(USART2->SR & USART_SR_TC_COMPLETED));
+}
+
+void usart2_write(char *buf) {
+  while (*buf) usart2_write_byte(*buf++);
+}

src/usart.h

@@ -0,0 +1,50 @@
+#ifndef USART_H_
+#define USART_H_
+
+#include <inttypes.h>
+
+struct usart {
+  volatile uint32_t SR;   // Status register
+  volatile uint32_t DR;   // Data register
+  volatile uint32_t BRR;  // Baud rate register
+  volatile uint32_t CR1;  // Control register 1
+  volatile uint32_t CR2;  // Control register 2
+  volatile uint32_t CR3;  // Control register 3
+  volatile uint32_t GTPR; // Guard time and prescaler register
+};
+
+#define USART2_BASE_ADDR (0x40004400U)
+#define USART2 ((struct usart *) USART2_BASE_ADDR)
+
+// SR Register
+// Transmission complete
+#define USART_SR_TC_BIT 6
+#define USART_SR_TC_COMPLETED (1 << USART_SR_TC_BIT)
+
+// CR Register
+// USART enable
+#define USART_CR1_UE_BIT 13
+#define USART_CR1_UE_ENABLE (1 << USART_CR1_UE_BIT)
+
+// Trasmitter enable
+#define USART_CR1_TE_BIT 3
+#define USART_CR1_TE_ENABLE (1 << USART_CR1_TE_BIT)
+
+// Receiver enable
+#define USART_CR1_RE_BIT 2
+#define USART_CR1_RE_ENABLE (1 << USART_CR1_RE_BIT)
+
+// BRR Register
+#define USART_BRR_MANTISSA_BIT 4 // Bits [15:4]
+#define USART_BRR_MANTISSA_MASK (0b111111111111)
+
+#define USART_BRR_FRACTION_BIT 0 // Bits [3:0]
+#define USART_BRR_FRACTION_MASK (0b111)
+
+void usart2_init(void);
+void usart2_start(void);
+
+void usart2_write_byte(uint8_t byte);
+void usart2_write(char *buf);
+
+#endif