Add Rx driver, move ppm output pins to free up INT0.

Makefile

@@ -1,7 +1,8 @@
 PORT = /dev/ttyUSB0
 ARDUINO = /usr/share/arduino-0017/hardware/cores/arduino
 TARGET = pilot
-SRC = $(ARDUINO)/pins_arduino.c $(TARGET).c uart.c adc.c timer1.c actuators.c
+SRC = $(ARDUINO)/pins_arduino.c $(TARGET).c uart.c adc.c timer1.c actuators.c \
+      rx.c
 MCU = atmega328p
 F_CPU = 16000000
 FORMAT = ihex

actuators.c

@@ -8,8 +8,8 @@
 #include "timer1.h"
 
 static void pin_mode(uint8_t pin, uint8_t mode) {
-	uint8_t bit = digitalPinToBitMask(pin + 2);
-	volatile uint8_t *reg = portModeRegister(digitalPinToPort(pin + 2));
+	uint8_t bit = digitalPinToBitMask(pin + 3);
+	volatile uint8_t *reg = portModeRegister(digitalPinToPort(pin + 3));
 
 	if (mode)
 		*reg |= bit;
@@ -18,8 +18,8 @@ static void pin_mode(uint8_t pin, uint8_t mode) {
 }
 
 static void pin_set(uint8_t pin, uint8_t val) {
-	uint8_t bit = digitalPinToBitMask(pin + 2);
-	volatile uint8_t *reg = portOutputRegister(digitalPinToPort(pin + 2));
+	uint8_t bit = digitalPinToBitMask(pin + 3);
+	volatile uint8_t *reg = portOutputRegister(digitalPinToPort(pin + 3));
 
 	if (val)
 		*reg |= bit;

rx.c

@@ -0,0 +1,111 @@
+/*
+ * E-sky Hobby ET6I tx and rx.
+ */
+
+#include <avr/io.h>
+#include <avr/interrupt.h>
+
+#include "timer1.h"
+
+volatile uint16_t rx_ch[10] = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, };
+volatile uint8_t rx_co_throttle = 0;
+volatile uint8_t rx_co_right = 0;
+volatile uint8_t rx_cy_front = 0;
+volatile uint8_t rx_cy_right = 0;
+volatile uint8_t rx_no_signal = 0;
+
+#if 0
+
+/* Rx sum signal connected to the PD2/INT0 pin.  */
+void rx_init(void) {
+	EICRA = 0x01;
+	EIMSK = 0x01;
+	DDRD &= ~0x04;
+}
+
+ISR(INT0_vect) {
+	/* Timer1 overflows every >4ms, so we should be fine using TCNT1 raw */
+	uint16_t now = TCNT1;
+
+	if (PIND & 0x04) {
+		if (now - rx_up > 40000)
+			rx_chnum = 0;
+		rx_up = now;
+		return;
+	}
+
+	rx_ch[rx_chnum ++] = now - rx_up;
+	if (rx_chnum >= sizeof(rx_ch) / sizeof(*rx_ch))
+		rx_chnum = 0;
+}
+#else
+
+/* Individual rx outputs connected to PB0, PB1, PB2, PB3 (and so on)
+ * Note: only unmask interrupts for the pins actually connected to Rx.  */
+void rx_init(void) {
+	DDRB &= ~0x0f;
+	PCMSK0 = 0x0f;
+	PCICR |= 0x01;
+}
+
+ISR(PCINT0_vect) {
+	static uint32_t rx_up = 0;
+	static uint8_t rx_chnum = 0;
+	uint32_t now = timer_read();
+
+	/* Note: could even just use TCNT1 above at the timer frequency
+	 * of 16MHz because the timer overflows at about 4ms only, but
+	 * then we might miss the long pause which might be 10ms or longer.
+	 * May be fixable though.
+	 */
+
+	/* The E-sky receiver starts pulsing the next channel almost
+	 * immediately before the falling edge of the previous channel
+	 * pulse, so the edges come about 0.01 ms apart only and
+	 * sometimes the second interrupts gets missed.  Since they are
+	 * so close we choose to unconditionally ignore the second one,
+	 * because the first one stands a better chance of being reported
+	 * accurately by the AVR core.  We ignore any edges < 0.5ms since
+	 * the previous edge.
+	 */
+
+	if ((uint32_t) (now - rx_up) > F_CPU / 400) {
+		/* Un-mix the channels.  The ET6I trasmitter is made
+		 * specifically for helicopters and mixes the channels on
+		 * the tx side to produce signals for: the motor, the 4
+		 * servos and the gyro.  We need to perform some simple
+		 * arithmetics to go back to the raw stick position values
+		 * for any processing in the autopilot.  This is currently
+		 * assuming the Idle Up setting ("aerobatic mode") is off.
+		 *
+		 * It's not ideal, but at least we're only using the 4
+		 * first channels and can re-use channels 5 and 6 for
+		 * something like triggering camera shutter or something
+		 * else.  Ideally we should calculate the collective
+		 * pitch by averaging the three servo angles, I think, and
+		 * for the left-right / front-back nick values we should
+		 * compare th three servo angles.
+		 */
+		uint16_t up_raw = rx_ch[2] >= ((256 + 31) << 6) ? 
+			((256 + 31) << 6) - 1 : rx_ch[2];
+		uint16_t back_raw = rx_ch[1] -
+			(rx_ch[2] >> 2) - (rx_ch[2] >> 4);
+		uint16_t left_raw = rx_ch[0] + (back_raw >> 1) -
+			(rx_ch[2] >> 2) - (rx_ch[2] >> 4);
+
+		rx_co_throttle = (up_raw >> 6) - 31;
+		rx_co_right = (rx_ch[3] >> 6) - 10;
+		rx_cy_front = ((~back_raw) >> 6) - 4;
+		rx_cy_right = ((~left_raw) >> 6) + 66;
+
+		rx_chnum = 0;
+		rx_no_signal = 0;
+	} else if ((uint32_t) (now - rx_up) > F_CPU / 2000)
+		/* XXX: should use F_CPU below */
+		rx_ch[rx_chnum ++] = now - rx_up - F_CPU / 1050;
+	else
+		return;
+
+	rx_up = now;
+}
+#endif

rx.h

@@ -0,0 +1,16 @@
+/*
+ * E-sky Hobby ET6I tx and rx.
+ */
+
+void rx_init(void);
+
+/* Raw channels */
+extern volatile uint16_t rx_ch[10];
+/* Processed */
+extern volatile uint8_t rx_no_signal;
+/* Collective */
+extern volatile uint8_t rx_co_throttle;
+extern volatile uint8_t rx_co_right;
+/* Cyclic */
+extern volatile uint8_t rx_cy_front;
+extern volatile uint8_t rx_cy_right;