fmtx: make transmitter handling interrupt-driven

(Hopefully) avoid glitches by making the transmitter handling
interrupt-driven.

USBCAS/fmtx.c

@@ -11,45 +11,36 @@
 #include "fmpat.h"
 
 static uint8_t parity = 0;
-static uint8_t data;
-static uint8_t left;
-
-static inline bool fmtx_send_ready(void) ATTR_ALWAYS_INLINE;
-static inline bool fmtx_send_ready(void)
-{
-	return !!(UCSR1A & (1 << UDRE1));
-}
 
 /*
- * Sends another chunk of output.
+ * Send stuff if we have anything to send
  */
-uint8_t fmtx_send_more(void)
+ISR(USART1_UDRE_vect)
 {
-	uint8_t l = left;
-
-	if (!fmtx_send_ready())
-		return l;	/* Not ready yet */
-
-	uint8_t d = data;
-	uint8_t pattern = fm_pat[d & PATTERN_BIT_MASK] ^ parity;
-
-	UCSR1A = (1 << TXC1);
+	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);
-	return left;
-}
-
-/*
- * Begin sending a new byte. Use when fmtx_send_*() has returned zero.
- */
-
-uint8_t fmtx_send_byte(uint8_t d)
-{
-	data = d;
-	left = 8;
-	return fmtx_send_more();
 }
 
 /*
@@ -57,7 +48,10 @@ uint8_t fmtx_send_byte(uint8_t d)
  */
 void fmtx_drain(void)
 {
-	while (!(UCSR1A & (1 << TXC1)))
+	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();
 }
 
@@ -84,16 +78,12 @@ uint32_t fmtx_real_baudrate(uint32_t baudrate)
 
 void fmtx_init_speed(uint32_t baudrate)
 {
-	uint32_t ubrrval;
-
-	ubrrval = FMTX_UBRRVAL(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);
 
-	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
@@ -104,12 +94,20 @@ void fmtx_init_speed(uint32_t baudrate)
 	/* UBRR must be zero when the transmitter is enabled */
 	UBRR1  = 0;
 
-	/* Enable transmitter, but not receiver */
+	/* Enable transmitter, but not receiver and not the ISR (yet) */
 	UCSR1B = (1 << TXEN1);
 
 	/* Configure baud rate */
-	UBRR1  = ubrrval;
+	UBRR1  = FMTX_UBRRVAL(baudrate);
 
-	/* Output the current idle state of the line */
+	/*
+	 * 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();
 }

USBCAS/usbcas.c

@@ -125,8 +125,6 @@ void do_usb_task(void)
  */
 int main(void)
 {
-	uint8_t tx_left = 0;	/* Bits left to send? */
-
 	SetupHardware();
 
 	RingBuffer_InitBuffer(&USBtoCAS_Buffer, USBtoCAS_Buffer_Data, sizeof(USBtoCAS_Buffer_Data));
@@ -142,8 +140,10 @@ int main(void)
 			int16_t ReceivedByte = CDC_Device_ReceiveByte(&VirtualSerial_CDC_Interface);
 
 			/* Store received byte into the USART transmit buffer */
-			if (!(ReceivedByte < 0))
-			  RingBuffer_Insert(&USBtoCAS_Buffer, ReceivedByte);
+			if (!(ReceivedByte < 0)) {
+				RingBuffer_Insert(&USBtoCAS_Buffer, ReceivedByte);
+				fmtx_enable(); /* Enable TX interrupt routine if disabled */
+			}
 		}
 
 		uint16_t BufferCount = RingBuffer_GetCount(&CAStoUSB_Buffer);
@@ -175,13 +175,6 @@ int main(void)
 			}
 		}
 
-		/* Transmit the next data chunk if we can */
-		if (tx_left) {
-			tx_left = fmtx_send_more();
-		} else if (!RingBuffer_IsEmpty(&USBtoCAS_Buffer)) {
-			tx_left = fmtx_send_byte(RingBuffer_Remove(&USBtoCAS_Buffer));
-		}
-
 		do_usb_task();
 	}
 }
@@ -250,6 +243,19 @@ void fmrx_recv_byte(uint8_t byte)
 		RingBuffer_Insert(&CAStoUSB_Buffer, byte);
 }
 
+/*
+ * Called from the transmit register empty ISR to fetch a new byte;
+ * returns -1 if the ring buffer is empty.
+ */
+int fmtx_next_byte(void)
+{
+	if (USB_DeviceState != DEVICE_STATE_Configured ||
+	    RingBuffer_IsEmpty(&USBtoCAS_Buffer))
+		return -1;
+
+	return RingBuffer_Remove(&USBtoCAS_Buffer);
+}
+
 /** Event handler for the CDC Class driver Line Encoding Changed event.
  *
  *  \param[in] CDCInterfaceInfo  Pointer to the CDC class interface configuration structure being referenced

USBCAS/usbcas.h

@@ -62,11 +62,14 @@ void EVENT_USB_Device_ControlRequest(void);
 void EVENT_CDC_Device_LineEncodingChanged(USB_ClassInfo_CDC_Device_t* const CDCInterfaceInfo);
 void EVENT_CDC_Device_ControLineStateChanged(USB_ClassInfo_CDC_Device_t* const CDCInterfaceInfo);
 
-uint8_t fmtx_send_byte(uint8_t byte);
-uint8_t fmtx_send_more(void);
+int fmtx_next_byte(void);
 void fmtx_drain(void);
 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);
+}
 
 void fmrx_init(void);
 void fmrx_set_speed(uint32_t baudrate);