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MicroAPRS/Modem/afsk.c

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//////////////////////////////////////////////////////
// First things first, all the includes we need //
//////////////////////////////////////////////////////
#include "afsk.h" // We need the header file for the modem
#include "config.h" // This stores basic configuration
#include "hardware.h" // Hardware functions are nice to have too :)
#include <drv/timer.h> // Timer driver from BertOS
//FIXME: is this needed ? #include <cfg/module.h>
#include <cpu/power.h> // Power management from BertOS
#include <cpu/pgm.h> // Access to PROGMEM from BertOS
#include <struct/fifobuf.h> // FIFO buffer implementation from BertOS
#include <string.h> // String operations, primarily used for memset function
//////////////////////////////////////////////////////
// Definitions and some useful macros //
//////////////////////////////////////////////////////
// Sine table for Direct Digital Synthesis DAC
// Since it would be inefficient to calculate a sine value each
// time we process a sample, we store the values in program memory
// as a look-up table. We only need to store values for a quarter
// wave, since we can easily reconstruct the entire 512 values
// from only these 128 values.
#define SIN_LEN 512
static const uint8_t PROGMEM sin_table[] =
{
128, 129, 131, 132, 134, 135, 137, 138, 140, 142, 143, 145, 146, 148, 149, 151,
152, 154, 155, 157, 158, 160, 162, 163, 165, 166, 167, 169, 170, 172, 173, 175,
176, 178, 179, 181, 182, 183, 185, 186, 188, 189, 190, 192, 193, 194, 196, 197,
198, 200, 201, 202, 203, 205, 206, 207, 208, 210, 211, 212, 213, 214, 215, 217,
218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230, 231, 232, 233,
234, 234, 235, 236, 237, 238, 238, 239, 240, 241, 241, 242, 243, 243, 244, 245,
245, 246, 246, 247, 248, 248, 249, 249, 250, 250, 250, 251, 251, 252, 252, 252,
253, 253, 253, 253, 254, 254, 254, 254, 254, 255, 255, 255, 255, 255, 255, 255,
}; STATIC_ASSERT(sizeof(sin_table) == SIN_LEN / 4);
// Calculate any sine value from quarter wave sine table
// The reason we declare this inline is to eliminate an extra
// call for the code. The code is essentially inserted directly
// in the calling functions code. This makes stuff faster :)
INLINE uint8_t sinSample(uint16_t i) {
// Make sure that the index asked for is in the correct range
ASSERT(i < SIN_LEN);
// First we make a new index value, and restrict it to only
// the first half-wave of the sine.
uint16_t newI = i % (SIN_LEN/2);
// We then check if this new index is larger than the first
// quarter wave. If it is, we don't have the value for this
// index directly, but we can figure it out by subtracting
// the new index from a half wave, effectively wrapping us
// back into the same place on the wave, whithin the quarter
// wave we have data for, only with the inverse sign. If the
// index was actually in the first quarter, we don't need to
// do anything.
newI = (newI >= (SIN_LEN/4)) ? (SIN_LEN/2 - newI -1) : newI;
// Now we just need to read the value from program memory
uint8_t sine = pgm_read8(&sin_table[newI]);
// And flip the sign (+/-) if the original index was greater
// than a half wave.
return (i >= (SIN_LEN/2)) ? (255 - sine) : sine;
}
// A very basic macro that just checks whether the last bit
// of a whatever is passed into it differ. This is used in the
// next macro.
#define BITS_DIFFER(bits1, bits2) (((bits1)^(bits2)) & 0x01)
// This macro is used to look for signal transitions. We need
// to identify these to keep the phase of our demodulator in
// sync with the incoming signal. Each time we find a signal
// transition on the physical medium, we adjust the phase of
// the demodulator.
// The macro effectively looks at the two least significant
// bits in a stream and returns true if they differ.
#define TRANSITION_FOUND(bits) BITS_DIFFER((bits), (bits) >> 1)
// Phase sync constants
#define PHASE_BITS 8
#define PHASE_INC 1
#define PHASE_MAX (SAMPLESPERBIT * PHASE_BITS)
#define PHASE_THRESHOLD (PHASE_MAX / 2)
// Modulation constants
#define MARK_FREQ 1200
#define MARK_INC (uint16_t)(DIV_ROUND(SIN_LEN * (uint32_t)MARK_FREQ, CONFIG_AFSK_DAC_SAMPLERATE))
#define SPACE_FREQ 2200
#define SPACE_INC (uint16_t)(DIV_ROUND(SIN_LEN * (uint32_t)SPACE_FREQ, CONFIG_AFSK_DAC_SAMPLERATE))
// HDLC flag bytes
#define HDLC_FLAG 0x7E // An HDLC_FLAG is used to signify the start or end of a frame
#define HDLC_RESET 0x7F // An HDLC_RESET is used to abruptly stop or reset a transmission
#define AX25_ESC 0x1B // We use the AX.25 escape character for escaping bit sequences in
// the actual data. This is similar to escaping an " character in a
// string enclosed by "s.
// Check that sample rate is divisible by bitrate.
// If this is not the case, all of our algorithms will
// fail horribly and we will cry.
STATIC_ASSERT(!(CONFIG_AFSK_DAC_SAMPLERATE % BITRATE));
// How many samples it takes to encode or decode one bit
// on the physical medium.
#define DAC_SAMPLESPERBIT (CONFIG_AFSK_DAC_SAMPLERATE / BITRATE)
//////////////////////////////////////////////////////
// Link Layer Control and Demodulation //
//////////////////////////////////////////////////////
// hdlcParse /////////////////////////////////////////
// This function looks at the raw bits demodulated from
// the physical medium and tries to parse actual data
// packets from the bitstream. Note that at this level,
// we don't really try to discriminate when a packet
// starts or ends, or where the payload is. We only try
// to detect that a transmission is taking place, then
// synchronise to the start and end of the transmitted
// bytes, and push these up to the data-link layer, in
// this example the MP.x protocol. It is then the
// protocols job to actually recreate the full packet.
// Also note that the data is not "pushed" per se, but
// stored in a FIFO buffer, that the protocol must
// continously read to recreate the received packets.
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;
LED_RX_ON();
} 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)
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;
}
return ret;
}
// adcISR ////////////////////////////////////////////
// This is the Interrupt Service Routine for the
// Analog to Digital Conversion. It is called 9600
// times each second to analyze the sample taken from
// the physical medium. The job of this routine is
// to detect whether we have a "mark" or "space"
// frequency present on the baseband (the physical
// medium). The result of this analysis will then
// be passed to the HDLC parser in form of a 1 or a 0
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 sample with a first
// order 600Hz filter. This is a Chebyshev filter.
afsk->iirX[0] = afsk->iirX[1];
afsk->iirX[1] = ((int8_t)fifo_pop(&afsk->delayFifo) * currentSample) >> 2;
afsk->iirY[0] = afsk->iirY[1];
afsk->iirY[1] = afsk->iirX[0] + afsk->iirX[1] + (afsk->iirY[0] >> 1);
// Put the sampled bit in a delay-line
afsk->sampledBits <<= 1; // Bitshift everything 1 left
afsk->sampledBits |= (afsk->iirY[1] > 0) ? 1 : 0;
// Put the current raw sample in the delay FIFO
fifo_push(&afsk->delayFifo, currentSample);
// If there is a signal transition, recalibrate
// sampling phase
if (TRANSITION_FOUND(afsk->sampledBits)) {
if (afsk->currentPhase < PHASE_THRESHOLD) {
afsk->currentPhase += PHASE_INC;
} else {
afsk->currentPhase -= PHASE_INC;
}
}
afsk->currentPhase += PHASE_BITS;
// Look at the raw samples to determine the transmitted bit
if (afsk->currentPhase >= PHASE_MAX) {
afsk->currentPhase %= PHASE_MAX;
// Bitshift to make room for next bit
afsk->actualBits <<= 1;
// Determine the actual bit value by reading the last
// 3 sampled bits. If there is two ore more 1's, the
// actual bit is a 1, otherwise a 0.
uint8_t bits = afsk->sampledBits & 0x07;
if (bits == 0x07 || // 111
bits == 0x06 || // 110
bits == 0x05 || // 101
bits == 0x03 // 011
) {
afsk->actualBits |= 1;
}
// Now we can pass the actual bit to the HDLC parser.
// We are using NRZI coding, so if 2 consecutive bits
// have the same value, we have a 1, otherwise a 0.
// We use the EDGE_FOUND function to determine this.
// 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 |= RX_OVERRUN;
}
}
}
//////////////////////////////////////////////////////
// Signal modulation and DAC //
//////////////////////////////////////////////////////
#define BIT_STUFF_LEN 5
#define SWITCH_TONE(inc) (((inc) == MARK_INC) ? SPACE_INC : MARK_INC)
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();
PTT_ON();
afsk->preambleLength = DIV_ROUND(CONFIG_AFSK_PREAMBLE_LEN * BITRATE, 8000);
AFSK_DAC_IRQ_START(afsk->dacPin);
}
ATOMIC(afsk->tailLength = DIV_ROUND(CONFIG_AFSK_TRAILER_LEN * BITRATE, 8000));
}
// This is the DAC ISR, called at sampling ratewhenever the DAC IRQ is on.
// It modulates the data to be transmitted and returns a value directly
// for output on the DAC
uint8_t afsk_dac_isr(Afsk *afsk) {
// Check whether we are at the beginning of a sample
if (afsk->sampleIndex == 0) {
if (afsk->txBit == 0) {
// If TX FIFO is empty and tail-length has decremented to 0
// we are done, stop the IRQ and reset
if (fifo_isempty(&afsk->txFifo) && afsk->tailLength == 0) {
AFSK_DAC_IRQ_STOP(afsk->dacPin);
afsk->sending = false;
LED_TX_OFF();
PTT_OFF();
return 0;
} else {
// Reset the bitstuff counter if we have just sent
// a bitstuffed byte
if (!afsk->bitStuff) afsk->bitstuffCount = 0;
// Reset bitstuff indicator to true
afsk->bitStuff = true;
// Check if we are in preamble or tail
if (afsk->preambleLength == 0) {
if (fifo_isempty(&afsk->txFifo)) {
afsk->tailLength--;
afsk->currentOutputByte = HDLC_FLAG;
} else {
// If preamble is already transmitted and TX
// buffer is not empty, we should get a byte
// for transmission
afsk->currentOutputByte = fifo_pop(&afsk->txFifo);
}
} else {
afsk->preambleLength--;
afsk->currentOutputByte = HDLC_FLAG;
}
// Handle escape sequences
if (afsk->currentOutputByte == AX25_ESC) {
if (fifo_isempty(&afsk->txFifo)) {
AFSK_DAC_IRQ_STOP(afsk->dacPin);
afsk->sending = false;
LED_TX_OFF();
PTT_OFF();
return 0;
} else {
afsk->currentOutputByte = fifo_pop(&afsk->txFifo);
}
} else if (afsk->currentOutputByte == HDLC_FLAG || afsk->currentOutputByte == HDLC_RESET) {
afsk->bitStuff = false;
}
}
// Start with LSB mask
afsk->txBit = 0x01;
}
// Check for bit stuffing
if (afsk->bitStuff && afsk->bitstuffCount >= BIT_STUFF_LEN) {
afsk->bitstuffCount = 0;
afsk->phaseInc = SWITCH_TONE(afsk->phaseInc);
} else {
// We are using NRZI so if we want to transmit a 1
// the modulated signal will stay the same. For a 0
// we make the signal transition
if (afsk->currentOutputByte & afsk->txBit) {
// We don't do anything, aka stay on the same
// tone as before. We have sent one 1, so we
// increment the bitstuff counter.
afsk->bitstuffCount++;
} else {
// We switch the tone, and reset the bitstuff
// counter, since we have now transmitted a
// zero
afsk->bitstuffCount = 0;
afsk->phaseInc = SWITCH_TONE(afsk->phaseInc);
}
// Move on to the next bit
afsk->txBit <<= 1;
}
afsk->sampleIndex = DAC_SAMPLESPERBIT;
}
// Retrieve af new sample index and DAC it
afsk->phaseAcc += afsk->phaseInc;
afsk->phaseAcc %= SIN_LEN;
afsk->sampleIndex--;
return sinSample(afsk->phaseAcc);
}
//////////////////////////////////////////////////////
// File operation overwrites for read/write //
// These functions make the "class" act like a file //
// pointer, which can be read from or written to. //
// Handy for sending and receiving data :) //
//////////////////////////////////////////////////////
static size_t afsk_read(KFile *fd, void *_buf, size_t size) {
Afsk *afsk = AFSK_CAST(fd);
uint8_t *buffer = (uint8_t *)_buf;
#if CONFIG_AFSK_RXTIMEOUT == 0
while (size-- && !fifo_isempty_locked(&afsk->rxFifo))
#else
while (size--)
#endif
{
#if CONFIG_AFSK_RXTIMEOUT != -1
ticks_t start = timer_clock();
#endif
while (fifo_isempty_locked(&afsk->rxFifo)) {
cpu_relax();
#if CONFIG_AFSK_RXTIMEOUT != -1
if (timer_clock() - start > ms_to_ticks(CONFIG_AFSK_RXTIMEOUT)) {
return buffer - (uint8_t *)_buf;
}
#endif
}
*buffer++ = fifo_pop_locked(&afsk->rxFifo);
}
return buffer - (uint8_t *)_buf;
}
static size_t afsk_write(KFile *fd, const void *_buf, size_t size) {
Afsk *afsk = AFSK_CAST(fd);
const uint8_t *buf = (const uint8_t *)_buf;
while (size--) {
while (fifo_isfull_locked(&afsk->txFifo)) {
cpu_relax();
}
fifo_push_locked(&afsk->txFifo, *buf++);
afsk_txStart(afsk);
}
return buf - (const uint8_t *)_buf;
}
static int afsk_flush(KFile *fd) {
Afsk *afsk = AFSK_CAST(fd);
while (afsk->sending) {
cpu_relax();
}
return 0;
}
static int afsk_error(KFile *fd) {
Afsk *afsk = AFSK_CAST(fd);
int err;
ATOMIC(err = afsk->status);
return err;
}
static void afsk_clearerr(KFile *fd) {
Afsk *afsk = AFSK_CAST(fd);
ATOMIC(afsk->status = 0);
}
//////////////////////////////////////////////////////
// Modem Initialization //
//////////////////////////////////////////////////////
void afsk_init(Afsk *afsk, int _adcPin, int _dacPin) {
// Allocate memory for struct
memset(afsk, 0, sizeof(*afsk));
// Configure pins
afsk->adcPin = _adcPin;
afsk->dacPin = _dacPin;
afsk->phaseInc = MARK_INC;
// Init 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);
}
// Init DAC & ADC
AFSK_ADC_INIT(_adcPin, afsk);
AFSK_DAC_INIT(_dacPin, afsk);
LED_TX_INIT();
LED_RX_INIT();
PTT_INIT();
DB(afsk->fd._type = KFT_AFSK);
afsk->fd.write = afsk_write;
afsk->fd.read = afsk_read;
afsk->fd.flush = afsk_flush;
afsk->fd.error = afsk_error;
afsk->fd.clearerr = afsk_clearerr;
}
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