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132 lines
4.3 KiB
HTML
132 lines
4.3 KiB
HTML
<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="UTF-8">
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<title>3.9 Reading a Resistor Value</title>
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<link type="text/css" rel="stylesheet" href="style.css">
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</head>
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<body>
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<h1>3.9 Reading a Resistor Value</h1>
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I used the ADC previously to read the internal sensors, so it's simple
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to move on to external ones. Once you can read the value of a resistor,
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there is a wide choice of analog applications: thermistors, photocells,
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potentiometers, sliders, joysticks …
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<h2>Voltage Divider Circuit</h2>
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Reading a resistor value is based on the voltage divider circuit.<br>
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<img alt="Voltage Divider Circuit" src="img/39_vdivider.png"><br>
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The relationship between the two voltages, <b>Vin</b> and <b>Vout</b> is
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<b><pre>
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Vout Vin
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────── = ──────────────
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Rref Rref + Rmeas
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</pre></b>
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<h2>ADC Readings</h2>
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Assuming the ADC is configured for 12 bit precision and return a value
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in the range [0 … 4095], I can express the two voltages in terms of ADC values.
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<br>
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<b>Vout = VADC = VDDA * ADCraw / 4095</b><br>
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<b>Vin = VDDA</b>
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<p>The voltage divider relationship now becomes<br>
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<b><i>VDDA</i> * ADCraw / 4095 = <i>VDDA</i> * Rref / (Rref + Rmeasured)</b>
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<p>which can be further simplified as<br>
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<b>ADCraw * Rmeasured = Rref * (4095 - ADCraw)</b>
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<p>The resistor value is then given by<br>
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<b>Rmeasured = Rref * (4095 - ADCraw) / ADCraw</b><br>
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or<br>
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<b>Rmeasured = Rref * (4095 / ADCraw - 1)</b>
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<h2>Devil in the whatnots</h2>
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Some of the things to pay attention to while coding
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<ul>
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<li> Avoiding division by zero.
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<li> Integer versus floating point calculation.
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<li> Choosing the reference resistor.
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<li> Calibration of the reference resistor.
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</ul>
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<h3>● Division by Zero</h3>
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In the case where ADC readings would return 0, I chose to return INT_MAX as
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a value to flag the error.
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<pre>
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#include <limits.h>
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int R = ADCraw ? Rref * (4095 / ADCraw - 1) : INT_MAX ;
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</pre>
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<h3>● Integer Calculation</h3>
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As integer based calculation tends to round intermediary results, I use the
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developped formula
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<pre>int R = ADCraw ? Rref * 4095 / ADCraw - Rref : INT_MAX ;</pre>
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<h3>● Reference Resistor Selection</h3>
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The pair of resistors, reference and measured, has to be selected according
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to the use case. For now I am experimenting with the measurement of a
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photoresistor and use a reference resistor of 10 kOhm.
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<h3>● Calibration</h3>
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For resistor reading, the reference voltage value VDDA does not influence the
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calculation. From a hardware design point of view, having a stable reference
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voltage still remains a key factor in securing reliable ADC readings.<br>
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For precise resistor value reading, the calibration of the reference resistor
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will be key. In other cases, the raw ADC reading can be used in combination
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with dynamic range acquisition.
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<h2>Sampling Code</h2>
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I create <b>adcext.c</b> to take readings every second.
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<pre>
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#include <limits.h>
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#include <stdio.h>
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#include "system.h" /* uptime, yield(), adc_init(), adc_convert() */
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#define RREF 10010 /* Rref is 10kOhm, measured @ 10.01 kOhm */
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int main( void) {
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unsigned last = 0 ;
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const unsigned short *calp ; /* TS_CAL, VREFINT_CAL */
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/* Initialize ADC and fetch calibration values */
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calp = adc_init( 2 | (1 << 17)) ; /* ADC read on GPIOA1 and VREF */
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short Vcal = calp[ 1] ; /* VREFINT_CAL */
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printf( "factory calibration: %u, %u, %u\n", calp[ 1], calp[ 0], calp[5]) ;
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for( ;;)
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if( uptime == last)
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yield() ;
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else {
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short Rsample, Vsample ;
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last = uptime ;
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Vsample = adc_convert() ;
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Rsample = adc_convert() ;
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printf( "%i, %i, %i, ", Vcal, Vsample, Rsample) ;
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int res = Rsample ? RREF * 4095 / Rsample - RREF : INT_MAX ;
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Vsample = 3300 * Vcal / Vsample ;
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printf( "%i, %i.%i\n", res, Vsample / 1000, Vsample % 1000) ;
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}
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}
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</pre>
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I add the composition in <b>Makefile</b>
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<pre>SRCS = startup.crc.c adc.c adcext.c</pre>
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<h2>Checkpoint</h2>
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<p>
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Building up on the previous ADC reading of internal sensors, sampling of
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an external resistor pair connected to one of the IO pin is straightforward.
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</p>
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<p>
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<a href="index.html#part4">Next</a>, I will show the temperature readings on a
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display.
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</p>
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<hr>© 2020-2025 Renaud Fivet
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</body>
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</html>
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