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hw tune minor fix, comments updated

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pwpiwi 2018-01-11 08:32:01 +01:00
commit 9b32625c2c
1 changed files with 7 additions and 10 deletions
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17
armsrc/appmain.c
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@ -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