/*
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation. See the file COPYING
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * Copyright (C) 2010 Frank van Maarseveen <frankvm@frankvm.com>
 */

#include <stdint.h>
#include <avr/io.h>
#include <avr/pgmspace.h>

#include "common.h"
#include "bits.h"
#include "timer0.h"
#include "hardware.h"
#include "transmit.h"

/*
 * Define the clock period T expressed in 16us ticks. Constraints:
 * -	receiver.c must be configured for this T too.
 * -	T must be even.
 * -	2T must be <= 128
 */
#define T 20					/* 320 us, 3125Hz */

static void tx_bit(uint8_t bit)
{
	if (bit)
		CLR(LED);
	else
		SET(LED);
	t0_tickwait((uint8_t)(0.5 * T + 0.5));
	TOGGLE(LED);
	t0_tickwait((uint8_t)(0.5 * T + 0.5));
}

/*
 * Transmit precomputed packets using Manchester/BPSK coding. Put idle
 * time around the data to mark frame boundaries.
 *
 * Initial level is '1' (LED on). The first transition is for the first bit
 * which by definition must be zero: At T/2 of every clock cycle there is a
 * transition: up means bit value '1', down means bit value '0'.
 */
void tx_packet(prog_uint8_t *buf, uint8_t len)
{
	prog_uint8_t *p, *end;
	uint8_t i, byte;

	end = buf + len;
	OUTPUT(LED);
	SET(LED);
	t0_tickwait(2 * T);
	for (p = buf; p < end; ++p) {
		byte = pgm_read_byte(p);
		for (i = 0; i < 8; ++i) {
			tx_bit(byte & 0x80);
			byte <<= 1;
		}
	}
	CLR(LED);
	t0_tickwait(2 * T);
}