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<title>3.9 Reading a Resistor Value</title>
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<h1>3.9 Reading a Resistor Value</h1>
I used the ADC previously to read the internal sensors, so it’s simple
to move on to external ones. Once you can read the value of a resistor,
there is a wide choice of analog applications: thermistors, photocells,
potentiometers, sliders, joysticks …
<h2>Voltage Divider Circuit</h2>
<img alt="Voltage Divider Circuit" src="img/39_vdivider.png">
<p>
<b>Vout = Vin * Rref / (Rref + Rmeasured)</b>
<h2>ADC Readings</h2>
Assuming the ADC is configured for 12 bit precision and return a value
in the range [0 … 4095]
<p>
<b>Vout = VADC = VDDA * ADCRAW / 4095</b>
<p>
<b>Vin = VDDA</b>
<p>
<b>~VDDA~ * ADCRAW / 4095 = ~VDDA~ * Rref / (Rref + Rmeasured)</b>
<p>
<b>ADCRAW * Rmeasured = Rref * (4095 – ADCRAW)</b>
<p>
<b>Rmeasured = Rref * (4095 – ADCRAW) / ADCRAW</b>
<p>
<b>Rmeasured = Rref * (4095 / ADCRAW – 1)</b>
<h2>Devil in the whatnots</h2>
<ul>
<li> Avoiding division by zero.
<li> Integer versus floating point calculation.
<li> Choosing the reference resistor.
<li> Calibration of the reference resistor.
</ul>
<pre>
#include <limits.h>
int R = ADCraw ? Rref * (4095 / ADCraw - 1) : INT_MAX ;
</pre>
<pre>int R = ADCraw ? Rref * 4095 / ADCraw - Rref : INT_MAX ;</pre>
<hr>© 2020-2024 Renaud Fivet
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