* Use interrupts and buffering for UART TX

* Set up P0.26 as an output, driving the on-board LED
* Toggle the LED while waiting for UART input to demonstrate activity

main.c

@@ -6,11 +6,19 @@
 #include "interrupt.h"
 
 #define PINSEL0 (*((volatile unsigned char *) 0xE002C000))
+#define FP0XDIR (*((volatile unsigned int *) 0x3FFFC000))
+#define FP0XVAL (*((volatile unsigned int *) 0x3FFFC014))
+
+#define SCS (*((volatile unsigned int *) 0xe01fc1a0))
+
 
 void init_pins(void)
 {
 	PINSEL0 = 0x00000055; /* P0.0 and P0.1 assigned to UART */
 			      /* P0.2 and P0.3 assigned to I2C  */
+	SCS = 1;
+	FP0XDIR = 0x04000000; /* P0.26 is an output */
+	FP0XVAL = 0x0;
 }
 
 void reply(char *str)

uart.c

@@ -1,7 +1,11 @@
 #include "uart.h"
+#include "types.h"
+#include "interrupt.h"
 
 #define UARTBASE 0xE000C000
 
+#define FP0XVAL (*((volatile unsigned int *) 0x3FFFC014))
+
 #define RBR 0x00
 #define THR 0x00
 #define DLL 0x00
@@ -22,6 +26,15 @@
 #define U0THRE ((UREG(LSR) & (1<<5))) /* UART0 transmitter holding register is empty */
 #define U0DR ((UREG(LSR) & (1<<0))) /* UART0 data ready */
 
+#define UART_BUFSIZE 128
+
+char uart_txbuf[UART_BUFSIZE];
+volatile int uart_txread;
+volatile int uart_txwrite;
+volatile bool tx_running;
+
+void __attribute__((interrupt("IRQ"))) uart_interrupt_handler(void);
+
 void init_uart(void)
 {
 	UREG(FDR) = 0x10; /* DivAddVal = 0, MulVal = 1 */
@@ -31,11 +44,72 @@ void init_uart(void)
 	UREG(DLL) = 0x08; /* 14745600 / (16*115200) */
 	UREG(LCR) = 0x13;
 	UREG(FCR) = 0x07;
+
+	uart_txread = 0;
+	uart_txwrite = 0;
+	tx_running = FALSE;
+	interrupt_register(UART0, uart_interrupt_handler);
+
+	UREG(IER) = 0x02; /* THRE interrupt enable */
 }
 
 void putch(char c) {
-	while (!U0THRE);
-	UREG(THR) = c;
+	/* Wait for space in the buffer */
+	while (uart_txread == ((uart_txwrite+1) % UART_BUFSIZE));
+
+	if (uart_txread == uart_txwrite) {
+		if (U0THRE) {
+			tx_running = TRUE;
+			UREG(THR) = c;
+			return;
+		}
+	}
+
+	uart_txbuf[uart_txwrite] = c;
+	uart_txwrite = (uart_txwrite + 1) % UART_BUFSIZE;
+
+	if (!tx_running) {
+		if (uart_txread != uart_txwrite) {
+			tx_running = TRUE;
+			uart_txread = (uart_txread + 1) % UART_BUFSIZE;
+			UREG(THR) = c;
+		}
+	}
+}
+
+void __attribute__((interrupt("IRQ"))) uart_interrupt_handler(void)
+{
+	bool active = FALSE;
+	int source;
+	int i;
+	/* uart_txread and uart_txwrite are volatile. We don't need
+	 * to treat them as such in this handler, so let the compiler
+	 * have an easier time.
+	 */
+	int local_txwrite = uart_txwrite;
+	int local_txread = uart_txread;
+
+	source = UREG(IIR);
+
+	/* For now we assume that all interrupts are actually THR empty
+	 * interrupts. Actually check this when we do more things with
+	 * the UART
+	 */
+
+	if (U0THRE) {
+		for (i = 0; (i < 16) && (local_txwrite != local_txread); i++) {
+			UREG(THR) = uart_txbuf[local_txread];
+			local_txread = (local_txread + 1) % UART_BUFSIZE;
+			active = TRUE;
+		}
+	}
+
+	uart_txread = local_txread;
+
+	if (!active)
+		tx_running = FALSE;
+
+	interrupt_clear();
 }
 
 void putstr(char *s) {
@@ -74,6 +148,8 @@ void puthex(unsigned int n) {
 }
 
 char getch(void) {
-	while (!U0DR);
+	while (!U0DR) {
+		FP0XVAL ^= 0x04000000;
+	}
 	return UREG(RBR);
 }