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"

#define TX_BUSY	0
#define TX_IDLE (1 << TXC1)

static inline ATTR_ALWAYS_INLINE void fmtx_disable(void)
{
	UCSR1B &= ~(1 << UDRIE1);
	PORTB |= (1 << 0);
}

static uint8_t parity;

/*
 * Send stuff if we have anything to send
 */
ISR(USART1_UDRE_vect)
{
	static uint8_t data, left, gap;
	static uint16_t block_bytes;
	uint8_t pattern;

	if (!left) {
		/* Fetch a new byte if there is one, unless motor is off */
		int nd;
		if (!motor_relay() || (nd = fmtx_next_byte()) < 0) {
			/*
			 * Nothing to send.  Hold the line for some
			 * number of byte-times in case we suffered an
			 * underrun. Afterwards, the hardware will
			 * drive the line high.
			 */
			UCSR1A = TX_IDLE;
			UCSR1A = parity;
			if (!++gap)
				fmtx_disable();
			return;
		}
		data = nd;
		if (!block_bytes) {
			if (data == 0x02) {
				/*
				 * Beginning of a data block, suppress
				 * FF = silence until the end of the block
				 *
				 * STX+header+data+ETX+csum
				 */
				block_bytes = 1+3+253+1+2;
			}
		} else {
			block_bytes--;
		}

		data = nd;
		left = 8;
		gap = 0;
	}

	if (!block_bytes && data == 0xff) {
		pattern = parity;
	} else {
		pattern = fm_pat[data & PATTERN_BIT_MASK] ^ parity;
		data >>= BITS_PER_PATTERN_BYTE;
		parity = -((int8_t)pattern < 0);
	}
	left -= BITS_PER_PATTERN_BYTE;

	UCSR1A = TX_IDLE;
	UDR1 = pattern;
}

/*
 * Synchronize transmitter before reconfiguration
 */
void fmtx_drain(void)
{
	/* Wait until input queue is idle and we have had at least one output */
	while ((UCSR1B & (1 << UDRIE1)) || !(UCSR1A & TX_IDLE))
		do_usb_tasks();
}

/*
 * 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)
{
	parity = 0xff;		/* We start out with a high idle */

	DDRD  |= 0x28; /* PD3 = outdata, PD5 = XCK/TXLED */
	PORTD |= 0x28; /* Drive data and clock high while idle */

	UCSR1B = 0;		/* Disable transmitter and ISR */

	/*
	 * 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);

	/*
	 * Enable transmitter ISR (which probably will immediately
	 * go to idle...)
	 */
	fmtx_enable();
}