Merge remote-tracking branch 'upstream/master'

diff --git a/armsrc/appmain.c b/armsrc/appmain.c
index cb1b9f73fee185ddf706e2e6720a348bb517799d..189f9d7a307ef54551a8887afff6ba49ce0f1401 100644 (file)
--- a/armsrc/appmain.c
+++ b/armsrc/appmain.c
@@ -135,12 +135,25 @@ static int ReadAdc(int ch)
 
        AT91C_BASE_ADC->ADC_CR = AT91C_ADC_SWRST;
        AT91C_BASE_ADC->ADC_MR =
-               ADC_MODE_PRESCALE(32) |
-               ADC_MODE_STARTUP_TIME(16) |
-               ADC_MODE_SAMPLE_HOLD_TIME(8);
+               ADC_MODE_PRESCALE(63  /* was 32 */) |                                                   // ADC_CLK = MCK / ((63+1) * 2) = 48MHz / 128 = 375kHz
+               ADC_MODE_STARTUP_TIME(1  /* was 16 */) |                                                // Startup Time = (1+1) * 8 / ADC_CLK = 16 / 375kHz = 42,7us     Note: must be > 20us
+               ADC_MODE_SAMPLE_HOLD_TIME(15  /* was 8 */);                                     // Sample & Hold Time SHTIM = 15 / ADC_CLK = 15 / 375kHz = 40us
+
+       // Note: ADC_MODE_PRESCALE and ADC_MODE_SAMPLE_HOLD_TIME are set to the maximum allowed value. 
+       // Both AMPL_LO and AMPL_HI are very high impedance (10MOhm) outputs, the input capacitance of the ADC is 12pF (typical). This results in a time constant
+       // of RC = 10MOhm * 12pF = 120us. Even after the maximum configurable sample&hold time of 40us the input capacitor will not be fully charged. 
+       // 
+       // The maths are:
+       // If there is a voltage v_in at the input, the voltage v_cap at the capacitor (this is what we are measuring) will be
+       //
+       //       v_cap = v_in * (1 - exp(-RC/SHTIM))  =   v_in * (1 - exp(-3))  =  v_in * 0,95                   (i.e. an error of 5%)
+       // 
+       // Note: with the "historic" values in the comments above, the error was 34%  !!!
+       
        AT91C_BASE_ADC->ADC_CHER = ADC_CHANNEL(ch);
 
        AT91C_BASE_ADC->ADC_CR = AT91C_ADC_START;
+
        while(!(AT91C_BASE_ADC->ADC_SR & ADC_END_OF_CONVERSION(ch)))
                ;
        d = AT91C_BASE_ADC->ADC_CDR[ch];
@@ -183,9 +196,7 @@ void MeasureAntennaTuning(void)
     WDT_HIT();
                FpgaSendCommand(FPGA_CMD_SET_DIVISOR, i);
                SpinDelay(20);
-               // Vref = 3.3V, and a 10000:240 voltage divider on the input
-               // can measure voltages up to 137500 mV
-               adcval = ((137500 * AvgAdc(ADC_CHAN_LF)) >> 10);
+               adcval = ((MAX_ADC_LF_VOLTAGE * AvgAdc(ADC_CHAN_LF)) >> 10);
                if (i==95)      vLf125 = adcval; // voltage at 125Khz
                if (i==89)      vLf134 = adcval; // voltage at 134Khz
 
@@ -205,11 +216,9 @@ void MeasureAntennaTuning(void)
        FpgaDownloadAndGo(FPGA_BITSTREAM_HF);
        FpgaWriteConfWord(FPGA_MAJOR_MODE_HF_READER_RX_XCORR);
        SpinDelay(20);
-       // Vref = 3300mV, and an 10:1 voltage divider on the input
-       // can measure voltages up to 33000 mV
-       vHf = (33000 * AvgAdc(ADC_CHAN_HF)) >> 10;
+       vHf = (MAX_ADC_HF_VOLTAGE * AvgAdc(ADC_CHAN_HF)) >> 10;
 
-       cmd_send(CMD_MEASURED_ANTENNA_TUNING,vLf125|(vLf134<<16),vHf,peakf|(peakv<<16),LF_Results,256);
+       cmd_send(CMD_MEASURED_ANTENNA_TUNING, vLf125 | (vLf134<<16), vHf, peakf | (peakv<<16), LF_Results, 256);
        FpgaWriteConfWord(FPGA_MAJOR_MODE_OFF);
        LED_A_OFF();
        LED_B_OFF();
@@ -222,19 +231,21 @@ void MeasureAntennaTuningHf(void)
 
        DbpString("Measuring HF antenna, press button to exit");
 
+       // Let the FPGA drive the high-frequency antenna around 13.56 MHz.
+       FpgaDownloadAndGo(FPGA_BITSTREAM_HF);
+       FpgaWriteConfWord(FPGA_MAJOR_MODE_HF_READER_RX_XCORR);
+
        for (;;) {
-               // Let the FPGA drive the high-frequency antenna around 13.56 MHz.
-               FpgaDownloadAndGo(FPGA_BITSTREAM_HF);
-               FpgaWriteConfWord(FPGA_MAJOR_MODE_HF_READER_RX_XCORR);
                SpinDelay(20);
-               // Vref = 3300mV, and an 10:1 voltage divider on the input
-               // can measure voltages up to 33000 mV
-               vHf = (33000 * AvgAdc(ADC_CHAN_HF)) >> 10;
+               vHf = (MAX_ADC_HF_VOLTAGE * AvgAdc(ADC_CHAN_HF)) >> 10;
 
                Dbprintf("%d mV",vHf);
                if (BUTTON_PRESS()) break;
        }
        DbpString("cancelled");
+
+       FpgaWriteConfWord(FPGA_MAJOR_MODE_OFF);
+
 }
 
 
@@ -512,26 +523,32 @@ static const int LIGHT_LEN = sizeof(LIGHT_SCHEME)/sizeof(LIGHT_SCHEME[0]);
 
 void ListenReaderField(int limit)
 {
-       int lf_av, lf_av_new, lf_baseline= 0, lf_count= 0, lf_max;
-       int hf_av, hf_av_new,  hf_baseline= 0, hf_count= 0, hf_max;
+       int lf_av, lf_av_new, lf_baseline= 0, lf_max;
+       int hf_av, hf_av_new,  hf_baseline= 0, hf_max;
        int mode=1, display_val, display_max, i;
 
-#define LF_ONLY                1
-#define HF_ONLY                2
+#define LF_ONLY                                                1
+#define HF_ONLY                                                2
+#define REPORT_CHANGE                          10    // report new values only if they have changed at least by REPORT_CHANGE
+
+
+       // switch off FPGA - we don't want to measure our own signal
+       FpgaDownloadAndGo(FPGA_BITSTREAM_HF);
+       FpgaWriteConfWord(FPGA_MAJOR_MODE_OFF);
 
        LEDsoff();
 
-       lf_av=lf_max=ReadAdc(ADC_CHAN_LF);
+       lf_av = lf_max = AvgAdc(ADC_CHAN_LF);
 
        if(limit != HF_ONLY) {
-               Dbprintf("LF 125/134 Baseline: %d", lf_av);
+               Dbprintf("LF 125/134kHz Baseline: %dmV", (MAX_ADC_LF_VOLTAGE * lf_av) >> 10);
                lf_baseline = lf_av;
        }
 
-       hf_av=hf_max=ReadAdc(ADC_CHAN_HF);
+       hf_av = hf_max = AvgAdc(ADC_CHAN_HF);
 
        if (limit != LF_ONLY) {
-               Dbprintf("HF 13.56 Baseline: %d", hf_av);
+               Dbprintf("HF 13.56MHz Baseline: %dmV", (MAX_ADC_HF_VOLTAGE * hf_av) >> 10);
                hf_baseline = hf_av;
        }
 
@@ -554,38 +571,38 @@ void ListenReaderField(int limit)
                WDT_HIT();
 
                if (limit != HF_ONLY) {
-                       if(mode==1) {
-                               if (abs(lf_av - lf_baseline) > 10) LED_D_ON();
-                               else                               LED_D_OFF();
+                       if(mode == 1) {
+                               if (abs(lf_av - lf_baseline) > REPORT_CHANGE) 
+                                       LED_D_ON();
+                               else
+                                       LED_D_OFF();
                        }
 
-                       ++lf_count;
-                       lf_av_new= ReadAdc(ADC_CHAN_LF);
+                       lf_av_new = AvgAdc(ADC_CHAN_LF);
                        // see if there's a significant change
-                       if(abs(lf_av - lf_av_new) > 10) {
-                               Dbprintf("LF 125/134 Field Change: %x %x %x", lf_av, lf_av_new, lf_count);
+                       if(abs(lf_av - lf_av_new) > REPORT_CHANGE) {
+                               Dbprintf("LF 125/134kHz Field Change: %5dmV", (MAX_ADC_LF_VOLTAGE * lf_av_new) >> 10);
                                lf_av = lf_av_new;
                                if (lf_av > lf_max)
                                        lf_max = lf_av;
-                               lf_count= 0;
                        }
                }
 
                if (limit != LF_ONLY) {
                        if (mode == 1){
-                               if (abs(hf_av - hf_baseline) > 10) LED_B_ON();
-                               else                               LED_B_OFF();
+                               if (abs(hf_av - hf_baseline) > REPORT_CHANGE)   
+                                       LED_B_ON();
+                               else
+                                       LED_B_OFF();
                        }
 
-                       ++hf_count;
-                       hf_av_new= ReadAdc(ADC_CHAN_HF);
+                       hf_av_new = AvgAdc(ADC_CHAN_HF);
                        // see if there's a significant change
-                       if(abs(hf_av - hf_av_new) > 10) {
-                               Dbprintf("HF 13.56 Field Change: %x %x %x", hf_av, hf_av_new, hf_count);
+                       if(abs(hf_av - hf_av_new) > REPORT_CHANGE) {
+                               Dbprintf("HF 13.56MHz Field Change: %5dmV", (MAX_ADC_HF_VOLTAGE * hf_av_new) >> 10);
                                hf_av = hf_av_new;
                                if (hf_av > hf_max)
                                        hf_max = hf_av;
-                               hf_count= 0;
                        }
                }
 

diff --git a/armsrc/apps.h b/armsrc/apps.h
index 58a2a62196bd0cb2b96fe2259d7e962c8527ec77..a15d8b8171c50119c662d7d621711862a36e13df 100644 (file)
--- a/armsrc/apps.h
+++ b/armsrc/apps.h
@@ -38,6 +38,10 @@ void DbpString(char *str);
 void Dbprintf(const char *fmt, ...);
 void Dbhexdump(int len, uint8_t *d, bool bAsci);
 
+// ADC Vref = 3300mV, and an (10M+1M):1M voltage divider on the HF input can measure voltages up to 36300 mV
+#define MAX_ADC_HF_VOLTAGE 36300
+// ADC Vref = 3300mV, and an (10000k+240k):240k voltage divider on the LF input can measure voltages up to 140800 mV
+#define MAX_ADC_LF_VOLTAGE 140800
 int AvgAdc(int ch);
 
 void ToSendStuffBit(int b);

diff --git a/armsrc/iso14443a.c b/armsrc/iso14443a.c
index b73495a3c024280dd48280df9707eb6b2cc31f35..103f25e691cfa6106cfabe7ffd47ced9e29720d9 100644 (file)
--- a/armsrc/iso14443a.c
+++ b/armsrc/iso14443a.c
@@ -310,26 +310,27 @@ static RAMFUNC bool MillerDecoding(uint8_t bit, uint32_t non_real_time)
 
        Uart.twoBits = (Uart.twoBits << 8) | bit;
        
-       if (Uart.state == STATE_UNSYNCD) {                                                                                              // not yet synced
+       if (Uart.state == STATE_UNSYNCD) {                                                                                      // not yet synced
        
-               if (Uart.highCnt < 7) {                                                                                                 // wait for a stable unmodulated signal
+               if (Uart.highCnt < 2) {                                                                                                 // wait for a stable unmodulated signal
                        if (Uart.twoBits == 0xffff) {
                                Uart.highCnt++;
                        } else {
                                Uart.highCnt = 0;
                        }
                } else {        
-                       Uart.syncBit = 0xFFFF; // not set
-                       // look for 00xx1111 (the start bit)
-                       if              ((Uart.twoBits & 0x6780) == 0x0780) Uart.syncBit = 7; 
-                       else if ((Uart.twoBits & 0x33C0) == 0x03C0) Uart.syncBit = 6;
-                       else if ((Uart.twoBits & 0x19E0) == 0x01E0) Uart.syncBit = 5;
-                       else if ((Uart.twoBits & 0x0CF0) == 0x00F0) Uart.syncBit = 4;
-                       else if ((Uart.twoBits & 0x0678) == 0x0078) Uart.syncBit = 3;
-                       else if ((Uart.twoBits & 0x033C) == 0x003C) Uart.syncBit = 2;
-                       else if ((Uart.twoBits & 0x019E) == 0x001E) Uart.syncBit = 1;
-                       else if ((Uart.twoBits & 0x00CF) == 0x000F) Uart.syncBit = 0;
-                       if (Uart.syncBit != 0xFFFF) {
+                       Uart.syncBit = 0xFFFF;                                                                                          // not set
+                                                                                                                                                               // we look for a ...1111111100x11111xxxxxx pattern (the start bit)
+                       if              ((Uart.twoBits & 0xDF00) == 0x1F00) Uart.syncBit = 8;           // mask is   11x11111 xxxxxxxx, 
+                                                                                                                                                               // check for 00x11111 xxxxxxxx
+                       else if ((Uart.twoBits & 0xEF80) == 0x8F80) Uart.syncBit = 7;           // both masks shifted right one bit, left padded with '1'
+                       else if ((Uart.twoBits & 0xF7C0) == 0xC7C0) Uart.syncBit = 6;           // ...
+                       else if ((Uart.twoBits & 0xFBE0) == 0xE3E0) Uart.syncBit = 5;
+                       else if ((Uart.twoBits & 0xFDF0) == 0xF1F0) Uart.syncBit = 4;
+                       else if ((Uart.twoBits & 0xFEF8) == 0xF8F8) Uart.syncBit = 3;
+                       else if ((Uart.twoBits & 0xFF7C) == 0xFC7C) Uart.syncBit = 2;
+                       else if ((Uart.twoBits & 0xFFBE) == 0xFE3E) Uart.syncBit = 1;
+                       if (Uart.syncBit != 0xFFFF) {                                                                           // found a sync bit
                                Uart.startTime = non_real_time?non_real_time:(GetCountSspClk() & 0xfffffff8);
                                Uart.startTime -= Uart.syncBit;
                                Uart.endTime = Uart.startTime;
@@ -342,11 +343,9 @@ static RAMFUNC bool MillerDecoding(uint8_t bit, uint32_t non_real_time)
                if (IsMillerModulationNibble1(Uart.twoBits >> Uart.syncBit)) {                  
                        if (IsMillerModulationNibble2(Uart.twoBits >> Uart.syncBit)) {          // Modulation in both halves - error
                                UartReset();
-                               Uart.highCnt = 6;
                        } else {                                                                                                                        // Modulation in first half = Sequence Z = logic "0"
                                if (Uart.state == STATE_MILLER_X) {                                                             // error - must not follow after X
                                        UartReset();
-                                       Uart.highCnt = 6;
                                } else {
                                        Uart.bitCount++;
                                        Uart.shiftReg = (Uart.shiftReg >> 1);                                           // add a 0 to the shiftreg
@@ -401,12 +400,13 @@ static RAMFUNC bool MillerDecoding(uint8_t bit, uint32_t non_real_time)
                                        if (Uart.len) {
                                                return TRUE;                                                                                    // we are finished with decoding the raw data sequence
                                        } else {
-                                               UartReset();                                    // Nothing receiver - start over
+                                               UartReset();                                                                                    // Nothing received - start over
+                                               Uart.highCnt = 1;
                                        }
                                }
                                if (Uart.state == STATE_START_OF_COMMUNICATION) {                               // error - must not follow directly after SOC
                                        UartReset();
-                                       Uart.highCnt = 6;
+                                       Uart.highCnt = 1;
                                } else {                                                                                                                // a logic "0"
                                        Uart.bitCount++;
                                        Uart.shiftReg = (Uart.shiftReg >> 1);                                           // add a 0 to the shiftreg
@@ -1425,6 +1425,7 @@ void CodeIso14443aAsReaderPar(const uint8_t *cmd, uint16_t len, const uint8_t *p
   CodeIso14443aBitsAsReaderPar(cmd, len*8, parity);
 }
 
+
 //-----------------------------------------------------------------------------
 // Wait for commands from reader
 // Stop when button is pressed (return 1) or field was gone (return 2)
@@ -1447,9 +1448,9 @@ static int EmGetCmd(uint8_t *received, uint16_t *len, uint8_t *parity)
        // Set ADC to read field strength
        AT91C_BASE_ADC->ADC_CR = AT91C_ADC_SWRST;
        AT91C_BASE_ADC->ADC_MR =
-                               ADC_MODE_PRESCALE(32) |
-                               ADC_MODE_STARTUP_TIME(16) |
-                               ADC_MODE_SAMPLE_HOLD_TIME(8);
+                               ADC_MODE_PRESCALE(63) |
+                               ADC_MODE_STARTUP_TIME(1) |
+                               ADC_MODE_SAMPLE_HOLD_TIME(15);
        AT91C_BASE_ADC->ADC_CHER = ADC_CHANNEL(ADC_CHAN_HF);
        // start ADC
        AT91C_BASE_ADC->ADC_CR = AT91C_ADC_START;
@@ -1459,7 +1460,7 @@ static int EmGetCmd(uint8_t *received, uint16_t *len, uint8_t *parity)
 
        // Clear RXRDY:
     uint8_t b = (uint8_t)AT91C_BASE_SSC->SSC_RHR;
-
+       
        for(;;) {
                WDT_HIT();
 
@@ -1471,7 +1472,7 @@ static int EmGetCmd(uint8_t *received, uint16_t *len, uint8_t *parity)
                        analogAVG += AT91C_BASE_ADC->ADC_CDR[ADC_CHAN_HF];
                        AT91C_BASE_ADC->ADC_CR = AT91C_ADC_START;
                        if (analogCnt >= 32) {
-                               if ((33000 * (analogAVG / analogCnt) >> 10) < MF_MINFIELDV) {
+                               if ((MAX_ADC_HF_VOLTAGE * (analogAVG / analogCnt) >> 10) < MF_MINFIELDV) {
                                        vtime = GetTickCount();
                                        if (!timer) timer = vtime;
                                        // 50ms no field --> card to idle state
@@ -1546,14 +1547,15 @@ static int EmSendCmd14443aRaw(uint8_t *resp, uint16_t respLen, bool correctionNe
        }
 
        // Ensure that the FPGA Delay Queue is empty before we switch to TAGSIM_LISTEN again:
-       for (i = 0; i < 2 ; ) {
+       uint8_t fpga_queued_bits = FpgaSendQueueDelay >> 3;
+       for (i = 0; i <= fpga_queued_bits/8 + 1; ) {
                if(AT91C_BASE_SSC->SSC_SR & (AT91C_SSC_TXRDY)) {
                        AT91C_BASE_SSC->SSC_THR = SEC_F;
                        FpgaSendQueueDelay = (uint8_t)AT91C_BASE_SSC->SSC_RHR;
                        i++;
                }
        }
-       
+
        LastTimeProxToAirStart = ThisTransferTime + (correctionNeeded?8:0);
 
        return 0;
@@ -1655,7 +1657,7 @@ static int GetIso14443aAnswerFromTag(uint8_t *receivedResponse, uint8_t *receive
 
        // clear RXRDY:
     uint8_t b = (uint8_t)AT91C_BASE_SSC->SSC_RHR;
-       
+
        c = 0;
        for(;;) {
                WDT_HIT();
@@ -2264,6 +2266,7 @@ void Mifare1ksim(uint8_t flags, uint8_t exitAfterNReads, uint8_t arg2, uint8_t *
 
        // free eventually allocated BigBuf memory but keep Emulator Memory
        BigBuf_free_keep_EM();
+
        // clear trace
     iso14a_clear_trace();
        iso14a_set_tracing(TRUE);
@@ -2328,10 +2331,8 @@ void Mifare1ksim(uint8_t flags, uint8_t exitAfterNReads, uint8_t arg2, uint8_t *
                WDT_HIT();
 
                // find reader field
-               // Vref = 3300mV, and an 10:1 voltage divider on the input
-               // can measure voltages up to 33000 mV
                if (cardSTATE == MFEMUL_NOFIELD) {
-                       vHf = (33000 * AvgAdc(ADC_CHAN_HF)) >> 10;
+                       vHf = (MAX_ADC_HF_VOLTAGE * AvgAdc(ADC_CHAN_HF)) >> 10;
                        if (vHf > MF_MINFIELDV) {
                                cardSTATE_TO_IDLE();
                                LED_A_ON();
@@ -2406,6 +2407,7 @@ void Mifare1ksim(uint8_t flags, uint8_t exitAfterNReads, uint8_t arg2, uint8_t *
                                        LogTrace(Uart.output, Uart.len, Uart.startTime*16 - DELAY_AIR2ARM_AS_TAG, Uart.endTime*16 - DELAY_AIR2ARM_AS_TAG, Uart.parity, TRUE);
                                        break;
                                }
+
                                uint32_t ar = bytes_to_num(receivedCmd, 4);
                                uint32_t nr = bytes_to_num(&receivedCmd[4], 4);
 
@@ -2512,6 +2514,7 @@ void Mifare1ksim(uint8_t flags, uint8_t exitAfterNReads, uint8_t arg2, uint8_t *
                                                ans = nonce ^ crypto1_word(pcs, cuid ^ nonce, 0); 
                                                num_to_bytes(ans, 4, rAUTH_AT);
                                        }
+
                                        EmSendCmd(rAUTH_AT, sizeof(rAUTH_AT));
                                        //Dbprintf("Sending rAUTH %02x%02x%02x%02x", rAUTH_AT[0],rAUTH_AT[1],rAUTH_AT[2],rAUTH_AT[3]);
                                        cardSTATE = MFEMUL_AUTH1;
@@ -2692,7 +2695,7 @@ void Mifare1ksim(uint8_t flags, uint8_t exitAfterNReads, uint8_t arg2, uint8_t *
                if(ar_nr_collected > 1) {
                        Dbprintf("Collected two pairs of AR/NR which can be used to extract keys from reader:");
                        Dbprintf("../tools/mfkey/mfkey32 %08x %08x %08x %08x %08x %08x",
-                                        ar_nr_responses[0], // UID
+                                       ar_nr_responses[0], // UID
                                        ar_nr_responses[1], //NT
                                        ar_nr_responses[2], //AR1
                                        ar_nr_responses[3], //NR1
@@ -2712,6 +2715,7 @@ void Mifare1ksim(uint8_t flags, uint8_t exitAfterNReads, uint8_t arg2, uint8_t *
                }
        }
        if (MF_DBGLEVEL >= 1)   Dbprintf("Emulator stopped. Tracing: %d  trace length: %d ",    tracing, traceLen);
+       
 }
 
 

diff --git a/client/cmddata.c b/client/cmddata.c
index 293b66ab92a6e3fd56d1064a898e92645b9d2d56..4d79eeaebda96bbe2ebf266c2eb39ec9646b6286 100644 (file)
--- a/client/cmddata.c
+++ b/client/cmddata.c
@@ -1914,24 +1914,31 @@ int CmdTuneSamples(const char *Cmd)
        PrintAndLog("# LF antenna: %5.2f V @   134.00 kHz", vLf134/1000.0);
        PrintAndLog("# LF optimal: %5.2f V @%9.2f kHz", peakv/1000.0, 12000.0/(peakf+1));
        PrintAndLog("# HF antenna: %5.2f V @    13.56 MHz", vHf/1000.0);
-       if (peakv<2000)
+
+#define LF_UNUSABLE_V          2948            // was 2000. Changed due to bugfix in voltage measurements. LF results are now 47% higher.
+#define LF_MARGINAL_V          14739           // was 10000. Changed due to bugfix bug in voltage measurements. LF results are now 47% higher.
+#define HF_UNUSABLE_V          3167            // was 2000. Changed due to bugfix in voltage measurements. HF results are now 58% higher.
+#define HF_MARGINAL_V          7917            // was 5000. Changed due to bugfix in voltage measurements. HF results are now 58% higher.
+
+       if (peakv < LF_UNUSABLE_V)
                PrintAndLog("# Your LF antenna is unusable.");
-       else if (peakv<10000)
+       else if (peakv < LF_MARGINAL_V)
                PrintAndLog("# Your LF antenna is marginal.");
-       if (vHf<2000)
+       if (vHf < HF_UNUSABLE_V)
                PrintAndLog("# Your HF antenna is unusable.");
-       else if (vHf<5000)
+       else if (vHf < HF_MARGINAL_V)
                PrintAndLog("# Your HF antenna is marginal.");
 
-       for (int i = 0; i < 256; i++) {
-               GraphBuffer[i] = resp.d.asBytes[i] - 128;
+       if (peakv >= LF_UNUSABLE_V)     {
+               for (int i = 0; i < 256; i++) {
+                       GraphBuffer[i] = resp.d.asBytes[i] - 128;
+               }
+               PrintAndLog("Displaying LF tuning graph. Divisor 89 is 134khz, 95 is 125khz.\n");
+               PrintAndLog("\n");
+               GraphTraceLen = 256;
+               ShowGraphWindow();
        }
 
-       PrintAndLog("Done! Divisor 89 is 134khz, 95 is 125khz.\n");
-       PrintAndLog("\n");
-       GraphTraceLen = 256;
-       ShowGraphWindow();
-
        return 0;
 }