Initial commit

src/main.c

@@ -0,0 +1,304 @@
+#include "common.h"
+
+#include <avr/io.h>
+#include <avr/interrupt.h>
+
+// Edge direction
+#define E_RISING   1
+#define E_FALLING  0
+
+// Decoder state
+#define S_IDLE  0
+#define S_SYNC  1
+#define S_DATA  2
+
+static volatile bool   txready;     // Aligned with clock?
+static volatile int    rxstate;     // Current encoder state
+
+static volatile byte   rxbuf[128];  // Data buffer for decoder
+static volatile byte * rxhead;      // Write position for decoder
+static volatile byte * rxtail;      // End of decoder buffer
+static volatile word   edgecap;     // Current edge capture time
+static volatile word   edgedir;     // Current edge direction
+static volatile int    lastbit;     // Previously read logic value
+static volatile word   numsync;     // Number of preamble bits read
+static volatile word   numdata;     // Number of data bits read
+static volatile bool   needmid;     // Expect short interval next
+
+static void SendByte(byte data);
+static void WaitPulse(void);
+static void HandleEdge(void);
+static void Synchronize(void);
+static void ReadPayload(void);
+static void ReadShortPeriod(void);
+static void ReadLongPeriod(void);
+static void WriteBit(int val);
+
+void RF_Init(void)
+{
+	// Set pins for line coded data
+	DDR(D) |=  BIT(5); // Modulator
+	DDR(B) &= ~BIT(5); // Demodulator
+
+	// Calculate end of decoder buffer
+	rxtail = rxbuf + sizeof(rxbuf);
+
+	// Data rate = 4000 Hz
+	// Baud rate = 8000 Hz (edge changes)
+	// Bit period = 1/f = 1/4000 = 250us
+	// T (mid-bit time) = 125us
+
+	// Initialize TIMER1 to generate encoder clock pulses
+	// at half of bit period which equals mid-bit time T.
+
+	TIMSK1  = 0x00;  // Disable timer interrupts
+	TIFR1   = 0x27;  // Clear all interrupt flags
+	TCCR1B  = 0x02;  // Prescale /8 = 1MHz = 1us per step
+	OCR1A   = 125;   // Generate interrupt every T steps
+	TCNT1   = 0;     // Reset counter value to zero
+	TCCR1A  = 0x00;  // Timer not connected to port
+	TCCR1C  = 0x00;  // Do not force compare match
+	TIMSK1  = 0x02;  // Enable compare interrupt
+
+	// Initialize TIMER3 to interrupt when a rising edge on
+	// PB5 is detected and when the counter value overflows.
+
+	TIMSK3  = 0x00;  // Disable timer interrupts
+	TIFR3   = 0x27;  // Clear all interrupt flags
+	TCCR3B  = 0x02;  // Prescale /8 = 1MHz = 1us per step
+	TCCR3B |= 0x40;  // Trigger capture event on rising edge
+	OCR3A   = 0;     // Not using output compare interrupt
+	TCNT3   = 0;     // Reset counter value to zero
+	TCCR3A  = 0x00;  // Timer not connected to port
+	TCCR3C  = 0x00;  // Do not force compare match
+	TIMSK3  = 0x20;  // Enable input capture interrupt
+	TIMSK3 |= 0x01;  // Enable overflow interrupt
+}
+
+void RF_Transmit(const byte *data, int size)
+{
+	const byte *head = data;
+	const byte *tail = data + size;
+
+	// The preamble with its alternating symbols is
+	// line coded with only the actual meat-and-potato
+	// transitions in the middle of the bit period and
+	// none of those pesky boundary transitions. This
+	// makes it possible for the decoder to align the
+	// clock phase before receiving any data.
+
+	// Preamble for clock synchronization
+	SendByte(0xAA); // AA = 1010 1010
+
+	while (head < tail) {
+		SendByte(*head++);
+	}
+}
+
+int RF_Receive(byte *data, int size)
+{
+	int n = 0;
+
+	while (1) {
+		if (n == size || n == (int) numdata) {
+			break; // Finished copying
+		}
+		data[n] = rxbuf[n];
+		n++;
+	}
+
+	return n;
+}
+
+static void SendByte(byte data)
+{
+	// Manchester code always has a transition at the
+	// middle of each bit period and may (depending on
+	// the information to be transmitted) have one at
+	// the start of the period also. The direction of
+	// the mid-bit transition indicates the data.
+
+	// Transitions at the period boundaries do not carry
+	// information. They only place the signal in the
+	// correct state to allow the mid-bit transition.
+
+	for (int bit = 0; bit < 8; bit++) {
+		if (data & (0x80 >> bit)) {
+			WaitPulse();
+			PORT(D) &= ~BIT(5);
+			WaitPulse();
+			// Rising edge
+			PORT(D) |= BIT(5);
+		} else {
+			WaitPulse();
+			PORT(D) |= BIT(5);
+			WaitPulse();
+			// Falling edge
+			PORT(D) &= ~BIT(5);
+		}
+	}
+}
+
+static void WaitPulse(void)
+{
+	txready = false;
+	while (!txready);
+}
+
+static void HandleEdge(void)
+{
+	if (edgedir != E_RISING) {
+		return; // Wrong edge
+	}
+
+	rxstate = S_SYNC;
+	numsync = 1;
+}
+
+static void Synchronize(void)
+{
+	// Preamble only has middle transitions
+	if (edgecap < 200 || edgecap > 300) {
+		rxstate = S_IDLE; // Wrong timing
+		return;
+	}
+
+	numsync++;
+	if (numsync == 8) {
+		rxstate = S_DATA;
+		rxhead  = rxbuf;
+		needmid = false;
+		numdata = 0;
+		lastbit = 0;
+	}
+}
+
+static void ReadPayload(void)
+{
+	if (edgecap >= 75 && edgecap <= 175) {
+		ReadShortPeriod();
+		return;
+	}
+
+	if (edgecap >= 200 && edgecap <= 300) {
+		ReadLongPeriod();
+		return;
+	}
+
+	// Wrong timing
+	rxstate = S_IDLE;
+}
+
+static void ReadShortPeriod(void)
+{
+	// The period length gives us enough information to
+	// know what the bit value is without even looking
+	// at the edge direction.
+
+	if (needmid) {
+		WriteBit(lastbit);
+		needmid = false;
+	} else {
+		needmid = true;
+	}
+}
+
+static void ReadLongPeriod(void)
+{
+	// If there was a boundary transition we must expect
+	// to receive another transition after mid-bit time,
+	// otherwise something went wrong...
+
+	if (needmid) {
+		rxstate = S_IDLE;
+		return;
+	}
+
+	lastbit = !lastbit;
+	WriteBit(lastbit);
+}
+
+static void WriteBit(int val)
+{
+	int bit;
+
+	if (rxhead == rxtail) {
+		return; // Discard
+	}
+
+	bit = numdata % 8;
+	numdata++;
+
+	*rxhead &= ~(0x80 >> bit);
+	*rxhead |= (val << (7 - bit));
+
+	if (bit == 7) {
+		rxhead++;
+	}
+}
+
+// Encoder clock pulse
+ISR(TIMER1_COMPA_vect)
+{
+	TCNT1 = 0;
+	txready = true;
+}
+
+// Decoder edge capture
+ISR(TIMER3_CAPT_vect)
+{
+	TCNT3 = 0;
+	edgecap = ICR3;
+	edgedir = (PIN(B) & BIT(5)) ? 1 : 0;
+
+	// Must not simply toggle the edge direction bit since
+	// we can miss very quick edge changes and run out of
+	// sync with the actual port state.
+
+	TCCR3B = (edgedir) ? 0x02 : 0x42;
+
+	if (rxstate == S_IDLE) {
+		HandleEdge();
+	} else if (rxstate == S_SYNC) {
+		Synchronize();
+	} else if (rxstate == S_DATA) {
+		ReadPayload();
+	}
+}
+
+// Decoder overflow
+ISR(TIMER3_OVF_vect)
+{
+	TCNT3 = 0;
+	edgecap = 0xFFFF;
+	if (rxstate == S_SYNC) {
+		Synchronize();
+	} else if (rxstate == S_DATA) {
+		ReadPayload();
+	}
+}
+
+int main(void)
+{
+	bool running = true;
+	const byte testmsg[] = "FOOBAR";
+	byte recvbuf[128];
+
+	RF_Init();
+	sei();
+
+	while (running) {
+		Info("Sending phase encoded message...");
+		RF_Transmit(testmsg, sizeof(testmsg));
+
+		Sleep(1000);
+
+		if (RF_Receive(recvbuf, sizeof(recvbuf))) {
+			Info("Received message '%s'.", recvbuf);
+		}
+
+		Sleep(1000);
+	}
+
+	return 0;
+}