usbcas/USBCAS/fmrx.c

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

#include "usbcas.h"

static uint16_t one_bit_lo, one_bit_hi; /* Range for an edge being a "1" */

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 */
	TIMSK1 = (1 << ICIE1);

	/* Clear interrupt flag if set */
	TIFR1 = (1 << ICF1);

	/* 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;
}

static uint16_t bytes;

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;

	bytes = 0;

	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 */

/* Interrupt routine for edge capture */
ISR(TIMER1_CAPT_vect)
{
	static uint16_t last_edge;
	static uint16_t data;
	static uint8_t  bits;

	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;
		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 {
			sprintf(tbuf, "[%04x]", data);
			say(tbuf);

			if (data == CAS_SYNC_PATTERN) {
				bits = 8;
				bytes = CAS_BLOCK_LEN;
				fmrx_led_on();
			}
		}
		data >>= 1;
	}
}