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" */

void fmrx_init(void)
{
	/* Stop counter */
	TCCR1B = 0;

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

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

	/* Clear timer counter */
	TCNT1 = 0;

	/* Input capture noise canceler on, no prescaler, normal mode;
	 * start timer
	 */
	TCCR1B = (1 << ICNC1) + (1 << CS10);
}

static uint16_t bytes;

void fmrx_set_speed(uint32_t baudrate)
{
	one_bit_lo = F_CPU / (4 * baudrate);
	one_bit_hi = 3 * one_bit_lo;

	bytes = 0;
}

/* We "hunt" for a block until we find this bit pattern; same as ABC800 */
#define CAS_SYNC_PATTERN 0x0216
#define CAS_BLOCK_LEN    (1+1+2+253+1+2) /* STX+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;
	last_edge = edge;

	if (delta < one_bit_lo) {
		/* Early edge, no action */
		return;
	} else if (delta < one_bit_hi) {
		/* A data bit edge */
		data |= 1;
	} else {
		/* Clock edge */
		if (bytes) {
			if (--bits == 0) {
				fmrx_recv_byte(data);
				data = 0; /* To avoid false sync */
				bits = 8;
				bytes--;
			}
		} else if (data == CAS_SYNC_PATTERN) {
			bits = 8;
			bytes = CAS_BLOCK_LEN;
		}
		data <<= 1;
	}
}