USBCAS: better handling of baudrate clip, add receive routine

USBCAS/Descriptors.h

@@ -107,4 +107,3 @@
 		                                    ATTR_WARN_UNUSED_RESULT ATTR_NON_NULL_PTR_ARG(3);
 
 #endif
-

USBCAS/Makefile

@@ -18,7 +18,7 @@ F_CPU        = 16000000
 F_USB        = $(F_CPU)
 OPTIMIZATION = s
 TARGET       = usbcas
-SRC          = $(TARGET).c Descriptors.c reboot.c fmtx.c \
+SRC          = $(TARGET).c Descriptors.c reboot.c fmtx.c fmrx.c \
 	       $(LUFA_SRC_USB) $(LUFA_SRC_USBCLASS)
 LUFA_PATH    = ../LUFA/LUFA
 CC_FLAGS     = -DUSE_LUFA_CONFIG_HEADER -IConfig/

USBCAS/fmrx.c

@@ -0,0 +1,79 @@
+/*
+ * Receive an FM-modulated signal by using a timer with input capture
+ */
+
+#include "usbcas.h"
+
+static uint16_t one_bit_lo, one_bit_hi; /* Range for an edge being a "1" */
+
+void fmrx_init(void)
+{
+	/* Stop counter */
+	TCCR1B = 0;
+
+	/* Normal mode, no comparators */
+	TCCR1A = 0;
+
+	/* Enable interrupts on input capture */
+	TIMSK1 = (1 << ICIE1);
+
+	/* Clear timer counter */
+	TCNT1 = 0;
+
+	/* Input capture noise canceler on, no prescaler, normal mode;
+	 * start timer
+	 */
+	TCCR1B = (1 << ICNC1) + (1 << CS10);
+}
+
+static uint16_t bytes;
+
+void fmrx_set_speed(uint32_t baudrate)
+{
+	one_bit_lo = F_CPU / (4 * baudrate);
+	one_bit_hi = 3 * one_bit_lo;
+
+	bytes = 0;
+}
+
+/* We "hunt" for a block until we find this bit pattern; same as ABC800 */
+#define CAS_SYNC_PATTERN 0x0216
+#define CAS_BLOCK_LEN    (1+1+2+253+1+2) /* STX+type+num+data+ETX+checksum */
+
+/* Interrupt routine for edge capture */
+ISR(TIMER1_CAPT_vect)
+{
+	static uint16_t last_edge;
+	static uint16_t data;
+	static uint8_t  bits;
+
+	uint16_t edge, delta;
+
+	TCCR1B ^= (1 << ICES1);	/* Next edge -> opposite polarity */
+	edge = ICR1;
+
+	delta = edge - last_edge;
+	last_edge = edge;
+
+	if (delta < one_bit_lo) {
+		/* Early edge, no action */
+		return;
+	} else if (delta < one_bit_hi) {
+		/* A data bit edge */
+		data |= 1;
+	} else {
+		/* Clock edge */
+		if (bytes) {
+			if (--bits == 0) {
+				fmrx_recv_byte(data);
+				data = 0; /* To avoid false sync */
+				bits = 8;
+				bytes--;
+			}
+		} else if (data == CAS_SYNC_PATTERN) {
+			bits = 8;
+			bytes = CAS_BLOCK_LEN;
+		}
+		data <<= 1;
+	}
+}

USBCAS/fmtx.c

@@ -64,22 +64,29 @@ void fmtx_drain(void)
 /*
  * Initialize the USART hardware and set transmission baudrate.
  * This is mostly the same as SerialSPI_Init().
+ * Returns the actual baud rate adjusted for underflow.
  */
-#define FMTX_UBRRVAL(b) (SERIAL_SPI_UBBRVAL(b * 8/BITS_PER_PATTERN_BYTE))
+#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)
+#define FMTX_MIN_BAUD (F_CPU / (2 * (8/BITS_PER_PATTERN_BYTE) * (UBRR_MAX+1UL)))
+#define FMTX_MAX_BAUD (F_CPU / (2 * (8/BITS_PER_PATTERN_BYTE)))
 
-#if FMTX_ABC80_UBRR > UBRR_MAX
+#if FMTX_MIN_BAUD > CAS_BAUDRATE_ABC80
 # error "UBRR overflows for standard ABC80 baud rate"
 #endif
 
+uint32_t fmtx_real_baudrate(uint32_t baudrate)
+{
+	if (baudrate < FMTX_MIN_BAUD || baudrate > FMTX_MAX_BAUD)
+		baudrate = CAS_BAUDRATE_ABC80;
+	return baudrate;
+}
+
 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;

USBCAS/usbcas.c

@@ -178,7 +178,7 @@ int main(void)
 	}
 }
 
-static uint32_t current_baudrate = CAS_BAUDRATE_ABC80;
+static uint32_t current_baudrate;
 
 /** Configures the board hardware and chip peripherals for the demo's functionality. */
 void SetupHardware(void)
@@ -199,7 +199,10 @@ void SetupHardware(void)
 	PORTB = (1 << 0) | (1 << 4);
 	DDRB  = ~(1 << 4);
 
-	/* Initialize transmitter (and receiver) */
+	/* Initialize receiver and transmitter */
+	fmrx_init();
+	current_baudrate = fmtx_real_baudrate(0);
+	fmrx_set_speed(current_baudrate);
 	fmtx_init_speed(current_baudrate);
 }
 
@@ -229,19 +232,15 @@ void EVENT_USB_Device_ControlRequest(void)
 	CDC_Device_ProcessControlRequest(&VirtualSerial_CDC_Interface);
 }
 
-#if 0
-/** ISR to manage the reception of data from the serial port, placing received bytes into a circular buffer
- *  for later transmission to the host.
+/*
+ * Called from the demodulation ISR to push a new byte into the buffer
  */
-ISR(USART1_RX_vect, ISR_BLOCK)
+void fmrx_recv_byte(uint8_t byte)
 {
-	uint8_t ReceivedByte = UDR1;
-
-	if ((USB_DeviceState == DEVICE_STATE_Configured) &&
-	    !(RingBuffer_IsFull(&CAStoUSB_Buffer)))
-		RingBuffer_Insert(&CAStoUSB_Buffer, ReceivedByte);
+	if (USB_DeviceState == DEVICE_STATE_Configured &&
+	    !RingBuffer_IsFull(&CAStoUSB_Buffer))
+		RingBuffer_Insert(&CAStoUSB_Buffer, byte);
 }
-#endif
 
 /** Event handler for the CDC Class driver Line Encoding Changed event.
  *
@@ -251,10 +250,13 @@ void EVENT_CDC_Device_LineEncodingChanged(USB_ClassInfo_CDC_Device_t* const CDCI
 {
 	uint32_t baudrate = CDCInterfaceInfo->State.LineEncoding.BaudRateBPS;
 
+	baudrate = fmtx_real_baudrate(baudrate);
+
 	if (baudrate != current_baudrate) {
 		current_baudrate = baudrate;
 		fmtx_drain();
-		fmtx_init_speed(baudrate);
+		fmrx_set_speed(current_baudrate);
+		fmtx_init_speed(current_baudrate);
 	}
 }
 

USBCAS/usbcas.h

@@ -64,8 +64,13 @@ void EVENT_CDC_Device_ControLineStateChanged(USB_ClassInfo_CDC_Device_t* const C
 uint8_t fmtx_send_byte(uint8_t byte);
 uint8_t fmtx_send_more(void);
 void fmtx_drain(void);
+uint32_t fmtx_real_baudrate(uint32_t baudrate);
 void fmtx_init_speed(uint32_t baudrate);
 
+void fmrx_init(void);
+void fmrx_set_speed(uint32_t baudrate);
+void fmrx_recv_byte(uint8_t byte);
+
 /* Parameters */
 #define CAS_BAUDRATE_ABC80	720
 #define CAS_BAUDRATE_ABC800	2400