serial.c

#include	"serial.h"

#include	<avr/interrupt.h>

#include	"config.h" // for XONXOFF
#include	"arduino.h"

#define		BUFSIZE			64
#define		BAUD				115200

#define		ASCII_XOFF	19
#define		ASCII_XON		17

volatile uint8_t rxhead = 0;
volatile uint8_t rxtail = 0;
volatile uint8_t rxbuf[BUFSIZE];

volatile uint8_t txhead = 0;
volatile uint8_t txtail = 0;
volatile uint8_t txbuf[BUFSIZE];

#define	buf_canread(buffer)			((buffer ## head - buffer ## tail    ) & (BUFSIZE - 1))
#define	buf_canwrite(buffer)		((buffer ## tail - buffer ## head - 1) & (BUFSIZE - 1))

#define	buf_push(buffer, data)	do { buffer ## buf[buffer ## head] = data; buffer ## head = (buffer ## head + 1) & (BUFSIZE - 1); } while (0)
#define	buf_pop(buffer, data)		do { data = buffer ## buf[buffer ## tail]; buffer ## tail = (buffer ## tail + 1) & (BUFSIZE - 1); } while (0)

/*
	ringbuffer logic:
	head = written data pointer
	tail = read data pointer

	when head == tail, buffer is empty
	when head + 1 == tail, buffer is full
	thus, number of available spaces in buffer is (tail - head) & bufsize

	can write:
	(tail - head - 1) & (BUFSIZE - 1)

	write to buffer:
	buf[head++] = data; head &= (BUFSIZE - 1);

	can read:
	(head - tail) & (BUFSIZE - 1)

	read from buffer:
	data = buf[tail++]; tail &= (BUFSIZE - 1);
*/

#ifdef	XONXOFF
#define		FLOWFLAG_STATE_XOFF	0
#define		FLOWFLAG_SEND_XON		1
#define		FLOWFLAG_SEND_XOFF	2
#define		FLOWFLAG_STATE_XON	4
// initially, send an XON
volatile uint8_t flowflags = FLOWFLAG_SEND_XON;
#endif

void serial_init()
{
#if BAUD > 38401
	UCSR0A = MASK(U2X0);
	UBRR0 = (((F_CPU / 8) / BAUD) - 0.5);
#else
	UCSR0A = 0;
	UBRR0 = (((F_CPU / 16) / BAUD) - 0.5);
#endif

	UCSR0B = MASK(RXEN0) | MASK(TXEN0);
	UCSR0C = MASK(UCSZ01) | MASK(UCSZ00);

	UCSR0B |= MASK(RXCIE0) | MASK(UDRIE0);
}

/*
	Interrupts
*/

#ifdef	USART_RX_vect
ISR(USART_RX_vect)
#else
ISR(USART0_RX_vect)
#endif
{
	if (buf_canwrite(rx))
		buf_push(rx, UDR0);
	else {
		uint8_t trash;

		// not reading the character makes the interrupt logic to swamp us with retries, so better read it and throw it away
		trash = UDR0;
	}

	#ifdef	XONXOFF
	if (flowflags & FLOWFLAG_STATE_XON && buf_canwrite(rx) <= 16) {
		// the buffer has only 16 free characters left, so send an XOFF
		// more characters might come in until the XOFF takes effect
		flowflags = FLOWFLAG_SEND_XOFF | FLOWFLAG_STATE_XON;
		// enable TX interrupt so we can send this character
		UCSR0B |= MASK(UDRIE0);
	}
	#endif
}

#ifdef	USART_UDRE_vect
ISR(USART_UDRE_vect)
#else
ISR(USART0_UDRE_vect)
#endif
{
	#ifdef	XONXOFF
	if (flowflags & FLOWFLAG_SEND_XON) {
		UDR0 = ASCII_XON;
		flowflags = FLOWFLAG_STATE_XON;
	}
	else if (flowflags & FLOWFLAG_SEND_XOFF) {
		UDR0 = ASCII_XOFF;
		flowflags = FLOWFLAG_STATE_XOFF;
	}
	else
	#endif
	if (buf_canread(tx))
		buf_pop(tx, UDR0);
	else
		UCSR0B &= ~MASK(UDRIE0);
}

/*
	Read
*/

uint8_t serial_rxchars()
{
	return buf_canread(rx);
}

uint8_t serial_popchar()
{
	uint8_t c = 0;

	// it's imperative that we check, because if the buffer is empty and we pop, we'll go through the whole buffer again
	if (buf_canread(rx))
		buf_pop(rx, c);

	#ifdef	XONXOFF
	if ((flowflags & FLOWFLAG_STATE_XON) == 0 && buf_canread(rx) <= 16) {
		// the buffer has (BUFSIZE - 16) free characters again, so send an XON
		flowflags = FLOWFLAG_SEND_XON;
		UCSR0B |= MASK(UDRIE0);
	}
	#endif

	return c;
}

/*
	Write
*/

void serial_writechar(uint8_t data)
{
	// check if interrupts are enabled
	if (SREG & MASK(SREG_I)) {
		// if they are, we should be ok to block since the tx buffer is emptied from an interrupt
		for (;buf_canwrite(tx) == 0;);
		buf_push(tx, data);
	}
	else {
		// interrupts are disabled- maybe we're in one?
		// anyway, instead of blocking, only write if we have room
		if (buf_canwrite(tx))
			buf_push(tx, data);
	}
	// enable TX interrupt so we can send this character
	UCSR0B |= MASK(UDRIE0);
}

void serial_writeblock(void *data, int datalen)
{
	int i;

	for (i = 0; i < datalen; i++)
		serial_writechar(((uint8_t *) data)[i]);
}

void serial_writestr(uint8_t *data)
{
	uint8_t i = 0, r;
	// yes, this is *supposed* to be assignment rather than comparison, so we break when r is assigned zero
	while ((r = data[i++]))
		serial_writechar(r);
}

/*
	Write from FLASH

	Extensions to output flash memory pointers. This prevents the data to
	become part of the .data segment instead of the .code segment. That means
	less memory is consumed for multi-character writes.

	For single character writes (i.e. '\n' instead of "\n"), using
	serial_writechar() directly is the better choice.
*/

void serial_writeblock_P(PGM_P data, int datalen)
{
	int i;

	for (i = 0; i < datalen; i++)
		serial_writechar(pgm_read_byte(&data[i]));
}

void serial_writestr_P(PGM_P data)
{
	uint8_t r, i = 0;
	// yes, this is *supposed* to be assignment rather than comparison, so we break when r is assigned zero
	while ((r = pgm_read_byte(&data[i++])))
		serial_writechar(r);
}