fmtx: hold the line for 1024 bit-times in case of underrun

USBCAS/fmtx.c

@@ -25,7 +25,7 @@ static uint8_t parity;
  */
 ISR(USART1_UDRE_vect)
 {
-	static uint8_t data, left;
+	static uint8_t data, left, gap;
 	uint8_t pattern;
 
 	if (!left) {
@@ -34,28 +34,31 @@ ISR(USART1_UDRE_vect)
 		nd = motor_relay ? fmtx_next_byte() : -1;
 		if (nd < 0) {
 			/*
-			 * Nothing to send. Disable the interrupt and
-			 * send out one last edge. Afterwards, the hardware
-			 * will drive the line high.
+			 * 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.
 			 */
-			fmtx_disable();
-
-			UCSR1A = TX_IDLE;
-			UDR1 = parity; /* One last edge before going high */
-			parity = 0;
+			if (++gap) {
+				UCSR1A = TX_IDLE;
+				UCSR1A = parity;
+			} else {
+				fmtx_disable();
+			}
 			return;
 		}
 		data = nd;
 		left = 8;
+		gap = 0;
 	}
 
-	pattern = fm_pat[data & PATTERN_BIT_MASK] ^ ~parity;
+	pattern = fm_pat[data & PATTERN_BIT_MASK] ^ parity;
 
 	UCSR1A = TX_IDLE;
 	UDR1 = pattern;
 	data >>= BITS_PER_PATTERN_BYTE;
 	left -= BITS_PER_PATTERN_BYTE;
-	parity = -((int8_t)pattern >= 0);
+	parity = -((int8_t)pattern < 0);
 }
 
 /*
@@ -91,7 +94,7 @@ uint32_t fmtx_real_baudrate(uint32_t baudrate)
 
 void fmtx_init_speed(uint32_t baudrate)
 {
-	parity = 0;		/* We start out with a high idle */
+	parity = 0xff;		/* We start out with a high idle */
 
 	UCSR1B = 0;		/* Disable transmitter and ISR */
 	UCSR1A = (1 << TXC1);	/* Clear TXC bit */