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AX25RX.cpp
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AX25RX.cpp
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/*
* Copyright (C) 2020 by Jonathan Naylor G4KLX
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
*/
#include "Config.h"
#if defined(MODE_AX25)
#include "Globals.h"
#include "AX25RX.h"
/*
* Generated with Scipy Filter, 152 coefficients, 1100-2300Hz bandpass,
* Hann window, starting and ending 0 value coefficients removed.
*
* np.array(
* firwin2(152,
* [
* 0.0,
* 1000.0/(sample_rate/2),
* 1100.0/(sample_rate/2),
* 2350.0/(sample_rate/2),
* 2500.0/(sample_rate/2),
* 1.0
* ],
* [0,0,1,1,0,0],
* antisymmetric = False,
* window='hann') * 32768,
* dtype=int)[10:-10]
*/
const uint32_t FILTER_LEN = 130U;
q15_t FILTER_COEFFS[] = {
5, 12, 18, 21, 19, 11, -2, -15, -25, -27,
-21, -11, -3, -5, -19, -43, -69, -83, -73, -35,
27, 98, 155, 180, 163, 109, 39, -20, -45, -26,
23, 74, 89, 39, -81, -247, -407, -501, -480, -334,
-92, 175, 388, 479, 429, 275, 99, 5, 68, 298,
626, 913, 994, 740, 115, -791, -1770, -2544, -2847, -2509,
-1527, -76, 1518, 2875, 3653, 3653, 2875, 1518, -76, -1527,
-2509, -2847, -2544, -1770, -791, 115, 740, 994, 913, 626,
298, 68, 5, 99, 275, 429, 479, 388, 175, -92,
-334, -480, -501, -407, -247, -81, 39, 89, 74, 23,
-26, -45, -20, 39, 109, 163, 180, 155, 98, 27,
-35, -73, -83, -69, -43, -19, -5, -3, -11, -21,
-27, -25, -15, -2, 11, 19, 21, 18, 12, 5
};
CAX25RX::CAX25RX() :
m_filter(),
m_state(),
m_demod1(3),
m_demod2(6),
m_demod3(9),
m_lastFCS(0U),
m_count(0U),
m_slotTime(30U),
m_slotCount(0U),
m_pPersist(128U),
m_dcd(false),
m_canTX(false),
m_x(1U),
m_a(0xB7U),
m_b(0x73U),
m_c(0xF6U)
{
m_filter.numTaps = FILTER_LEN;
m_filter.pState = m_state;
m_filter.pCoeffs = FILTER_COEFFS;
initRand();
}
void CAX25RX::samples(q15_t* samples, uint8_t length)
{
q15_t output[RX_BLOCK_SIZE];
::arm_fir_fast_q15(&m_filter, samples, output, RX_BLOCK_SIZE);
m_count++;
CAX25Frame frame;
bool ret = m_demod1.process(output, length, frame);
if (ret) {
if (frame.m_fcs != m_lastFCS || m_count > 2U) {
m_lastFCS = frame.m_fcs;
m_count = 0U;
serial.writeAX25Data(frame.m_data, frame.m_length - 2U);
}
DEBUG1("Decoder 1 reported");
}
ret = m_demod2.process(output, length, frame);
if (ret) {
if (frame.m_fcs != m_lastFCS || m_count > 2U) {
m_lastFCS = frame.m_fcs;
m_count = 0U;
serial.writeAX25Data(frame.m_data, frame.m_length - 2U);
}
DEBUG1("Decoder 2 reported");
}
ret = m_demod3.process(output, length, frame);
if (ret) {
if (frame.m_fcs != m_lastFCS || m_count > 2U) {
m_lastFCS = frame.m_fcs;
m_count = 0U;
serial.writeAX25Data(frame.m_data, frame.m_length - 2U);
}
DEBUG1("Decoder 3 reported");
}
m_slotCount += RX_BLOCK_SIZE;
if (m_slotCount >= m_slotTime) {
m_slotCount = 0U;
bool dcd1 = m_demod1.isDCD();
bool dcd2 = m_demod2.isDCD();
bool dcd3 = m_demod3.isDCD();
if (dcd1 || dcd2 || dcd3) {
if (!m_dcd) {
io.setDecode(true);
io.setADCDetection(true);
m_dcd = true;
}
m_canTX = false;
} else {
if (m_dcd) {
io.setDecode(false);
io.setADCDetection(false);
m_dcd = false;
}
m_canTX = m_pPersist >= rand();
}
}
}
bool CAX25RX::canTX() const
{
return m_canTX;
}
void CAX25RX::setParams(int8_t twist, uint8_t slotTime, uint8_t pPersist)
{
m_demod1.setTwist(twist - 3);
m_demod2.setTwist(twist);
m_demod3.setTwist(twist + 3);
m_slotTime = slotTime * 240U; // Slot time in samples
m_pPersist = pPersist;
}
// Taken from https://www.electro-tech-online.com/threads/ultra-fast-pseudorandom-number-generator-for-8-bit.124249/
//X ABC Algorithm Random Number Generator for 8-Bit Devices:
//This is a small PRNG, experimentally verified to have at least a 50 million byte period
//by generating 50 million bytes and observing that there were no overapping sequences and repeats.
//This generator passes serial correlation, entropy , Monte Carlo Pi value, arithmetic mean,
//And many other statistical tests. This generator may have a period of up to 2^32, but this has
//not been verified.
//
// By XORing 3 bytes into the a,b, and c registers, you can add in entropy from
//an external source easily.
//
//This generator is free to use, but is not suitable for cryptography due to its short period(by //cryptographic standards) and simple construction. No attempt was made to make this generator
// suitable for cryptographic use.
//
//Due to the use of a constant counter, the generator should be resistant to latching up.
//A significant performance gain is had in that the x variable is only ever incremented.
//
//Only 4 bytes of ram are needed for the internal state, and generating a byte requires 3 XORs , //2 ADDs, one bit shift right , and one increment. Difficult or slow operations like multiply, etc
//were avoided for maximum speed on ultra low power devices.
void CAX25RX::initRand() //Can also be used to seed the rng with more entropy during use.
{
m_a = (m_a ^ m_c ^ m_x);
m_b = (m_b + m_a);
m_c = (m_c + (m_b >> 1) ^ m_a);
}
uint8_t CAX25RX::rand()
{
m_x++; //x is incremented every round and is not affected by any other variable
m_a = (m_a ^ m_c ^ m_x); //note the mix of addition and XOR
m_b = (m_b + m_a); //And the use of very few instructions
m_c = (m_c + (m_b >> 1) ^ m_a); //the right shift is to ensure that high-order bits from b can affect
return uint8_t(m_c); //low order bits of other variables
}
#endif