usbcas/USBCAS/fmtx.c

/*
 * Transmit an FM-modulated signal using the USART in SPI master mode.
 * As per ABC standard, this signal is transmitted LSB first.
 */

#include "usbcas.h"

/*
 * FM nybble patters; these should be transmitted LSB first
 * and XOR'd with the MSB of the pattern byte last transmitted.
 */
static const uint8_t fm_nyb[16] = {
	0x33, 0xcd, 0xcb, 0x35, 0xd3, 0x2d, 0x2b, 0xd5,
	0xb3, 0x4d, 0x4b, 0xb5, 0x53, 0xad, 0xab, 0x55
};
static uint8_t parity = 0;

/*
 * Send the least significant nybble of this data item and return
 * the next nybble, if any. Check with fmtx_send_ready() before
 * calling this function.
 */
uint8_t fmtx_send_nybble(uint8_t d)
{
	uint8_t nyb = fm_nyb[d & 15] ^ parity;
	UCSR1A = (1 << TXC1);
	UDR1 = nyb;
	parity = -((int8_t)nyb < 0);
	return d >> 4;
}

/*
 * Synchronize transmitter before reconfiguration
 */
void fmtx_drain(void)
{
	while (!(UCSR1A & (1 << TXC1)))
		;
}

/*
 * Initialize the USART hardware and set transmission baudrate.
 * This is mostly the same as SerialSPI_Init().
 */
void fmtx_init_speed(uint32_t baudrate)
{
	DDRD |= 0x28; /* PD3 = outdata, PD5 = XCK/TXLED */
	PORTD = (PORTD & ~0x28) | (parity & 0x20) | 0x08;

	UCSR1B = 0;	/* Disable USART */
	UCSR1A = 0;	/* Clear error bits */

	/*
	 * SPI master mode, mode 1, LSB first
	 * UCPHA doesn't matter, but have UCPOL=1 to that the clock
	 * is 1 when idle and the TXLED is OFF.
	 */
	UCSR1C = (3 << UMSEL10) | (1 << UDORD1) | (1 << UCPOL1);

	/* UBRR must be zero when the transmitter is enabled */
	UBRR1  = 0;

	/* Enable transmitter, but not receiver */
	UCSR1B = (1 << TXEN1);

	/* Configure baud rate (UBBRVAL is a typo in LUFA) */
	/* We need two SPI ticks per symbol, hence double... */
	UBRR1  = SERIAL_SPI_UBBRVAL(baudrate << 1);

	/* Output the current idle state of the line */
	UDR1   = parity;
}