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/* Bob Anderson (bob.anderson.ok@gmail.com)
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This program...
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1) Uses AnalogIn but speeds it up by setting
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the ADC clock to 12MHz (13MHz is max from spec sheet)
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2) Produces a histogram of deviations around a center
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value that is determined at the beginning of a run
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by a "vote" taken from three successive readings.
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3) Introduces a "glitch" suppression method that appears
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to be effective.
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With a 100nf capacitor from the analog pin to ground and
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a reasonably low impedance source ( < 5K ) and glitch suppression
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enabled, the LPC1768/9 onboard analog to digital converter subsystem
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becomes quite useable. Without the capacitor and glitch suppression,
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it is not a reliable ADC --- it is glitch prone, even with the
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cleanest of inputs. This program will help you determine for
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yourself whether that statement is true. Play with it.
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*/
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#include "mbed.h"
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#include "Serial.h"
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#include "AnalogIn.h"
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Serial pc( USBTX, USBRX );
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AnalogIn ain( p20 );
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#define NUM_SAMPLES 1000000
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#define HGRAM_MAX_INDEX 60
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int histoGram[ HGRAM_MAX_INDEX + 1 ];
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int glitchCount = 0;
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bool glitchSuppressionWanted = false;
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void setADCclockToMaxConversionRate(void);
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int glitchSuppressedAnalogIn( void );
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int max( int a, int b ) {
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if ( a > b ) return a;
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return b;
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}
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int min( int a, int b ) {
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if ( a < b ) return a;
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return b;
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}
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// middle( a, b, c ) returns true if a is between b and c
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bool middle( int a, int b, int c ) {
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return (a <= max(b,c)) && (a >= min(b,c));
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}
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int main() {
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int referenceValue;
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int delta;
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int newValue;
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// Speedup the AnalogIn conversion rate by readjusting
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// the ADC clock to 12 MHz (assuming SystemCoreClock = 96MHz)
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setADCclockToMaxConversionRate();
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while(1) {
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printf( "\nPress the s key to start stats gathering\n" );
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printf( "Press the g key to toggle glitch suppression...\n" );
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bool waitingForKeyInput = true;
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while ( waitingForKeyInput ){
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switch (pc.getc()) {
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case 'g':
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case 'G':
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glitchSuppressionWanted = ! glitchSuppressionWanted;
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if ( glitchSuppressionWanted )
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printf( "...glitch suppression on...\n" );
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else
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printf( "...glitch suppression off...\n" );
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break;
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case 's':
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case 'S': waitingForKeyInput = false;
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}
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}
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// Establish center point for histogram by taking three readings
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// and selecting the one in the middle as the reference value around
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// which deviations will be calculated.
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int value1 = ain.read_u16() >> 4;
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int value2 = ain.read_u16() >> 4;
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int value3 = ain.read_u16() >> 4;
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if ( middle(value1,value2,value3) )
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referenceValue = value1;
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else if ( middle(value2,value1,value3) )
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referenceValue = value2;
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else
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referenceValue = value3;
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printf( "\nreferenceValue: %d value1: %d value2: %d value3: %d\n",
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referenceValue, value1, value2, value3 );
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printf( "\n...now gathering...\n\n" );
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// Clear the histogram array.
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for ( int k = 0; k <= HGRAM_MAX_INDEX; k++ ) histoGram[k] = 0;
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glitchCount = 0;
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for ( int i = 0; i < NUM_SAMPLES; i++ ) {
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newValue = glitchSuppressedAnalogIn();
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delta = newValue - referenceValue + (HGRAM_MAX_INDEX / 2);
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if ( delta < 0 ) histoGram[0]++;
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else if ( delta > HGRAM_MAX_INDEX ) histoGram[HGRAM_MAX_INDEX]++;
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else histoGram[delta]++;
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}
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// Output histoGram
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for ( int j = 0; j <= HGRAM_MAX_INDEX; j++ ) {
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printf( "%4d %8d\n", j, histoGram[j] );
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}
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printf( "glitchCount: %d\n", glitchCount );
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}
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}
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void setADCclockToMaxConversionRate(void) {
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// Set pclk = cclk / 4 (pclk = 96Mhz/4 = 24Mhz)
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LPC_SC->PCLKSEL0 &= ~(0x3 << 24); // Clear bits 25:24
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// software-controlled ADC settings
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LPC_ADC->ADCR = (0 << 0) // SEL: 0 = no channels selected
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| (1 << 8 ) // CLKDIV: ADCCLK = PCLK / (CLKDIV + 1) (12MHz)
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| (0 << 16) // BURST: 0 = software control
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| (0 << 17) // CLKS: not applicable
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| (1 << 21) // PDN: 1 = operational
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| (0 << 24) // START: 0 = no start
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| (0 << 27); // EDGE: not applicable}
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return;
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}
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int glitchSuppressedAnalogIn( void ) {
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int v1,v2,delta;
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v1 = ain.read_u16() >> 4;
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if ( ! glitchSuppressionWanted ) return v1;
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// While this has the possiblity of never returning,
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// the probability of that is vanishingly small. We assume
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// that we will eventually find two successive readings that are
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// within the tolerance band --- i.e., no glitch
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while(1){
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v2 = ain.read_u16() >> 4;
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delta = v1 - v2;
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if ( (delta > -10) && (delta < 10)) return (v1+v2)/2;
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v1 = v2;
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glitchCount++;
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}
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} |
Bob Anderson