chg: updated the README.txt

[proxmark3-svn] / common / lfdemod.c

//-----------------------------------------------------------------------------
// Copyright (C) 2014 
//
// This code is licensed to you under the terms of the GNU GPL, version 2 or,
// at your option, any later version. See the LICENSE.txt file for the text of
// the license.
//-----------------------------------------------------------------------------
// Low frequency commands
//-----------------------------------------------------------------------------

#include <stdlib.h>
#include <string.h>
#include "lfdemod.h"

//by marshmellow
//takes 1s and 0s and searches for EM410x format - output EM ID
uint64_t Em410xDecode(uint8_t *BitStream,uint32_t BitLen)
{
  //no arguments needed - built this way in case we want this to be a direct call from "data " cmds in the future
  //  otherwise could be a void with no arguments
  //set defaults
  int high=0, low=128;
  uint64_t lo=0; //hi=0,

  uint32_t i = 0;
  uint32_t initLoopMax = 65;
  if (initLoopMax>BitLen) initLoopMax=BitLen;

  for (;i < initLoopMax; ++i) //65 samples should be plenty to find high and low values
  {
    if (BitStream[i] > high)
      high = BitStream[i];
    else if (BitStream[i] < low)
      low = BitStream[i];
  }
  if (((high !=1)||(low !=0))){  //allow only 1s and 0s 
   // PrintAndLog("no data found"); 
    return 0;
  }
  uint8_t parityTest=0;
   // 111111111 bit pattern represent start of frame
  uint8_t frame_marker_mask[] = {1,1,1,1,1,1,1,1,1};
  uint32_t idx = 0;
  uint32_t ii=0;
  uint8_t resetCnt = 0;
  while( (idx + 64) < BitLen) {
 restart:
    // search for a start of frame marker
    if ( memcmp(BitStream+idx, frame_marker_mask, sizeof(frame_marker_mask)) == 0)
    { // frame marker found
      idx+=9;//sizeof(frame_marker_mask);
      for (i=0; i<10;i++){
        for(ii=0; ii<5; ++ii){
          parityTest += BitStream[(i*5)+ii+idx];        
        }
        if (parityTest== ((parityTest>>1)<<1)){
          parityTest=0;
          for (ii=0; ii<4;++ii){
            //hi = (hi<<1)|(lo>>31);
            lo=(lo<<1LL)|(BitStream[(i*5)+ii+idx]);
          }
          //PrintAndLog("DEBUG: EM parity passed parity val: %d, i:%d, ii:%d,idx:%d, Buffer: %d%d%d%d%d,lo: %d",parityTest,i,ii,idx,BitStream[idx+ii+(i*5)-5],BitStream[idx+ii+(i*5)-4],BitStream[idx+ii+(i*5)-3],BitStream[idx+ii+(i*5)-2],BitStream[idx+ii+(i*5)-1],lo);          
        }else {//parity failed
          //PrintAndLog("DEBUG: EM parity failed parity val: %d, i:%d, ii:%d,idx:%d, Buffer: %d%d%d%d%d",parityTest,i,ii,idx,BitStream[idx+ii+(i*5)-5],BitStream[idx+ii+(i*5)-4],BitStream[idx+ii+(i*5)-3],BitStream[idx+ii+(i*5)-2],BitStream[idx+ii+(i*5)-1]);
          parityTest=0;
          idx-=8;
          if (resetCnt>5)return 0;
          resetCnt++;
          goto restart;//continue;
        }
      }
      //skip last 5 bit parity test for simplicity.
      return lo;
    }else{
      idx++;
    }
  }
  return 0;
}

//by marshmellow
//takes 2 arguments - clock and invert both as integers
//attempts to demodulate ask while decoding manchester 
//prints binary found and saves in graphbuffer for further commands
int askmandemod(uint8_t * BinStream,uint32_t *BitLen,int *clk, int *invert)
{
  int i;
  int high = 0, low = 128;
  *clk=DetectASKClock(BinStream,(size_t)*BitLen,*clk); //clock default

  if (*clk<8) *clk =64;
  if (*clk<32) *clk=32;
  if (*invert != 1) *invert=0;
  
  uint32_t initLoopMax = 200;
  if (initLoopMax>*BitLen) initLoopMax=*BitLen;
  
  // Detect high and lows 
  for (i = 0; i < initLoopMax; ++i) //200 samples should be enough to find high and low values
  {
    if (BinStream[i] > high)
      high = BinStream[i];
    else if (BinStream[i] < low)
      low = BinStream[i];
  }
  if ((high < 158) ){  //throw away static 
    return -2;
  }
  //25% fuzz in case highs and lows aren't clipped [marshmellow]
  high=(int)((high-128)*.75)+128;
  low= (int)((low-128)*.75)+128;
 
  //PrintAndLog("DEBUG - valid high: %d - valid low: %d",high,low);
  int lastBit = 0;  //set first clock check
  uint32_t bitnum = 0;     //output counter
  int tol = 0;  //clock tolerance adjust - waves will be accepted as within the clock if they fall + or - this value + clock from last valid wave
  if (*clk==32)tol=1;    //clock tolerance may not be needed anymore currently set to + or - 1 but could be increased for poor waves or removed entirely 
  int iii = 0;
  uint32_t gLen = *BitLen;
  if (gLen > 3000) gLen=3000;
  uint8_t errCnt =0;
  uint32_t bestStart = *BitLen;
  uint32_t bestErrCnt = (*BitLen/1000);
  uint32_t maxErr = (*BitLen/1000);

  //loop to find first wave that works
  for (iii=0; iii < gLen; ++iii){
    if ((BinStream[iii]>=high)||(BinStream[iii]<=low)){
      lastBit=iii-*clk;    
      errCnt=0;
      //loop through to see if this start location works
      for (i = iii; i < *BitLen; ++i) {   
        if ((BinStream[i] >= high) && ((i-lastBit)>(*clk-tol))){
          lastBit+=*clk;
        } else if ((BinStream[i] <= low) && ((i-lastBit)>(*clk-tol))){
          //low found and we are expecting a bar
          lastBit+=*clk;
        } else {
          //mid value found or no bar supposed to be here
          if ((i-lastBit)>(*clk+tol)){
            //should have hit a high or low based on clock!!
           
            errCnt++;
            lastBit+=*clk;//skip over until hit too many errors
            if (errCnt>(maxErr)) break;  //allow 1 error for every 1000 samples else start over
          }
        }
        if ((i-iii) >(400 * *clk)) break; //got plenty of bits
      }
      //we got more than 64 good bits and not all errors
      if ((((i-iii)/ *clk) > (64+errCnt)) && (errCnt<maxErr)) {
        //possible good read
        if (errCnt==0){
                        bestStart=iii;
                        bestErrCnt=errCnt;
                        break;  //great read - finish
        } 
        if (errCnt<bestErrCnt){  //set this as new best run
          bestErrCnt=errCnt;
          bestStart = iii;
        }
      }
    }
  }
  if (bestErrCnt<maxErr){
        //best run is good enough set to best run and set overwrite BinStream
        iii=bestStart;
        lastBit=bestStart-*clk;
        bitnum=0;
    for (i = iii; i < *BitLen; ++i) {   
      if ((BinStream[i] >= high) && ((i-lastBit)>(*clk-tol))){
        lastBit+=*clk;
        BinStream[bitnum] =  *invert;
        bitnum++;
      } else if ((BinStream[i] <= low) && ((i-lastBit)>(*clk-tol))){
        //low found and we are expecting a bar
        lastBit+=*clk;
        BinStream[bitnum] = 1-*invert; 
        bitnum++;
      } else {
        //mid value found or no bar supposed to be here
        if ((i-lastBit)>(*clk+tol)){
          //should have hit a high or low based on clock!!

          if (bitnum > 0){
            BinStream[bitnum]=77;
            bitnum++;
          }
          
          lastBit+=*clk;//skip over error
        }
      }
      if (bitnum >=400) break;
    }
    *BitLen=bitnum;
        } else{
        *invert=bestStart;
        *clk=iii;
        return -1; 
  }     
  return bestErrCnt;
}

//by marshmellow
//take 10 and 01 and manchester decode
//run through 2 times and take least errCnt
int manrawdecode(uint8_t * BitStream, int *bitLen)
{
  int bitnum=0;
  int errCnt =0;
  int i=1;
  int bestErr = 1000;
  int bestRun = 0;
  int ii=1;
  for (ii=1;ii<3;++ii){
        i=1;
                for (i=i+ii;i<*bitLen-2;i+=2){
                  if(BitStream[i]==1 && (BitStream[i+1]==0)){
                  } else if((BitStream[i]==0)&& BitStream[i+1]==1){
            } else {
                      errCnt++;
            }
            if(bitnum>300) break;
                }
                if (bestErr>errCnt){
                  bestErr=errCnt;
                  bestRun=ii;
                }       
                errCnt=0;
  }
  errCnt=bestErr;
  if (errCnt<20){
        ii=bestRun;
        i=1;
                for (i=i+ii;i<*bitLen-2;i+=2){
                  if(BitStream[i]==1 && (BitStream[i+1]==0)){
                    BitStream[bitnum++]=0;
                  } else if((BitStream[i]==0)&& BitStream[i+1]==1){
                    BitStream[bitnum++]=1;
            } else {
              BitStream[bitnum++]=77;
                      //errCnt++;
            }
            if(bitnum>300) break;
                }
        *bitLen=bitnum;
        }   
  return errCnt;
}


//by marshmellow
//take 01 or 10 = 0 and 11 or 00 = 1
int BiphaseRawDecode(uint8_t * BitStream, int *bitLen, int offset)
{
  uint8_t bitnum = 0;
  uint32_t errCnt = 0;
  uint32_t i = 1;
  i=offset;
        for (;i<*bitLen-2;i+=2){
          if((BitStream[i]==1 && BitStream[i+1]==0)||(BitStream[i]==0 && BitStream[i+1]==1)){
            BitStream[bitnum++]=1;
          } else if((BitStream[i]==0 && BitStream[i+1]==0)||(BitStream[i]==1 && BitStream[i+1]==1)){
            BitStream[bitnum++]=0;
    } else {
            BitStream[bitnum++]=77;
      errCnt++;
    }
    if(bitnum>250) break;
        }  
  *bitLen=bitnum;
  return errCnt;
}

//by marshmellow
//takes 2 arguments - clock and invert both as integers
//attempts to demodulate ask only
//prints binary found and saves in graphbuffer for further commands
int askrawdemod(uint8_t *BinStream, int *bitLen,int *clk, int *invert)
{
  uint32_t i;
 // int invert=0;  //invert default
  int high = 0, low = 128;
  *clk=DetectASKClock(BinStream,*bitLen,*clk); //clock default
  uint8_t BitStream[502] = {0};

  if (*clk<8) *clk =64;
  if (*clk<32) *clk=32;
  if (*invert != 1) *invert = 0;
  
  uint32_t initLoopMax = 200;
  if (initLoopMax>*bitLen) initLoopMax=*bitLen;
  // Detect high and lows 
  for (i = 0; i < initLoopMax; ++i) //200 samples should be plenty to find high and low values
  {
    if (BinStream[i] > high)
      high = BinStream[i];
    else if (BinStream[i] < low)
      low = BinStream[i];
  }
  if ((high < 158)){  //throw away static
    return -2;
  }
  //25% fuzz in case highs and lows aren't clipped [marshmellow]
  high=(int)((high-128)*.75)+128;
  low= (int)((low-128)*.75)+128;

  int lastBit = 0;  //set first clock check
  uint32_t bitnum = 0;     //output counter
  uint8_t tol = 0;  //clock tolerance adjust - waves will be accepted as within the clock if they fall + or - this value + clock from last valid wave
  if (*clk==32) tol=1;    //clock tolerance may not be needed anymore currently set to + or - 1 but could be increased for poor waves or removed entirely 
  uint32_t iii = 0;
  uint32_t gLen = *bitLen;
  if (gLen > 500) gLen=500;
  uint8_t errCnt =0;
  uint32_t bestStart = *bitLen;
  uint32_t bestErrCnt = (*bitLen/1000);
  uint8_t midBit=0;

  //loop to find first wave that works
  for (iii=0; iii < gLen; ++iii){
    if ((BinStream[iii]>=high)||(BinStream[iii]<=low)){
      lastBit=iii-*clk;    
      //loop through to see if this start location works
      for (i = iii; i < *bitLen; ++i) {  
        if ((BinStream[i] >= high) && ((i-lastBit)>(*clk-tol))){
          lastBit+=*clk;
          BitStream[bitnum] =  *invert;
          bitnum++;
          midBit=0;
        } else if ((BinStream[i] <= low) && ((i-lastBit)>(*clk-tol))){
          //low found and we are expecting a bar
          lastBit+=*clk;
          BitStream[bitnum] = 1-*invert; 
          bitnum++;
          midBit=0;
        } else if ((BinStream[i]<=low) && (midBit==0) && ((i-lastBit)>((*clk/2)-tol))){
          //mid bar?
          midBit=1;
          BitStream[bitnum]= 1-*invert;
          bitnum++;
        } else if ((BinStream[i]>=high)&&(midBit==0) && ((i-lastBit)>((*clk/2)-tol))){
          //mid bar?
          midBit=1;
          BitStream[bitnum]= *invert;
          bitnum++;
        } else if ((i-lastBit)>((*clk/2)+tol)&&(midBit==0)){
          //no mid bar found
          midBit=1;
          BitStream[bitnum]= BitStream[bitnum-1];
          bitnum++;
        } else {
          //mid value found or no bar supposed to be here

          if ((i-lastBit)>(*clk+tol)){
            //should have hit a high or low based on clock!!

            if (bitnum > 0){
              BitStream[bitnum]=77;
              bitnum++;
            }

            errCnt++;
            lastBit+=*clk;//skip over until hit too many errors
            if (errCnt>((*bitLen/1000))){  //allow 1 error for every 1000 samples else start over
              errCnt=0;
              bitnum=0;//start over
              break;
            }
          }          
        }
        if (bitnum>500) break;
      }
      //we got more than 64 good bits and not all errors
      if ((bitnum > (64+errCnt)) && (errCnt<(*bitLen/1000))) {
        //possible good read
        if (errCnt==0) break;  //great read - finish
        if (bestStart == iii) break;  //if current run == bestErrCnt run (after exhausted testing) then finish 
        if (errCnt<bestErrCnt){  //set this as new best run
          bestErrCnt=errCnt;
          bestStart = iii;
        }
      }
    }
    if (iii>=gLen){ //exhausted test
      //if there was a ok test go back to that one and re-run the best run (then dump after that run)
      if (bestErrCnt < (*bitLen/1000)) iii=bestStart;
    }
  }
        if (bitnum>16){

                for (i=0; i < bitnum; ++i){
                        BinStream[i]=BitStream[i];
                }
                *bitLen = bitnum;
        } else {
                return -1;
        }
  return errCnt;
}
//translate wave to 11111100000 (1 for each short wave 0 for each long wave) 
size_t fsk_wave_demod(uint8_t * dest, size_t size, uint8_t fchigh, uint8_t fclow)
{
        uint32_t last_transition = 0;
        uint32_t idx = 1;
        uint32_t maxVal=0;
        if (fchigh==0) fchigh=10;
        if (fclow==0) fclow=8;
        // we do care about the actual theshold value as sometimes near the center of the
        // wave we may get static that changes direction of wave for one value
        // if our value is too low it might affect the read.  and if our tag or
        // antenna is weak a setting too high might not see anything. [marshmellow]
        if (size<100) return 0;
        for(idx=1; idx<100; idx++){
    if(maxVal<dest[idx]) maxVal = dest[idx];
  }
    // set close to the top of the wave threshold with 25% margin for error
    // less likely to get a false transition up there. 
    // (but have to be careful not to go too high and miss some short waves)
        uint8_t threshold_value = (uint8_t)(((maxVal-128)*.75)+128); 
        
        // sync to first lo-hi transition, and threshold
        // Need to threshold first sample
        
        if(dest[0] < threshold_value) dest[0] = 0;
        else dest[0] = 1;

        size_t numBits = 0;
        // count cycles between consecutive lo-hi transitions, there should be either 8 (fc/8)
        // or 10 (fc/10) cycles but in practice due to noise etc we may end up with with anywhere
        // between 7 to 11 cycles so fuzz it by treat anything <9 as 8 and anything else as 10
        for(idx = 1; idx < size; idx++) {
                // threshold current value

                if (dest[idx] < threshold_value) dest[idx] = 0;
                else dest[idx] = 1;

                // Check for 0->1 transition
                if (dest[idx-1] < dest[idx]) { // 0 -> 1 transition
                        if ((idx-last_transition)<(fclow-2)){            //0-5 = garbage noise
                                //do nothing with extra garbage
                        } else if ((idx-last_transition) < (fchigh-1)) { //6-8 = 8 waves
                                dest[numBits]=1;
                        } else {                                                        //9+ = 10 waves
                                dest[numBits]=0;
                        }
                        last_transition = idx;
                        numBits++;
                }
        }
        return numBits; //Actually, it returns the number of bytes, but each byte represents a bit: 1 or 0
}

uint32_t myround2(float f)
{
  if (f >= 2000) return 2000;//something bad happened
  return (uint32_t) (f + (float)0.5);
}

//translate 11111100000 to 10 
size_t aggregate_bits(uint8_t *dest,size_t size,  uint8_t rfLen, uint8_t maxConsequtiveBits, uint8_t invert,uint8_t fchigh,uint8_t fclow )// uint8_t h2l_crossing_value,uint8_t l2h_crossing_value, 
{
        uint8_t lastval=dest[0];
        uint32_t idx=0;
        size_t numBits=0;
        uint32_t n=1;

        for( idx=1; idx < size; idx++) {

                if (dest[idx]==lastval) {
                        n++;
                        continue;
                }
                //if lastval was 1, we have a 1->0 crossing
                if ( dest[idx-1]==1 ) {
                        n=myround2((float)(n+1)/((float)(rfLen)/(float)fclow));
                        //n=(n+1) / h2l_crossing_value;
                } else {// 0->1 crossing
                        n=myround2((float)(n+1)/((float)(rfLen-2)/(float)fchigh));  //-2 for fudge factor
                        //n=(n+1) / l2h_crossing_value;
                }
                if (n == 0) n = 1;

                if(n < maxConsequtiveBits) //Consecutive 
                {
                        if(invert==0){ //invert bits 
                                memset(dest+numBits, dest[idx-1] , n);
                        }else{
                                memset(dest+numBits, dest[idx-1]^1 , n);        
                        }                       
                        numBits += n;
                }
                n=0;
                lastval=dest[idx];
        }//end for
        return numBits;
}
//by marshmellow  (from holiman's base)
// full fsk demod from GraphBuffer wave to decoded 1s and 0s (no mandemod)
int fskdemod(uint8_t *dest, size_t size, uint8_t rfLen, uint8_t invert, uint8_t fchigh, uint8_t fclow)
{
  // FSK demodulator
  size = fsk_wave_demod(dest, size, fchigh, fclow);
  size = aggregate_bits(dest, size,rfLen,192,invert,fchigh,fclow);
  return size;
}
// loop to get raw HID waveform then FSK demodulate the TAG ID from it
int HIDdemodFSK(uint8_t *dest, size_t size, uint32_t *hi2, uint32_t *hi, uint32_t *lo)
{
        
        size_t idx=0; //, found=0; //size=0,
        // FSK demodulator
        size = fskdemod(dest, size,50,0,10,8);

        // final loop, go over previously decoded manchester data and decode into usable tag ID
        // 111000 bit pattern represent start of frame, 01 pattern represents a 1 and 10 represents a 0
        uint8_t frame_marker_mask[] = {1,1,1,0,0,0};
        int numshifts = 0;
        idx = 0;
        //one scan
        while( idx + sizeof(frame_marker_mask) < size) {
        // search for a start of frame marker
                if ( memcmp(dest+idx, frame_marker_mask, sizeof(frame_marker_mask)) == 0)
                { // frame marker found
                        idx+=sizeof(frame_marker_mask);
                        while(dest[idx] != dest[idx+1] && idx < size-2)
                        {       
                                // Keep going until next frame marker (or error)
                                // Shift in a bit. Start by shifting high registers
                                *hi2 = (*hi2<<1)|(*hi>>31);
                                *hi = (*hi<<1)|(*lo>>31);
                                //Then, shift in a 0 or one into low
                                if (dest[idx] && !dest[idx+1])  // 1 0
                                        *lo=(*lo<<1)|0;
                                else // 0 1
                                        *lo=(*lo<<1)|1;
                                numshifts++;
                                idx += 2;
                        }
                        // Hopefully, we read a tag and  hit upon the next frame marker
                        if(idx + sizeof(frame_marker_mask) < size)
                        {
                                if ( memcmp(dest+idx, frame_marker_mask, sizeof(frame_marker_mask)) == 0)
                                {
                                        //good return 
                                        return idx;
                                }
                        }
                        // reset
                        *hi2 = *hi = *lo = 0;
                        numshifts = 0;
                }else   {
                        idx++;
                }
        }
        return -1;
}

uint32_t bytebits_to_byte(uint8_t* src, int numbits)
{
        uint32_t num = 0;
        for(int i = 0 ; i < numbits ; i++)      {
                num = (num << 1) | (*src);
                src++;
        }
        return num;
}

int IOdemodFSK(uint8_t *dest, size_t size)
{
  uint32_t idx=0;
        //make sure buffer has data
        if (size < 66) return -1;
        //test samples are not just noise
        uint8_t testMax=0;
        for(idx=0;idx<65;idx++){
                if (testMax<dest[idx]) testMax=dest[idx];
        }
        idx=0;
        //if not just noise
        if (testMax>170){
                // FSK demodulator
                size = fskdemod(dest, size,64,1,10,8);  //  RF/64 and invert
                if (size < 65) return -1;  //did we get a good demod?
                //Index map
                //0           10          20          30          40          50          60
                //|           |           |           |           |           |           |
                //01234567 8 90123456 7 89012345 6 78901234 5 67890123 4 56789012 3 45678901 23
                //-----------------------------------------------------------------------------
                //00000000 0 11110000 1 facility 1 version* 1 code*one 1 code*two 1 ???????? 11
                //
                //XSF(version)facility:codeone+codetwo
                //Handle the data
          uint8_t mask[] = {0,0,0,0,0,0,0,0,0,1};
                for( idx=0; idx < (size - 65); idx++) {
        if ( memcmp(dest + idx, mask, sizeof(mask))==0) {
                //frame marker found
                if (!dest[idx+8] && dest[idx+17]==1 && dest[idx+26]==1 && dest[idx+35]==1 && dest[idx+44]==1 && dest[idx+53]==1){
                        //confirmed proper separator bits found
                        //return start position
                                        return (int) idx;
                                }
                        }               
                }
        }       
        return 0;
}

// by marshmellow
// not perfect especially with lower clocks or VERY good antennas (heavy wave clipping)
// maybe somehow adjust peak trimming value based on samples to fix?
int DetectASKClock(uint8_t dest[], size_t size, int clock)
{
  int i=0;
  int peak=0;
  int low=128;
  int clk[]={16,32,40,50,64,100,128,256};
  int loopCnt = 256;  //don't need to loop through entire array...
  if (size<loopCnt) loopCnt = size;

  //if we already have a valid clock quit
  for (;i<8;++i)
        if (clk[i]==clock) return clock;

  //get high and low peak
  for (i=0;i<loopCnt;++i){
    if(dest[i]>peak){
      peak = dest[i]; 
    }
    if(dest[i]<low){
      low = dest[i];
    }
  }
  peak=(int)((peak-128)*.75)+128;
  low= (int)((low-128)*.75)+128;
  int ii;
  int clkCnt;
  int tol = 0;
  int bestErr=1000;
  int errCnt[]={0,0,0,0,0,0,0,0};
  //test each valid clock from smallest to greatest to see which lines up
  for(clkCnt=0; clkCnt<6;++clkCnt){
    if (clk[clkCnt]==32){
      tol=1;
    }else{
      tol=0;
    }
    bestErr=1000;
    //try lining up the peaks by moving starting point (try first 256) 
    for (ii=0; ii<loopCnt; ++ii){
      if ((dest[ii]>=peak) || (dest[ii]<=low)){
        errCnt[clkCnt]=0;
        // now that we have the first one lined up test rest of wave array
        for (i=0; i<((int)(size/clk[clkCnt])-1); ++i){
          if (dest[ii+(i*clk[clkCnt])]>=peak || dest[ii+(i*clk[clkCnt])]<=low){
          }else if(dest[ii+(i*clk[clkCnt])-tol]>=peak || dest[ii+(i*clk[clkCnt])-tol]<=low){
          }else if(dest[ii+(i*clk[clkCnt])+tol]>=peak || dest[ii+(i*clk[clkCnt])+tol]<=low){
          }else{  //error no peak detected
            errCnt[clkCnt]++;
          }    
        }
        //if we found no errors this is correct one - return this clock
        if(errCnt[clkCnt]==0) return clk[clkCnt];
        //if we found errors see if it is lowest so far and save it as best run
        if(errCnt[clkCnt]<bestErr) bestErr=errCnt[clkCnt];
      }
    } 
  }
  int iii=0;
  int best=0;
  for (iii=0; iii<6;++iii){
    if (errCnt[iii]<errCnt[best]){
      best = iii;
    }
  }
  return clk[best];
}