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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 <avr/io.h>
#include <stdlib.h>
#include "modem.h"
Modem modem;
/*
* 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 (modem.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;
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();
}
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