usbcas/USBCAS/fmtx.c

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

#include "usbcas.h"

#define BITS_PER_PATTERN_BYTE	2
#define PATTERN_BIT_MASK	((1 << BITS_PER_PATTERN_BYTE) - 1)

#include "fmpat.h"

static uint8_t parity = 0;

/*
 * Send stuff if we have anything to send
 */
ISR(USART1_UDRE_vect)
{
	static uint8_t data, left;
	uint8_t pattern;

	if (!left) {
		/* Fetch a new byte if there is one */
		int nd = fmtx_next_byte();
		if (nd < 0) {
			/*
			 * Nothing to send. Disable interrupt.
			 */
			UCSR1B &= ~(1 << UDRIE1);
			return;
		}
		data = nd;
		left = 8;
	}

	pattern = fm_pat[data & PATTERN_BIT_MASK] ^ parity;

	UCSR1A = (1 << TXC1); /* Make TXC1 bit useful */
	UDR1 = pattern;
	data >>= BITS_PER_PATTERN_BYTE;
	left -= BITS_PER_PATTERN_BYTE;
	parity = -((int8_t)pattern < 0);
}

/*
 * Synchronize transmitter before reconfiguration
 */
void fmtx_drain(void)
{
	const uint8_t ucsr1a_mask = (1 << UDRE1) + (1 << TXC1);

	/* Wait until UDRE1 and TXC1 are both set */
	while ((UCSR1A & ucsr1a_mask) != ucsr1a_mask)
		do_usb_task();
}

/*
 * Initialize the USART hardware and set transmission baudrate.
 * This is mostly the same as SerialSPI_Init().
 * Returns the actual baud rate adjusted for underflow.
 */
#define FMTX_UBRRVAL(b) (SERIAL_SPI_UBBRVAL((b) * (8/BITS_PER_PATTERN_BYTE)))
#define UBRR_MAX 4095
#define FMTX_MIN_BAUD (F_CPU / (2 * (8/BITS_PER_PATTERN_BYTE) * (UBRR_MAX+1UL)))
#define FMTX_MAX_BAUD (F_CPU / (2 * (8/BITS_PER_PATTERN_BYTE)))

#if FMTX_MIN_BAUD > CAS_BAUDRATE_ABC80
# error "UBRR overflows for standard ABC80 baud rate"
#endif

uint32_t fmtx_real_baudrate(uint32_t baudrate)
{
	if (baudrate < FMTX_MIN_BAUD || baudrate > FMTX_MAX_BAUD)
		baudrate = CAS_BAUDRATE_ABC80;
	return baudrate;
}

void fmtx_init_speed(uint32_t baudrate)
{
	UCSR1B = 0;		/* Disable transmitter and ISR */
	UCSR1A = (1 << TXC1);	/* Clear TXC bit */

	DDRD |= 0x28; /* PD3 = outdata, PD5 = XCK/TXLED */
	PORTD = (PORTD & ~0x28) | 0x20 | (parity & 0x08);

	/*
	 * 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 and not the ISR (yet) */
	UCSR1B = (1 << TXEN1);

	/* Configure baud rate */
	UBRR1  = FMTX_UBRRVAL(baudrate);

	/*
	 * Output the current idle state of the line. This will also
	 * cause the TXC bit to get set afterwards.
	 */
	UDR1   = parity;

	/*
	 * Enable transmitter ISR (which may immediately disable itself...)
	 */
	fmtx_enable();
}