#include <io8515.h>
#include <macros.h>

#pragma interrupt_handler iTxd:11 

typedef struct {
	int head;
	int tail;
	int nchar;
	int size;
}IODESC;

volatile IODESC txdesc;
char txbuff[32];

void InitUart(int baud)
{
	UCR = 0x18;	/*	enable uart rec and xmit	*/
	UBRR = baud;
	txdesc.head = 0;
	txdesc.tail = 0;
	txdesc.nchar = 0;
	txdesc.size = 32;
}

void Disable(void)		/*	disable interrupts	*/
{
	SREG &= ~0x80;
}

void Enable(void)		/* enable interrupts	*/
{
	SREG |= 0x80;		
}

void EnableTxIRQ(void)
{
	UCR |= 0x20;	/*	enable transmit data register empty interrupt	*/
}

void DisableTxIRQ(void)
{
	UCR &= ~0x20;	/*	disable transmit data register empty interrupt	*/
}

void iTxd(void)		/* interrupt handler for uart	*/
{
	UDR = txbuff[txdesc.tail++];		/*	get character from buffer	*/
	if(--txdesc.nchar == 0)				/*	one less character to send	*/
		DisableTxIRQ();					/*	stop transmitting data	*/
	if(txdesc.tail == txdesc.size) txdesc.tail = 0;	/* wrap tail pointer	*/		
}

void LED_On(int i)
{
	PORTB = ~BIT(i);	/* low output to turn LED on */
}

void PutC(int i)
{
	while(txdesc.nchar == txdesc.size);	/*	pend on buffer full	*/
	Disable();	/*	Disable interrupts	*/
	txbuff[txdesc.head++] = (char)i;	/*	put data into buffer	*/
	if(txdesc.head == txdesc.size) txdesc.head = 0;	/*	wrap head pointer	*/
	if(txdesc.nchar == 0)	/*	first char in buff?	*/
		EnableTxIRQ();		/*	start transmit interrupt	*/
	txdesc.nchar++;
	Enable();	/*	enable interrupts	*/	
}

main()
{
	int i,j;

	InitUart(25);
	DDRB = 0xFF;	/* output */
	PORTB = 0xFF;	/* all off */

	j=0;
	while(1)
	{
		LED_On(j);
		if(++j == 8) j = 0;
		for(i=' ';i<'z';++i)
		{
			PutC(i);
		}
		PutC('\r');
		PutC('\n');
		while((PIND & 0x80) == 0);	/*	stop if button 7 is pushed	*/
	}
	return 0;
}