USBCAS: we overflow UBRR for 720 baud, so need to duplicate bits

USBCAS/fmnyb.pl

@@ -1,21 +0,0 @@
-#!/usr/bin/perl
-
-use integer;
-use strict;
-
-print "static const uint8_t fm_nyb[16] = {\n";
-
-for (my $i = 0; $i < 16; $i++) {
-    my $pat = 0;
-    my $inc = 0;		# Previous bit
-
-    for (my $j = 0; $j < 4; $j++) {
-	my $bp = ($i & (1 << $j)) ? 0x1 : 0x3;
-	$bp ^= $inc * 3;
-	$inc = $bp >> 1;
-	$pat |= $bp << ($j*2);
-    }
-
-    printf "%s0x%02x", ($i ? ', ' : "\t"), $pat;
-}
-print "\n};\n";

USBCAS/fmpat.h

@@ -0,0 +1,24 @@
+#if BITS_PER_PATTERN_BYTE == 1
+
+static const uint8_t fm_pat[2] = {
+	0xff, 0x0f
+};
+
+#elif BITS_PER_PATTERN_BYTE == 2
+
+static const uint8_t fm_pat[4] = {
+	0x0f, 0xf3, 0xcf, 0x33
+};
+
+
+#elif BITS_PER_PATTERN_BYTE == 4
+
+static const uint8_t fm_pat[16] = {
+	0x33, 0xcd, 0xcb, 0x35, 0xd3, 0x2d, 0x2b, 0xd5, 0xb3, 0x4d, 0x4b, 0xb5, 0x53, 0xad, 0xab, 0x55
+};
+
+#else
+
+#error "Invalid BITS_PER_PATTERN_BYTE"
+
+#endif

USBCAS/fmpat.pl

@@ -0,0 +1,54 @@
+#!/usr/bin/perl
+
+use integer;
+use strict;
+
+# 1 bit/byte patterns
+print "#if BITS_PER_PATTERN_BYTE == 1\n\n";
+
+print "static const uint8_t fm_pat[2] = {\n";
+print "\t0xff, 0x0f";
+
+print "\n};\n\n";
+
+# 2 bits/byte patterns
+print "#elif BITS_PER_PATTERN_BYTE == 2\n\n";
+
+print "static const uint8_t fm_pat[4] = {\n";
+for (my $i = 0; $i < 4; $i++) {
+    my $pat = 0;
+    my $inc = 0;		# Previous bit
+
+    for (my $j = 0; $j < 2; $j++) {
+	my $bp = ($i & (1 << $j)) ? 0x3 : 0xf;
+	$bp ^= $inc * 15;
+	$inc = $bp >> 3;
+	$pat |= $bp << ($j*4);
+    }
+
+    printf "%s0x%02x", ($i ? ', ' : "\t"), $pat;
+}
+print "\n};\n\n";
+
+print "\n#elif BITS_PER_PATTERN_BYTE == 4\n\n";
+
+# 4 bits/byte patterns
+print "static const uint8_t fm_pat[16] = {\n";
+for (my $i = 0; $i < 16; $i++) {
+    my $pat = 0;
+    my $inc = 0;		# Previous bit
+
+    for (my $j = 0; $j < 4; $j++) {
+	my $bp = ($i & (1 << $j)) ? 0x1 : 0x3;
+	$bp ^= $inc * 3;
+	$inc = $bp >> 1;
+	$pat |= $bp << ($j*2);
+    }
+
+    printf "%s0x%02x", ($i ? ', ' : "\t"), $pat;
+}
+print "\n};\n\n";
+
+print "#else\n\n";
+print "#error \"Invalid BITS_PER_PATTERN_BYTE\"\n";
+print "\n#endif\n";

USBCAS/fmtx.c

@@ -5,28 +5,51 @@
 
 #include "usbcas.h"
 
-/*
- * FM nybble patters; these should be transmitted LSB first
- * and XOR'd with the MSB of the pattern byte last transmitted.
- */
-static const uint8_t fm_nyb[16] = {
-	0x33, 0xcd, 0xcb, 0x35, 0xd3, 0x2d, 0x2b, 0xd5,
-	0xb3, 0x4d, 0x4b, 0xb5, 0x53, 0xad, 0xab, 0x55
-};
+#define BITS_PER_PATTERN_BYTE	2
+#define PATTERN_BIT_MASK	((1 << BITS_PER_PATTERN_BYTE) - 1)
+
+#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));
+}
 
 /*
- * Send the least significant nybble of this data item and return
- * the next nybble, if any. Check with fmtx_send_ready() before
- * calling this function.
+ * Sends another chunk of output.
  */
-uint8_t fmtx_send_nybble(uint8_t d)
+uint8_t fmtx_send_more(void)
 {
-	uint8_t nyb = fm_nyb[d & 15] ^ parity;
+	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);
-	UDR1 = nyb;
-	parity = -((int8_t)nyb < 0);
-	return d >> 4;
+	UDR1 = pattern;
+	data = d >> 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();
 }
 
 /*
@@ -42,8 +65,22 @@ void fmtx_drain(void)
  * Initialize the USART hardware and set transmission baudrate.
  * This is mostly the same as SerialSPI_Init().
  */
+#define FMTX_UBRRVAL(b) (SERIAL_SPI_UBBRVAL(b * 8/BITS_PER_PATTERN_BYTE))
+#define UBRR_MAX 4095
+#define FMTX_ABC80_UBRR FMTX_UBRRVAL(CAS_BAUDRATE_ABC80)
+
+#if FMTX_ABC80_UBRR > UBRR_MAX
+# error "UBRR overflows for standard ABC80 baud rate"
+#endif
+
 void fmtx_init_speed(uint32_t baudrate)
 {
+	uint32_t ubrrval;
+
+	ubrrval = FMTX_UBRRVAL(baudrate);
+	if (ubrrval > UBRR_MAX)
+		ubrrval = FMTX_ABC80_UBRR;
+
 	DDRD |= 0x28; /* PD3 = outdata, PD5 = XCK/TXLED */
 	PORTD = (PORTD & ~0x28) | (parity & 0x20) | 0x08;
 
@@ -63,9 +100,8 @@ void fmtx_init_speed(uint32_t baudrate)
 	/* Enable transmitter, but not receiver */
 	UCSR1B = (1 << TXEN1);
 
-	/* Configure baud rate (UBBRVAL is a typo in LUFA) */
-	/* We need two SPI ticks per symbol, hence double... */
-	UBRR1  = SERIAL_SPI_UBBRVAL(baudrate << 1);
+	/* Configure baud rate */
+	UBRR1  = ubrrval;
 
 	/* Output the current idle state of the line */
 	UDR1   = parity;

USBCAS/usbcas.c

@@ -113,7 +113,7 @@ static void update_modem_status(USB_ClassInfo_CDC_Device_t* const CDCInterfaceIn
  */
 int main(void)
 {
-	int8_t hinybble = -1;
+	uint8_t tx_left = 0;	/* Bits left to send? */
 
 	SetupHardware();
 
@@ -163,16 +163,11 @@ int main(void)
 			}
 		}
 
-		/* Transmit the next data nybble if we can */
-		if (fmtx_send_ready()) {
-			if (hinybble >= 0) {
-				/* Send high nybble of previous data */
-				fmtx_send_nybble(hinybble);
-				hinybble = -1;
-			} else if (!RingBuffer_IsEmpty(&USBtoCAS_Buffer)) {
-				/* Send low nybble of new data */
-				hinybble = fmtx_send_nybble(RingBuffer_Remove(&USBtoCAS_Buffer));
-			}
+		/* 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));
 		}
 
 		/* Update modem flags if needed */
@@ -256,10 +251,6 @@ void EVENT_CDC_Device_LineEncodingChanged(USB_ClassInfo_CDC_Device_t* const CDCI
 {
 	uint32_t baudrate = CDCInterfaceInfo->State.LineEncoding.BaudRateBPS;
 
-	/* Hack to map ABC80 standard speed to a common baud rate */
-	if (baudrate == CAS_BAUDRATE_ABC80_FAKE)
-		baudrate = CAS_BAUDRATE_ABC80;
-
 	if (baudrate != current_baudrate) {
 		current_baudrate = baudrate;
 		fmtx_drain();

USBCAS/usbcas.h

@@ -61,17 +61,12 @@ 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_nybble(uint8_t byte);
+uint8_t fmtx_send_byte(uint8_t byte);
+uint8_t fmtx_send_more(void);
 void fmtx_drain(void);
 void fmtx_init_speed(uint32_t baudrate);
-static inline bool fmtx_send_ready(void) ATTR_ALWAYS_INLINE;
-static inline bool fmtx_send_ready(void)
-{
-	return !!(UCSR1A & (1 << UDRE1));
-}
 
 /* Parameters */
-#define CAS_BAUDRATE_ABC80_FAKE	600
 #define CAS_BAUDRATE_ABC80	720
 #define CAS_BAUDRATE_ABC800	2400