fmtx: explicitly drive the output line to 0 when not in use

The output line is forced high when there is no output, so force
it to GND explicitly.

USBCAS/fmtx.c

@@ -11,6 +11,9 @@
 #include "fmpat.h"
 
 static uint8_t parity = 0;
+static uint8_t tx_idle;
+#define TX_BUSY	0
+#define TX_IDLE (1 << TXC1)
 
 /*
  * Send stuff if we have anything to send
@@ -22,16 +25,19 @@ ISR(USART1_UDRE_vect)
 
 	if (!left) {
 		/* Fetch a new byte if there is one, unless motor is off */
-		int nd = motor_relay ? fmtx_next_byte() : -1;
+		int nd;
+		nd = motor_relay ? fmtx_next_byte() : -1;
 		if (nd < 0) {
 			/*
 			 * Nothing to send. Disable the interrupt and
 			 * send out an idle zero to make sure the output
 			 * is low when not used; otherwise the motor
-			 * detect won't work.
+			 * detect won't work. Unfortunately we have to
+			 * keep doing so and can't simply disable the
+			 * interrupt routine.
 			 */
-			UCSR1B &= ~(1 << UDRIE1);
-			UCSR1A  = (1 << TXC1); /* Make TXC1 bit useful */
+			if (!tx_idle)
+				UCSR1A = tx_idle = TX_IDLE;
 			UDR1    = parity = 0;
 			return;
 		}
@@ -39,6 +45,8 @@ ISR(USART1_UDRE_vect)
 		left = 8;
 	}
 
+	tx_idle = TX_BUSY;
+
 	pattern = fm_pat[data & PATTERN_BIT_MASK] ^ parity;
 
 	UDR1 = pattern;
@@ -52,10 +60,8 @@ ISR(USART1_UDRE_vect)
  */
 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)
+	/* Wait until input queue is idle and we have had at least one output */
+	while (!(UCSR1A & tx_idle))
 		do_usb_task();
 }
 
@@ -108,5 +114,5 @@ void fmtx_init_speed(uint32_t baudrate)
 	 * Enable transmitter ISR (which probably will immediately
 	 * go to idle...)
 	 */
-	fmtx_enable();
+	UCSR1B |= (1 << UDRIE1);
 }

USBCAS/usbcas.h

@@ -68,7 +68,7 @@ uint32_t fmtx_real_baudrate(uint32_t baudrate);
 void fmtx_init_speed(uint32_t baudrate);
 /* Enable the TX ISR which enables sending data */
 static inline ATTR_ALWAYS_INLINE void fmtx_enable(void) {
-	UCSR1B |= (1 << UDRIE1);
+	/* UCSR1B |= (1 << UDRIE1); */
 }
 extern bool motor_relay;