Portable AX.25 Radio TNC

#include <Arduino.h>
#include <math.h>
#include <pico/time.h>
#include <hardware/sync.h>
// Pin table confirmed: GP14 PTT via rb, GP15 PWM via rf, GP26 RX via ri.
// Bell 202, 1200 baud. Core 1 owns all real-time audio and PTT.
constexpr int RX=26, AUDIO=15, PTT=14, FS=26400, SPB=22, MAXFRAME=330;
volatile bool audioReady=false, txReady=false, rxReady=false, busy=false;
uint8_t txBits[6000], rxFrame[MAXFRAME+2];
volatile unsigned txCount=0, rxLength=0;
uint8_t pending[MAXFRAME]; unsigned pendingLen=0;
uint8_t txDelay=30, persistence=63, slotTime=10, txTail=3;
bool fullDuplex=false;
float sinTab[FS/100], cm[22],sm[22],cs[22],ss[22];
uint16_t crcStep(uint16_t c,uint8_t b){c^=b;for(int i=0;i<8;i++)c=(c>>1)^((c&1)?0x8408:0);return c;}
void kissByte(uint8_t b){if(b==0xc0){Serial.write(0xdb);Serial.write(0xdc);}else if(b==0xdb){Serial.write(0xdb);Serial.write(0xdd);}else Serial.write(b);}
// Decode a whole HDLC frame only after a closing flag; strip stuffing then FCS.
uint8_t raw[6000]; unsigned rawN=0; bool inFrame=false; uint8_t flagShift=0;
void receiveBit(bool bit){
 flagShift=(flagShift>>1)|(bit?128:0);
 if(inFrame){if(rawN<sizeof(raw))raw[rawN++]=bit;else{inFrame=false;rawN=0;}}
 if(flagShift!=0x7e)return;
 if(inFrame && rawN>=8){
  unsigned n=rawN-8,len=0,bc=0,ones=0;uint8_t b=0;uint16_t crc=0xffff;bool ok=true;
  uint8_t frame[MAXFRAME+2];
  for(unsigned i=0;i<n;i++){
   bool x=raw[i];if(ones==5){if(x){ok=false;break;}ones=0;continue;}
   if(x)ones++;else ones=0;
   b|=uint8_t(x)<<bc;if(++bc==8){if(len>=sizeof(frame)){ok=false;break;}frame[len++]=b;crc=crcStep(crc,b);b=0;bc=0;}
  }
  if(ok && bc==0 && len>=17 && crc==0xf0b8 && !rxReady){memcpy(rxFrame,frame,len-2);rxLength=len-2;__dmb();rxReady=true;}
 }
 rawN=0;inFrame=true;
}
void setup1(){while(!audioReady)delay(1);}
void loop1(){
 static bool init=false;static uint64_t next=0;static unsigned frac=0;
 static float hist[22]={},dc=2048;static int hp=0,phase=0;static bool tone=false,lastTone=false,nrzi=false;
 static unsigned txIndex=0,txSample=0,osc=0;static bool transmitting=false,txTone=false;
 if(!init){for(int i=0;i<264;i++)sinTab[i]=sinf(2*PI*i/264);for(int i=0;i<22;i++){cm[i]=cosf(2*PI*1200*i/FS);sm[i]=sinf(2*PI*1200*i/FS);cs[i]=cosf(2*PI*2200*i/FS);ss[i]=sinf(2*PI*2200*i/FS);}next=time_us_64();init=true;}
 while((int64_t)(time_us_64()-next)<0)tight_loop_contents();
 // Rational period avoids sample-rate drift from rounded microseconds.
 next+=37;frac+=23200;if(frac>=26400){next++;frac-=26400;}
 if(time_us_64()>next+200){next=time_us_64();rawN=0;inFrame=false;}
 if(txReady && !transmitting){__dmb();transmitting=true;digitalWrite(PTT,HIGH);txIndex=0;txSample=0;txTone=false;}
 if(transmitting){
  if(txSample==0){if(txIndex>=txCount){analogWrite(AUDIO,128);digitalWrite(PTT,LOW);transmitting=false;__dmb();txReady=false;rawN=0;inFrame=false;return;}if(!txBits[txIndex++])txTone=!txTone;}
  osc=(osc+(txTone?22:12))%264;analogWrite(AUDIO,128+int(115*sinTab[osc]));txSample=(txSample+1)%22;return;
 }
 float x=analogRead(RX);dc+=(x-dc)*0.001f;hist[hp]=x-dc;hp=(hp+1)%22;
 float a=0,b=0,c=0,d=0,power=0;
 for(int i=0;i<22;i++){float v=hist[(hp+i)%22];a+=v*cm[i];b+=v*sm[i];c+=v*cs[i];d+=v*ss[i];power+=v*v;}
 float mark=a*a+b*b,space=c*c+d*d;
 static unsigned quiet=0;
 if(power>22*16 && (mark+space)>power*3){quiet=0;busy=true;}else if(++quiet>1320)busy=false;
 if(mark>space*1.12f)tone=false;else if(space>mark*1.12f)tone=true;
 if(tone!=lastTone){phase=0;lastTone=tone;}else phase=(phase+1)%22;
 if(phase==11){receiveBit(tone==nrzi);nrzi=tone;}
}
void prepareTx(){
 unsigned n=0;auto byte=[&](uint8_t b){for(int j=0;j<8;j++)txBits[n++]=(b>>j)&1;};
 // 12 bits per TXDELAY unit of 10 milliseconds. Round up to flags.
 unsigned flags=(unsigned(txDelay)*12+7)/8; if(flags<2)flags=2;
 for(unsigned i=0;i<flags;i++)byte(0x7e);
 uint16_t crc=0xffff;for(unsigned i=0;i<pendingLen;i++)crc=crcStep(crc,pending[i]);crc=~crc;
 unsigned ones=0;for(unsigned i=0;i<pendingLen+2;i++){
 uint8_t b=i<pendingLen?pending[i]:uint8_t(crc>>(8*(i-pendingLen)));
 for(int j=0;j<8;j++){bool v=(b>>j)&1;txBits[n++]=v;if(v){if(++ones==5){txBits[n++]=0;ones=0;}}else ones=0;}}
 byte(0x7e);for(unsigned i=0;i<(unsigned(txTail)*12+7)/8;i++)byte(0x7e);
 txCount=n;__dmb();txReady=true;pendingLen=0;
}
void hostByte(uint8_t b){
 static uint8_t buf[MAXFRAME+1];static unsigned len=0;static bool esc=false,drop=false;
 if(b==0xc0){
 if(!drop && len>0 && (buf[0]>>4)==0){unsigned cmd=buf[0]&15;
 if(cmd==0 && len>1 && !pendingLen && !txReady){memcpy(pending,buf+1,len-1);pendingLen=len-1;}
 else if(len==2){switch(cmd){case 1:txDelay=min(buf[1],uint8_t(60));break;case 2:persistence=buf[1];break;case 3:slotTime=max(buf[1],uint8_t(1));break;case 4:txTail=min(buf[1],uint8_t(10));break;case 5:fullDuplex=buf[1]!=0;break;}}}
 len=0;esc=false;drop=false;return;}
 if(drop)return;
 if(esc){esc=false;if(b==0xdc)b=0xc0;else if(b==0xdd)b=0xdb;else{drop=true;return;}}else if(b==0xdb){esc=true;return;}
 if(len<sizeof(buf))buf[len++]=b;else drop=true;
}
void setup(){
 digitalWrite(PTT,LOW);pinMode(PTT,OUTPUT);analogReadResolution(12);
 analogWriteFreq(100000);analogWriteRange(255);analogWrite(AUDIO,128);
 Serial.begin(115200);__dmb();audioReady=true;
}
void loop(){
 for(int i=0;i<64 && Serial.available();i++)hostByte(Serial.read());
 if(rxReady){__dmb();if(Serial){Serial.write(0xc0);Serial.write(uint8_t(0));for(unsigned i=0;i<rxLength;i++)kissByte(rxFrame[i]);Serial.write(0xc0);}__dmb();rxReady=false;}
 static uint32_t last=0;uint32_t now=millis();
 if(pendingLen && !txReady && now-last>=unsigned(slotTime)*10){last=now;if(fullDuplex || (!busy && random(256)<=persistence))prepareTx();}
}