////////////////////////////////////////////////////// // 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 // Timer driver from BertOS #include // Power management from BertOS #include // Access to PROGMEM from BertOS #include // FIFO buffer implementation from BertOS #include // 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) // We use this macro to check if the signal transitioned // from one bit (tone) to another. This is used in the phase // synchronisation. We look at the last four bits in the // stream of demodulated bits and if they differ in sets of // two bits, we assume a signal transition occured. We look // at pairs of bits to eliminate false positives where a // single erroneously demodulated bit will trigger an // incorrect phase syncronisation. #define DUAL_XOR(bits1, bits2) ((((bits1)^(bits2)) & 0x03) == 0x03) #define SIGNAL_TRANSITIONED(bits) DUAL_XOR((bits), (bits) >> 2) // Phase sync constants #define PHASE_BITS 8 // How much to increment phase counter each sample #define PHASE_INC 1 // Nudge by an eigth of a sample each adjustment #define PHASE_MAX (SAMPLESPERBIT * PHASE_BITS) // Resolution of our phase counter = 64 #define PHASE_THRESHOLD (PHASE_MAX / 2) // Target transition point of our phase window // Modulation constants #define MARK_FREQ 1200 // The tone frequency signifying a binary one #define SPACE_FREQ 2200 // The tone frequency signifying a binary zero // We calculate the amount we need to increment the index // in our sine table for each sample of the two tones #define MARK_INC (uint16_t)(DIV_ROUND(SIN_LEN * (uint32_t)MARK_FREQ, CONFIG_AFSK_DAC_SAMPLERATE)) #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) // 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; } 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 samples with a // Chebyshev filter. The lowpass filtering serves // to "smooth out" the variations in the samples. 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); // Chebyshev filter // 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) ? 1 : 0; // 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 three 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 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 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 |= RX_OVERRUN; } } } ////////////////////////////////////////////////////// // Signal modulation and DAC // ////////////////////////////////////////////////////// // Defines how many consecutive ones we send // before we need to "stuff" in a zero #define BIT_STUFF_LEN 5 // A macro for switching what tone is being // synthesized by the DAC. We basically just // change how quickly we go through the sine // table each time we send out a sample. This // is done by changing the phaseInc variable #define SWITCH_TONE(inc) (((inc) == MARK_INC) ? SPACE_INC : MARK_INC) // This function starts the transmission static void afsk_txStart(Afsk *afsk) { if (!afsk->sending) { // Initialize the phase increment to // that of the mark frequency (zero) afsk->phaseInc = MARK_INC; // Reset the phase accumulator to 0 afsk->phaseAcc = 0; // And also the bitstuff counter afsk->bitstuffCount = 0; // Indicate we are now sending afsk->sending = true; // And turn on the blingy LED LED_TX_ON(); // We also need to calculate how many HDLC_FLAG // bytes we need to send in preamble afsk->preambleLength = DIV_ROUND(CONFIG_AFSK_PREAMBLE_LEN * BITRATE, 8000); AFSK_DAC_IRQ_START(); } // We make the same calculation for the tail length, // but this needs to be atomic, since the txStart // function could potentially be called while we // are already transmitting. ATOMIC(afsk->tailLength = DIV_ROUND(CONFIG_AFSK_TRAILER_LEN * BITRATE, 8000)); } // This is the DAC ISR, called at sampling rate whenever 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 we are, we should figure out what we are // actually going to modulate and transmit :) if (afsk->txBit == 0) { // txBit is a bitmask that is ANDed to the // byte we are sending. It is bitshifted one // position left each time we shift the next // bit. If it is 0, we know we are at the // beginning of the next byte, and nothing // has been transmitted yet. // 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->sending = false; LED_TX_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, signifying // that it's ok to bit stuff. afsk->bitStuff = true; // Check if we are in preamble or tail if (afsk->preambleLength == 0) { // We are not in preamble if (fifo_isempty(&afsk->txFifo)) { // If the TX buffer is empty, we must // be in the TX tail then. // Decrement the tail counter and send // a HDLC_FLAG 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 { // We are in preamble. We'll decrement // the preamble counter and transmit a // HDLC_FLAG afsk->preambleLength--; afsk->currentOutputByte = HDLC_FLAG; } // This handles escape sequences and control // characters. First we check if the current // byte is an escape character. If it is, we // know the next byte, even though it looks // like an HDLC control character, in fact is // not. Therefore we'll fetch it and transmit // it as data using bit stuffing. if (afsk->currentOutputByte == AX25_ESC) { // First make sure that the TX buffer is // not empty for some strange reason if (fifo_isempty(&afsk->txFifo)) { AFSK_DAC_IRQ_STOP(); afsk->sending = false; LED_TX_OFF(); return 0; } else { // If it is not, fetch the next byte afsk->currentOutputByte = fifo_pop(&afsk->txFifo); } } else if (afsk->currentOutputByte == HDLC_FLAG || afsk->currentOutputByte == HDLC_RESET) { // If there was not an escape character and // this byte is an HDLC control character, // we know that it is an _actual_ control // character, and we indicate that it should // not be bitstuffed. afsk->bitStuff = false; } } // Since we are at the beginning of a byte, // we'll initialize the txBit mask to: // 00000001. It will then be bit-shifted one // position to the left each time we send the // next bit. By ANDing this mask to the byte // we are sending, we can quickly figure out // what tone we should transmit. For example: // // If we are sending bit number 4 of the // byte: 01101011 // The bit mask would be: 00001000 // If we AND the byte and the // mask, we get: 00001000 // Since this is not zero, we know we should // transmit a one. afsk->txBit = 0x01; } // First we need to check for bit-stuffing if (afsk->bitStuff && afsk->bitstuffCount >= BIT_STUFF_LEN) { // If we are allowed to bit-stuff, and we have // reached the maximum number of consecutive // ones, we'll reset the bit-stuff counter and // insert a zero into the bitstream afsk->bitstuffCount = 0; afsk->phaseInc = SWITCH_TONE(afsk->phaseInc); } else { // If we don't need to bit-stuff now, we can get // on with the actual transmission. // // We are using NRZ 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); } // Bitshift the mast to allow for the next // bit in the byte to be transmitted afsk->txBit <<= 1; } // We set sampleIndex to DAC_SAMPLESPERBIT, // so we will transmit this bit for the number // of samples one bit requires to transmit at // the chosen bitrate. afsk->sampleIndex = DAC_SAMPLESPERBIT; } // We increment the phase accumulator // by the amount needed for the tone afsk->phaseAcc += afsk->phaseInc; // We then make sure that we have not // exceeded the length of our sine table afsk->phaseAcc %= SIN_LEN; // Finally we decrement the sample counter afsk->sampleIndex--; // ... and return the sample to for it to // be written out return sinSample(afsk->phaseAcc); } ////////////////////////////////////////////////////// // File operation functions 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 :) // ////////////////////////////////////////////////////// // Read from the modem 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; } // Write to the modem 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; } // Waits for the write operation to finish static int afsk_flush(KFile *fd) { Afsk *afsk = AFSK_CAST(fd); while (afsk->sending) { cpu_relax(); } return 0; } // Check whether there was any errors // while reading or writing static int afsk_error(KFile *fd) { Afsk *afsk = AFSK_CAST(fd); int err; ATOMIC(err = afsk->status); return err; } // Allows resetting the error-state static void afsk_clearerr(KFile *fd) { Afsk *afsk = AFSK_CAST(fd); ATOMIC(afsk->status = 0); } ////////////////////////////////////////////////////// // Modem Initialization // ////////////////////////////////////////////////////// void afsk_init(Afsk *afsk, int _adcPin) { // Allocate memory for struct memset(afsk, 0, sizeof(*afsk)); // Configure ADC pin afsk->adcPin = _adcPin; // Initialise phase increment to that // of the mark frequency afsk->phaseInc = MARK_INC; // Initialize 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; idelayFifo, 0); } // Initialize hardware AFSK_ADC_INIT(_adcPin, afsk); AFSK_DAC_INIT(); LED_TX_INIT(); LED_RX_INIT(); // And register the modem file-pointer // functions for reading from and // writing to it. 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; }