1 files changed, 117 insertions, 0 deletions
diff --git a/src/modem.cc b/src/modem.cc
new file mode 100644
index 0000000..e34910a
--- /dev/null
+++ b/src/modem.cc
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+/* Name: modem.c
+ *
+ * Audio modem for Attiny85 & other AVR chips with modifications
+ *
+ * Author: Jari Tulilahti
+ * Copyright: 2014 Rakettitiede Oy and 2016 Daniel Friesel
+ * License: LGPLv3, see COPYING, and COPYING.LESSER -files for more info
+ */
+
+#include <avr/io.h>
+#include <stdlib.h>
+#include "modem.h"
+#include "fecmodem.h"
+
+extern FECModem modem;
+
+bool Modem::newTransmission()
+{
+	if (new_transmission) {
+		new_transmission = false;
+		return true;
+	}
+	return false;
+}
+
+/*
+ * Returns number of available bytes in ringbuffer or 0 if empty
+ */
+uint8_t Modem::buffer_available() {
+	return buffer_head - buffer_tail;
+}
+
+/*
+ * Store 1 byte in ringbuffer
+ */
+inline void Modem::buffer_put(const uint8_t c) {
+	if (buffer_available() != MODEM_BUFFER_SIZE) {
+		buffer[buffer_head++ % MODEM_BUFFER_SIZE] = c;
+	}
+}
+
+/*
+ * Fetch 1 byte from ringbuffer
+ */
+uint8_t Modem::buffer_get() {
+	uint8_t b = 0;
+	if (buffer_available() != 0) {
+		b = buffer[buffer_tail++ % MODEM_BUFFER_SIZE];
+	}
+	return b;
+}
+
+/*
+ * Start the modem by enabling Pin Change Interrupts & Timer
+ */
+void Modem::enable()  {
+	/* Enable R1 */
+	DDRA  |= _BV(PA3);
+	PORTA |= _BV(PA3);
+
+	/* Modem pin as input */
+	MODEM_DDR &= ~_BV(MODEM_PIN);
+
+	/* Enable Pin Change Interrupts and PCINT for MODEM_PIN */
+	MODEM_PCMSK |= _BV(MODEM_PCINT);
+	PCICR |= _BV(MODEM_PCIE);
+
+	/* Timer: TCCR1: CS10 and CS11 bits: 8MHz clock with Prescaler 64 = 125kHz timer clock */
+	TCCR1B = _BV(CS11) | _BV(CS10);
+}
+
+void Modem::disable()
+{
+	PORTA &= ~_BV(PA3);
+	DDRA  &= ~_BV(PA3);
+}
+
+void Modem::receive() {
+	/* Static variables instead of globals to keep scope inside ISR */
+	static uint8_t modem_bit = 0;
+	static uint8_t modem_bitlen = 0;
+	static uint8_t modem_byte = 0;
+
+	/* Read & Zero Timer/Counter 1 value */
+	uint8_t modem_pulselen = MODEM_TIMER;
+	MODEM_TIMER = 0;
+
+	/*
+	 * Check if we received Start/Sync -pulse.
+	 * Calculate bit signal length middle point from pulse.
+	 * Return from ISR immediately.
+	 */
+	if (modem_pulselen > MODEM_SYNC_LEN) {
+		modem_bitlen = (modem_pulselen >> 2);
+		modem_bit = 0;
+		new_transmission = true;
+		return;
+	}
+
+	/*
+	 * Shift byte and set high bit according to the pulse length.
+	 * Long pulse = 1, Short pulse = 0
+	 */
+	modem_byte = (modem_byte >> 1) | (modem_pulselen < modem_bitlen ? 0x00 : 0x80);
+
+	/* Check if we received complete byte and store it in ring buffer */
+	if (!(++modem_bit % 0x08)) {
+		buffer_put(modem_byte);
+	}
+}
+
+/*
+ * Pin Change Interrupt Vector. This is The Modem.
+ */
+ISR(PCINT3_vect) {
+	modem.receive();
+}