1 files changed, 95 insertions, 0 deletions
diff --git a/modem.c b/modem.c
new file mode 100644
index 0000000..db4c296
--- /dev/null
+++ b/modem.c
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+/* Name: modem.c
+ *
+ * Audio modem for Attiny85 & other AVR chips with modifications
+ *
+ * Author: Jari Tulilahti
+ * Copyright: 2014 Rakettitiede Oy
+ * License: LGPLv3, see COPYING, and COPYING.LESSER -files for more info
+ */
+
+#include "modem.h"
+
+/* Ring buffer global variables */
+static volatile uint8_t modem_buffer_head = 0, modem_buffer_tail = 0;
+static volatile uint8_t modem_buffer[MODEM_BUFFER_SIZE];
+
+/*
+ * Returns number of available bytes in ringbuffer or 0 if empty
+ */
+uint8_t modem_buffer_available() {
+	return modem_buffer_head - modem_buffer_tail;
+}
+
+/*
+ * Store 1 byte in ringbuffer
+ */
+static inline void modem_buffer_put(const uint8_t c) {
+	if (modem_buffer_available() != MODEM_BUFFER_SIZE) { 
+		modem_buffer[modem_buffer_head++ % MODEM_BUFFER_SIZE] = c;
+	} 
+}
+
+/*
+ * Fetch 1 byte from ringbuffer
+ */
+uint8_t modem_buffer_get() {
+	uint8_t b = 0;
+	if (modem_buffer_available() != 0) {
+		b = modem_buffer[modem_buffer_tail++ % MODEM_BUFFER_SIZE];
+	}
+	return b;
+}
+
+/*
+ * Pin Change Interrupt Vector. This is The Modem.
+ */
+ISR(PCINT0_vect) {
+	/* 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;
+		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)) {
+		modem_buffer_put(modem_byte);
+	}
+}
+
+/*
+ * Start the modem by enabling Pin Change Interrupts & Timer
+ */
+void modem_init()  {
+	/* Modem pin as input */
+	MODEM_DDR &= ~(1 << MODEM_PIN);
+
+	/* Enable Pin Change Interrupts and PCINT for MODEM_PIN */
+	GIMSK |= (1 << PCIE);
+	PCMSK |= (1 << MODEM_PIN);
+
+	/* Timer: TCCR1: CS10, CS11 and CS12 bits: 8MHz clock with Prescaler 64 = 125kHz timer clock */
+	TCCR1 = (1 << CS10) | (1 << CS11) | (1 << CS12);
+
+	/* Enable interrupts */
+	sei();
+}