usbcas/USBCAS/fmrx.c

/*
 * Receive an FM-modulated signal by using a timer with input capture
 */

#include "usbcas.h"

static inline void fmrx_led_on(void)
{
	PORTB &= ~(1 << 0);
}

static inline void fmrx_led_off(void)
{
	PORTB |= (1 << 0);
}

void fmrx_init(void)
{
	uint8_t tccr1b;

	/* ICP1/PD4 is input, with pullup */
	DDRD  &= ~(1 << 4);
	PORTD |=  (1 << 4);

	/* PB0 is RXLED */
	DDRB |= (1 << 0);
	fmrx_led_off();

	/* Stop counter */
	TCCR1B = 0;

	/* Normal mode, no comparators */
	TCCR1A = 0;

	/* Enable interrupts on input capture */
	TIFR1 = TIMSK1 = (1 << ICIE1);

	/* Clear timer counter */
	TCNT1 = 0;

	/*
	 * Input capture noise canceler on, normal mode; not counting yet
	 */
	tccr1b = (1 << ICNC1) + (0 << CS10);
	if (!(PIND & (1 << 4)))
		tccr1b |= 1 << ICES1;
	TCCR1B = tccr1b;
}

/* Timing constants: start and end of data bit interval, and timeout */
static uint16_t one_bit_lo, one_bit_hi, one_bit_timeout;

static uint16_t bytes;
static uint16_t last_edge;
static uint16_t data;		/* 16 bit for sync pattern detect */
static uint8_t  bits;
static uint16_t bytes;
static bool started;

/* Bit receive timeout on/off */
static inline void enable_timeout(uint16_t when)
{
	OCR1A = when;
	TIFR1 = TIMSK1 = (1 << ICIE1) | (1 << OCIE1A);
}

static inline void disable_timeout(void)
{
	TIMSK1 = (1 << ICIE1);
}

void fmrx_set_speed(uint32_t baudrate)
{
	uint8_t prescale = 1;	/* No prescaling */

	/* Enable prescaler for low baud rates to reduce wraparound */
	if (baudrate < 976) {
		baudrate <<= 3;	/* ... for calculation purposes ... */
		prescale++;	/* Prescale by 8 */
	}

	one_bit_lo      = F_CPU / (4 * baudrate);
	one_bit_hi      =  3 * one_bit_lo;
	one_bit_timeout = 16 * one_bit_lo; /* 4 total bit times... */

	disable_timeout();
	started = false;
	bits = bytes = 0;
	last_edge = TCNT1;

	TCCR1B = (TCCR1B & ~7) | prescale;
}

/* We "hunt" for a block until we find this bit pattern; same as ABC800 */
#define CAS_SYNC_PATTERN 0x0216 /* SYNC + STX */
#define CAS_BLOCK_LEN    (1+2+253+1+2) /* type+num+data+ETX+checksum */

/* Finish a bit due to clock edge or timeout */
static void finish_bit(void)
{
	if (!started)
		return;

	if (bytes) {
		if (--bits == 0) {
			uint8_t outbyte = data >> 8;
			fmrx_recv_byte(outbyte);
			data = 0; /* To avoid false sync */
			bits = 8;
			if (--bytes == 0)
				fmrx_led_off();
		}
	} else if (data == CAS_SYNC_PATTERN) {
		bits = 8;
		bytes = CAS_BLOCK_LEN;
		fmrx_led_on();
	}
}

/* Interrupt routine for edge capture */
ISR(TIMER1_CAPT_vect)
{
	uint16_t edge, delta;

	TCCR1B ^= (1 << ICES1);	/* Next edge -> opposite polarity */
	edge = ICR1;

	delta = edge - last_edge;

	if (delta < one_bit_lo) {
		/* Early edge, no action */
		return;
	} else if (delta < one_bit_hi) {
		/* A data bit edge */
		data |= (1 << 15);
	} else {
		/* Clock edge */
		last_edge = edge;
		enable_timeout(edge + one_bit_timeout);
		finish_bit();
		started = true;
		data >>= 1;
	}
}

/* Interrupt routine for bit timeout */
ISR(TIMER1_COMPA_vect)
{
	disable_timeout();
	finish_bit();
	started = false;
}