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samples: shields: lmp90100_evb: rtd: improve readability, fix off-by-one

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Improve the readability of the LMP90100-EVB RTD sample by adding
comments and further definitions instead of magic values within the
code.

This furthermore fixes an off-by-one error for the maximum ADC value
used in the formula for calculating the resistance of the RTD (8388607
vs. 8388608).

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>
This commit is contained in:
Henrik Brix Andersen 2021-02-20 18:58:11 +01:00 • committed by Carles Cufí
commit 0449e086c6
1 changed files with 12 additions and 2 deletions
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14
samples/shields/lmp90100_evb/rtd/src/main.c
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@ -13,8 +13,18 @@
#include <logging/log.h> #include <logging/log.h>
LOG_MODULE_REGISTER(main); LOG_MODULE_REGISTER(main);
/* Nominal RTD (PT100) resistance in ohms */
#define RTD_NOMINAL_RESISTANCE 100 #define RTD_NOMINAL_RESISTANCE 100
/* ADC resolution in bits */
#define ADC_RESOLUTION 24U
/* ADC maximum value (taking sign bit into consideration) */
#define ADC_MAX BIT_MASK(ADC_RESOLUTION - 1)
/* Bottom resistor value in ohms */
#define BOTTOM_RESISTANCE 2000
static double sqrt(double value) static double sqrt(double value)
{ {
double sqrt = value / 3; double sqrt = value / 3;
@ -65,7 +75,7 @@ void main(void)
.channels = BIT(0), .channels = BIT(0),
.buffer = &buffer, .buffer = &buffer,
.buffer_size = sizeof(buffer), .buffer_size = sizeof(buffer),
.resolution = 24, .resolution = ADC_RESOLUTION,
.oversampling = 0, .oversampling = 0,
.calibrate = 0 .calibrate = 0
}; };
@ -87,7 +97,7 @@ void main(void)
if (err) { if (err) {
LOG_ERR("failed to read ADC (err %d)", err); LOG_ERR("failed to read ADC (err %d)", err);
} else { } else {
resistance = (buffer / 8388608.0) * 2000; resistance = (buffer / (double)ADC_MAX) * BOTTOM_RESISTANCE;
printf("R: %.02f ohm\n", resistance); printf("R: %.02f ohm\n", resistance);
printf("T: %.02f degC\n", printf("T: %.02f degC\n",
rtd_temperature(RTD_NOMINAL_RESISTANCE, rtd_temperature(RTD_NOMINAL_RESISTANCE,