hw tune minor fix, comments updated

diff --git a/armsrc/appmain.c b/armsrc/appmain.c
index 65fc6de6f6ed76faaebd49aa2dce39ab59e90c2f..1c3d93982441300451cc0d2a26811c83f819c77e 100644 (file)
--- a/armsrc/appmain.c
+++ b/armsrc/appmain.c
@@ -137,21 +137,19 @@ void Dbhexdump(int len, uint8_t *d, bool bAsci) {
 static int ReadAdc(int ch)
 {      
        // 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. 
+       // AMPL_HI is are high impedance (10MOhm || 1MOhm) output, the input capacitance of the ADC is 12pF (typical). This results in a time constant
+       // of RC = (0.91MOhm) * 12pF = 10.9us. 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%  !!!
+       //       v_cap = v_in * (1 - exp(-SHTIM/RC))  =   v_in * (1 - exp(-40us/10.9us))  =  v_in * 0,97                   (i.e. an error of 3%)
 
        AT91C_BASE_ADC->ADC_CR = AT91C_ADC_SWRST;
        AT91C_BASE_ADC->ADC_MR =
-               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
+               ADC_MODE_PRESCALE(63) |                                                 // ADC_CLK = MCK / ((63+1) * 2) = 48MHz / 128 = 375kHz
+               ADC_MODE_STARTUP_TIME(1) |                                              // Startup Time = (1+1) * 8 / ADC_CLK = 16 / 375kHz = 42,7us     Note: must be > 20us
+               ADC_MODE_SAMPLE_HOLD_TIME(15);                                  // Sample & Hold Time SHTIM = 15 / ADC_CLK = 15 / 375kHz = 40us
 
        AT91C_BASE_ADC->ADC_CHER = ADC_CHANNEL(ch);
        AT91C_BASE_ADC->ADC_CR = AT91C_ADC_START;
@@ -175,8 +173,7 @@ int AvgAdc(int ch) // was static - merlok
 
 void MeasureAntennaTuningLfOnly(int *vLf125, int *vLf134, int *peakf, int *peakv, uint8_t LF_Results[])
 {
-       uint8_t i;
-       int adcval = 0, peak = 0;
+       int i, adcval = 0, peak = 0;
 
 /*
  * Sweeps the useful LF range of the proxmark from