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OpenModem/hardware/AFSK.c

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#include <string.h>
#include "AFSK.h"
#include "util/time.h"
extern volatile ticks_t _clock;
extern unsigned long custom_preamble;
extern unsigned long custom_tail;
bool hw_afsk_dac_isr = false;
bool hw_5v_ref = false;
Afsk *AFSK_modem;
// Forward declerations
int afsk_getchar(void);
void afsk_putchar(char c);
void AFSK_hw_refDetect(void) {
// This is manual for now
#if ADC_REFERENCE == REF_5V
hw_5v_ref = true;
#else
hw_5v_ref = false;
#endif
}
void AFSK_hw_init(void) {
// Set up ADC
AFSK_hw_refDetect();
TCCR1A = 0;
TCCR1B = _BV(CS10) | _BV(WGM13) | _BV(WGM12);
ICR1 = (((CPU_FREQ+FREQUENCY_CORRECTION)) / 9600) - 1;
if (hw_5v_ref) {
ADMUX = _BV(REFS0) | 0;
} else {
ADMUX = 0;
}
ADC_DDR &= ~_BV(0);
ADC_PORT &= ~_BV(0);
DIDR0 |= _BV(0);
ADCSRB = _BV(ADTS2) |
_BV(ADTS1) |
_BV(ADTS0);
ADCSRA = _BV(ADEN) |
_BV(ADSC) |
_BV(ADATE)|
_BV(ADIE) |
_BV(ADPS2);
AFSK_DAC_INIT();
LED_TX_INIT();
LED_RX_INIT();
}
void AFSK_init(Afsk *afsk) {
// Allocate modem struct memory
memset(afsk, 0, sizeof(*afsk));
AFSK_modem = afsk;
// Set phase increment
afsk->phaseInc = MARK_INC;
// Initialise FIFO buffers
fifo_init(&afsk->delayFifo, (uint8_t *)afsk->delayBuf, sizeof(afsk->delayBuf));
fifo_init(&afsk->rxFifo, afsk->rxBuf, sizeof(afsk->rxBuf));
fifo_init(&afsk->txFifo, afsk->txBuf, sizeof(afsk->txBuf));
// Fill delay FIFO with zeroes
for (int i = 0; i<SAMPLESPERBIT / 2; i++) {
fifo_push(&afsk->delayFifo, 0);
}
AFSK_hw_init();
// Set up streams
FILE afsk_fd = FDEV_SETUP_STREAM(afsk_putchar, afsk_getchar, _FDEV_SETUP_RW);
afsk->fd = afsk_fd;
}
static void AFSK_txStart(Afsk *afsk) {
if (!afsk->sending) {
afsk->phaseInc = MARK_INC;
afsk->phaseAcc = 0;
afsk->bitstuffCount = 0;
afsk->sending = true;
LED_TX_ON();
afsk->preambleLength = DIV_ROUND(custom_preamble * BITRATE, 8000);
AFSK_DAC_IRQ_START();
}
ATOMIC_BLOCK(ATOMIC_RESTORESTATE) {
afsk->tailLength = DIV_ROUND(custom_tail * BITRATE, 8000);
}
}
void afsk_putchar(char c) {
AFSK_txStart(AFSK_modem);
while(fifo_isfull_locked(&AFSK_modem->txFifo)) { /* Wait */ }
fifo_push_locked(&AFSK_modem->txFifo, c);
}
int afsk_getchar(void) {
if (fifo_isempty_locked(&AFSK_modem->rxFifo)) {
return EOF;
} else {
return fifo_pop_locked(&AFSK_modem->rxFifo);
}
}
void AFSK_transmit(char *buffer, size_t size) {
fifo_flush(&AFSK_modem->txFifo);
int i = 0;
while (size--) {
afsk_putchar(buffer[i++]);
}
}
uint8_t AFSK_dac_isr(Afsk *afsk) {
if (afsk->sampleIndex == 0) {
if (afsk->txBit == 0) {
if (fifo_isempty(&afsk->txFifo) && afsk->tailLength == 0) {
AFSK_DAC_IRQ_STOP();
afsk->sending = false;
LED_TX_OFF();
return 0;
} else {
if (!afsk->bitStuff) afsk->bitstuffCount = 0;
afsk->bitStuff = true;
if (afsk->preambleLength == 0) {
if (fifo_isempty(&afsk->txFifo)) {
afsk->tailLength--;
afsk->currentOutputByte = HDLC_FLAG;
} else {
afsk->currentOutputByte = fifo_pop(&afsk->txFifo);
}
} else {
afsk->preambleLength--;
afsk->currentOutputByte = HDLC_FLAG;
}
if (afsk->currentOutputByte == AX25_ESC) {
if (fifo_isempty(&afsk->txFifo)) {
AFSK_DAC_IRQ_STOP();
afsk->sending = false;
LED_TX_OFF();
return 0;
} else {
afsk->currentOutputByte = fifo_pop(&afsk->txFifo);
}
} else if (afsk->currentOutputByte == HDLC_FLAG || afsk->currentOutputByte == HDLC_RESET) {
afsk->bitStuff = false;
}
}
afsk->txBit = 0x01;
}
if (afsk->bitStuff && afsk->bitstuffCount >= BIT_STUFF_LEN) {
afsk->bitstuffCount = 0;
afsk->phaseInc = SWITCH_TONE(afsk->phaseInc);
} else {
if (afsk->currentOutputByte & afsk->txBit) {
afsk->bitstuffCount++;
} else {
afsk->bitstuffCount = 0;
afsk->phaseInc = SWITCH_TONE(afsk->phaseInc);
}
afsk->txBit <<= 1;
}
afsk->sampleIndex = SAMPLESPERBIT;
}
afsk->phaseAcc += afsk->phaseInc;
afsk->phaseAcc %= SIN_LEN;
afsk->sampleIndex--;
return sinSample(afsk->phaseAcc);
}
static bool hdlcParse(Hdlc *hdlc, bool bit, FIFOBuffer *fifo) {
// Initialise a return value. We start with the
// assumption that all is going to end well :)
bool ret = true;
// Bitshift our byte of demodulated bits to
// the left by one bit, to make room for the
// next incoming bit
hdlc->demodulatedBits <<= 1;
// And then put the newest bit from the
// demodulator into the byte.
hdlc->demodulatedBits |= bit ? 1 : 0;
// Now we'll look at the last 8 received bits, and
// check if we have received a HDLC flag (01111110)
if (hdlc->demodulatedBits == HDLC_FLAG) {
// If we have, check that our output buffer is
// not full.
if (!fifo_isfull(fifo)) {
// If it isn't, we'll push the HDLC_FLAG into
// the buffer and indicate that we are now
// receiving data. For bling we also turn
// on the RX LED.
fifo_push(fifo, HDLC_FLAG);
hdlc->receiving = true;
#if OPEN_SQUELCH == false
LED_RX_ON();
#endif
} else {
// If the buffer is full, we have a problem
// and abort by setting the return value to
// false and stopping the here.
ret = false;
hdlc->receiving = false;
LED_RX_OFF();
}
// Everytime we receive a HDLC_FLAG, we reset the
// storage for our current incoming byte and bit
// position in that byte. This effectively
// synchronises our parsing to the start and end
// of the received bytes.
hdlc->currentByte = 0;
hdlc->bitIndex = 0;
return ret;
}
// Check if we have received a RESET flag (01111111)
// In this comparison we also detect when no transmission
// (or silence) is taking place, and the demodulator
// returns an endless stream of zeroes. Due to the NRZ
// coding, the actual bits send to this function will
// be an endless stream of ones, which this AND operation
// will also detect.
if ((hdlc->demodulatedBits & HDLC_RESET) == HDLC_RESET) {
// If we have, something probably went wrong at the
// transmitting end, and we abort the reception.
hdlc->receiving = false;
LED_RX_OFF();
return ret;
}
// If we have not yet seen a HDLC_FLAG indicating that
// a transmission is actually taking place, don't bother
// with anything.
if (!hdlc->receiving)
return ret;
// First check if what we are seeing is a stuffed bit.
// Since the different HDLC control characters like
// HDLC_FLAG, HDLC_RESET and such could also occur in
// a normal data stream, we employ a method known as
// "bit stuffing". All control characters have more than
// 5 ones in a row, so if the transmitting party detects
// this sequence in the _data_ to be transmitted, it inserts
// a zero to avoid the receiving party interpreting it as
// a control character. Therefore, if we detect such a
// "stuffed bit", we simply ignore it and wait for the
// next bit to come in.
//
// We do the detection by applying an AND bit-mask to the
// stream of demodulated bits. This mask is 00111111 (0x3f)
// if the result of the operation is 00111110 (0x3e), we
// have detected a stuffed bit.
if ((hdlc->demodulatedBits & 0x3f) == 0x3e)
return ret;
// If we have an actual 1 bit, push this to the current byte
// If it's a zero, we don't need to do anything, since the
// bit is initialized to zero when we bitshifted earlier.
if (hdlc->demodulatedBits & 0x01)
hdlc->currentByte |= 0x80;
// Increment the bitIndex and check if we have a complete byte
if (++hdlc->bitIndex >= 8) {
// If we have a HDLC control character, put a AX.25 escape
// in the received data. We know we need to do this,
// because at this point we must have already seen a HDLC
// flag, meaning that this control character is the result
// of a bitstuffed byte that is equal to said control
// character, but is actually part of the data stream.
// By inserting the escape character, we tell the protocol
// layer that this is not an actual control character, but
// data.
if ((hdlc->currentByte == HDLC_FLAG ||
hdlc->currentByte == HDLC_RESET ||
hdlc->currentByte == AX25_ESC)) {
// We also need to check that our received data buffer
// is not full before putting more data in
if (!fifo_isfull(fifo)) {
fifo_push(fifo, AX25_ESC);
} else {
// If it is, abort and return false
hdlc->receiving = false;
LED_RX_OFF();
ret = false;
}
}
// Push the actual byte to the received data FIFO,
// if it isn't full.
if (!fifo_isfull(fifo)) {
fifo_push(fifo, hdlc->currentByte);
} else {
// If it is, well, you know by now!
hdlc->receiving = false;
LED_RX_OFF();
ret = false;
}
// Wipe received byte and reset bit index to 0
hdlc->currentByte = 0;
hdlc->bitIndex = 0;
} else {
// We don't have a full byte yet, bitshift the byte
// to make room for the next bit
hdlc->currentByte >>= 1;
}
//digitalWrite(13, LOW);
return ret;
}
void AFSK_adc_isr(Afsk *afsk, int8_t currentSample) {
// To determine the received frequency, and thereby
// the bit of the sample, we multiply the sample by
// a sample delayed by (samples per bit / 2).
// We then lowpass-filter the samples with a
// Chebyshev filter. The lowpass filtering serves
// to "smooth out" the variations in the samples.
afsk->iirX[0] = afsk->iirX[1];
#if FILTER_CUTOFF == 600
afsk->iirX[1] = ((int8_t)fifo_pop(&afsk->delayFifo) * currentSample) >> 2;
// The above is a simplification of:
// afsk->iirX[1] = ((int8_t)fifo_pop(&afsk->delayFifo) * currentSample) / 3.558147322;
#elif FILTER_CUTOFF == 800
afsk->iirX[1] = ((int8_t)fifo_pop(&afsk->delayFifo) * currentSample) >> 2;
// The above is a simplification of:
// afsk->iirX[1] = ((int8_t)fifo_pop(&afsk->delayFifo) * currentSample) / 2.899043379;
#elif FILTER_CUTOFF == 1200
afsk->iirX[1] = ((int8_t)fifo_pop(&afsk->delayFifo) * currentSample) >> 1;
// The above is a simplification of:
// afsk->iirX[1] = ((int8_t)fifo_pop(&afsk->delayFifo) * currentSample) / 2.228465666;
#elif FILTER_CUTOFF == 1600
afsk->iirX[1] = ((int8_t)fifo_pop(&afsk->delayFifo) * currentSample) >> 1;
// The above is a simplification of:
// afsk->iirX[1] = ((int8_t)fifo_pop(&afsk->delayFifo) * currentSample) / 1.881349100;
#else
#error Unsupported filter cutoff!
#endif
afsk->iirY[0] = afsk->iirY[1];
#if FILTER_CUTOFF == 600
afsk->iirY[1] = afsk->iirX[0] + afsk->iirX[1] + (afsk->iirY[0] >> 1);
// The above is a simplification of a first-order 600Hz chebyshev filter:
// afsk->iirY[1] = afsk->iirX[0] + afsk->iirX[1] + (afsk->iirY[0] * 0.4379097269);
#elif FILTER_CUTOFF == 800
afsk->iirY[1] = afsk->iirX[0] + afsk->iirX[1] + (afsk->iirY[0] / 3);
// The above is a simplification of a first-order 800Hz chebyshev filter:
// afsk->iirY[1] = afsk->iirX[0] + afsk->iirX[1] + (afsk->iirY[0] * 0.3101172565);
#elif FILTER_CUTOFF == 1200
afsk->iirY[1] = afsk->iirX[0] + afsk->iirX[1] + (afsk->iirY[0] / 10);
// The above is a simplification of a first-order 800Hz chebyshev filter:
// afsk->iirY[1] = afsk->iirX[0] + afsk->iirX[1] + (afsk->iirY[0] * 0.1025215106);
#elif FILTER_CUTOFF == 1600
afsk->iirY[1] = afsk->iirX[0] + afsk->iirX[1] + -1*(afsk->iirY[0] / 17);
// The above is a simplification of a first-order 800Hz chebyshev filter:
// afsk->iirY[1] = afsk->iirX[0] + afsk->iirX[1] + (afsk->iirY[0] * -0.0630669239);
#else
#error Unsupported filter cutoff!
#endif
// We put the sampled bit in a delay-line:
// First we bitshift everything 1 left
afsk->sampledBits <<= 1;
// And then add the sampled bit to our delay line
afsk->sampledBits |= (afsk->iirY[1] > 0) ? 0 : 1;
// Put the current raw sample in the delay FIFO
fifo_push(&afsk->delayFifo, currentSample);
// We need to check whether there is a signal transition.
// If there is, we can recalibrate the phase of our
// sampler to stay in sync with the transmitter. A bit of
// explanation is required to understand how this works.
// Since we have PHASE_MAX/PHASE_BITS = 8 samples per bit,
// we employ a phase counter (currentPhase), that increments
// by PHASE_BITS everytime a sample is captured. When this
// counter reaches PHASE_MAX, it wraps around by modulus
// PHASE_MAX. We then look at the last three samples we
// captured and determine if the bit was a one or a zero.
//
// This gives us a "window" looking into the stream of
// samples coming from the ADC. Sort of like this:
//
// Past Future
// 0000000011111111000000001111111100000000
// |________|
// ||
// Window
//
// Every time we detect a signal transition, we adjust
// where this window is positioned little. How much we
// adjust it is defined by PHASE_INC. If our current phase
// phase counter value is less than half of PHASE_MAX (ie,
// the window size) when a signal transition is detected,
// add PHASE_INC to our phase counter, effectively moving
// the window a little bit backward (to the left in the
// illustration), inversely, if the phase counter is greater
// than half of PHASE_MAX, we move it forward a little.
// This way, our "window" is constantly seeking to position
// it's center at the bit transitions. Thus, we synchronise
// our timing to the transmitter, even if it's timing is
// a little off compared to our own.
if (SIGNAL_TRANSITIONED(afsk->sampledBits)) {
if (afsk->currentPhase < PHASE_THRESHOLD) {
afsk->currentPhase += PHASE_INC;
} else {
afsk->currentPhase -= PHASE_INC;
}
}
// We increment our phase counter
afsk->currentPhase += PHASE_BITS;
// Check if we have reached the end of
// our sampling window.
if (afsk->currentPhase >= PHASE_MAX) {
// If we have, wrap around our phase
// counter by modulus
afsk->currentPhase %= PHASE_MAX;
// Bitshift to make room for the next
// bit in our stream of demodulated bits
afsk->actualBits <<= 1;
// We determine the actual bit value by reading
// the last 3 sampled bits. If there is two or
// more 1's, we will assume that the transmitter
// sent us a one, otherwise we assume a zero
uint8_t bits = afsk->sampledBits & 0x07;
if (bits == 0x07 || // 111
bits == 0x06 || // 110
bits == 0x05 || // 101
bits == 0x03 // 011
) {
afsk->actualBits |= 1;
}
//// Alternative using five bits ////////////////
// uint8_t bits = afsk->sampledBits & 0x0f;
// uint8_t c = 0;
// c += bits & BV(1);
// c += bits & BV(2);
// c += bits & BV(3);
// c += bits & BV(4);
// c += bits & BV(5);
// if (c >= 3) afsk->actualBits |= 1;
/////////////////////////////////////////////////
// Now we can pass the actual bit to the HDLC parser.
// We are using NRZ-S coding, so if 2 consecutive bits
// have the same value, we have a 1, otherwise a 0.
// We use the TRANSITION_FOUND function to determine this.
//
// This is smart in combination with bit stuffing,
// since it ensures a transmitter will never send more
// than five consecutive 1's. When sending consecutive
// ones, the signal stays at the same level, and if
// this happens for longer periods of time, we would
// not be able to synchronize our phase to the transmitter
// and would start experiencing "bit slip".
//
// By combining bit-stuffing with NRZ-S coding, we ensure
// that the signal will regularly make transitions
// that we can use to synchronize our phase.
//
// We also check the return of the Link Control parser
// to check if an error occured.
if (!hdlcParse(&afsk->hdlc, !TRANSITION_FOUND(afsk->actualBits), &afsk->rxFifo)) {
afsk->status |= 1;
if (fifo_isfull(&afsk->rxFifo)) {
fifo_flush(&afsk->rxFifo);
afsk->status = 0;
}
}
}
}
ISR(ADC_vect) {
TIFR1 = _BV(ICF1);
AFSK_adc_isr(AFSK_modem, ((int16_t)((ADC) >> 2) - 128));
if (hw_afsk_dac_isr) {
DAC_PORT = (AFSK_dac_isr(AFSK_modem) & 0xF0) | _BV(3);
} else {
DAC_PORT = 128;
}
++_clock;
}
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