fmtx: support emitting interblock silence

ABC80 needs silence - without carrier - for some time between
blocks. This will be encoded as FF FF FF ..., but obviously not inside
the blocks themselves.

TODO: uplevel the interface, either using an escape byte or actually
encode the data blocks directly.

USBCAS/fmtx.c

@@ -27,6 +27,7 @@ static uint8_t parity;
 ISR(USART1_UDRE_vect)
 {
 	static uint8_t data, left, gap;
+	static uint16_t block_bytes;
 	uint8_t pattern;
 
 	if (!left) {
@@ -45,18 +46,37 @@ ISR(USART1_UDRE_vect)
 				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;
 	}
 
-	pattern = fm_pat[data & PATTERN_BIT_MASK] ^ parity;
+	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;
-	data >>= BITS_PER_PATTERN_BYTE;
-	left -= BITS_PER_PATTERN_BYTE;
-	parity = -((int8_t)pattern < 0);
 }
 
 /*