Implement USART with bot RX and TX interrupts

src/bus/twi.c

@@ -0,0 +1,12 @@
+#include "bus/twi.h"
+#include <avr/io.h>
+
+int TWI_Init(void)
+{
+	// Set SCL bit rate to 400kHz
+
+	TWSR = 0x00; // TWI status register
+	TWBR = 0x0C; // TWI bit rate register
+
+	return 0;
+}

src/bus/twi.h

@@ -0,0 +1,6 @@
+#ifndef MAD_CORE_BUS_TWI_H
+#define MAD_CORE_BUS_TWI_H
+
+int TWI_Init(void);
+
+#endif // MAD_CORE_BUS_TWI_H

src/bus/usart.c

@@ -0,0 +1,93 @@
+#include "common.h"
+#include "bus/usart.h"
+#include <avr/io.h>
+#include <avr/interrupt.h>
+//#include <util/atomic.h>
+
+#define USART_BAUDRATE    9600
+#define USART_RXBUF_SIZE  512
+#define USART_TXBUF_SIZE  512
+
+#define USART_RXBUF_MASK (USART_RXBUF_SIZE - 1)
+#define USART_TXBUF_MASK (USART_TXBUF_SIZE - 1)
+#define USART_BAUD_PRESCALE ((((F_CPU / 16) + \
+        (USART_BAUDRATE / 2)) / (USART_BAUDRATE)) - 1)
+
+static volatile char rxbuf[USART_RXBUF_SIZE]; // RX ring buffer
+static volatile char txbuf[USART_TXBUF_SIZE]; // TX ring buffer
+static volatile short rxhead, txhead; // Current write position
+static volatile short rxtail, txtail; // Current read position
+
+int USART_Init(void)
+{
+	rxhead = 0;
+	rxtail = 0;
+	txhead = 0;
+	txtail = 0;
+
+	UCSRB = BIT(RXCIE); // Handle RXC interrupts
+	UCSRB |= BIT(RXEN) | BIT(TXEN); // Enable RX and TX circuitry
+	UCSRC = BIT(URSEL) | BIT(UCSZ0) | BIT(UCSZ1); // Using 8-bit chars
+	UBRRH = (USART_BAUD_PRESCALE >> 8); // Set baud rate upper byte
+	UBRRL = USART_BAUD_PRESCALE; // Set baud rate lower byte
+
+	return 0;
+}
+
+char USART_Getc(void)
+{
+	if (rxhead == rxtail) {
+		return -1;
+	}
+
+	rxtail = (rxtail + 1) & USART_RXBUF_MASK;
+	return rxbuf[rxtail];
+}
+
+void USART_Putc(char ch)
+{
+	short head;
+
+	head = (txhead + 1) & USART_TXBUF_MASK;
+	while (head == txtail); // Wait for space
+
+	txbuf[head] = ch;
+	txhead = head;
+
+	// Enable interrupt
+	UCSRB |= BIT(UDRIE);
+}
+
+// INT: Rx complete
+ISR(USART_RXC_vect)
+{
+	short head;
+
+	// Next byte ready to read from serial
+	head = (rxhead + 1) & USART_RXBUF_MASK;
+	// Free space in RX buffer?
+	if (head != rxtail) {
+		// Copy from register
+		rxbuf[head] = UDR;
+		rxhead = head;
+	} else {
+		// XXX: Discard overflow
+	}
+}
+
+// INT: Data register empty
+ISR(USART_UDRE_vect)
+{
+	short tail;
+
+	// Anything in TX buffer?
+	if (txhead != txtail) {
+		// Write next byte to data register
+		tail = (txtail + 1) & USART_TXBUF_MASK;
+		UDR = txbuf[tail];
+		txtail = tail;
+	} else {
+		// Disable interrupt
+		UCSRB &= ~BIT(UDRIE);
+	}
+}

src/bus/usart.h

@@ -0,0 +1,8 @@
+#ifndef MAD_CORE_BUS_USART_H
+#define MAD_CORE_BUS_USART_H
+
+int   USART_Init(void);
+void  USART_Putc(char ch);
+char  USART_Getc(void);
+
+#endif // MAD_CORE_BUS_USART_H