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#include <stddef.h>
#include "temperature.h"
/*
* formula by applying Ohms law:
*
* Rt: Thermistor resistance
* Ra: Voltage divider resistor
* Vt: Tap voltage
* Vcc: Input Voltage (3.31V)
*
* Rt = Ra * Vcc / Vt - Ra
*
* Using 64-bit arithmetic as 32-bit would overflow.
* On WeMOS D1 Mini the A0 reads 3.2V as 1023; our Vcc is 3.31V
* The correct value of the reference volume obtained by measuring
* resistances with a multimeter, taking the ADC reading and applying
* Ohm's law.
* We shift the range by one to avoid division by zero.
*/
struct temptuple {
short res; /* Resistance in ohms. */
short temp; /* Temperatures in 0.1°K. */
};
int get_resistance(int64_t vt, int64_t ra)
{
++vt;
return ra * 1081 / vt - ra;
}
int get_temperature(int res)
{
int ret;
size_t i;
static const struct temptuple lt[] = {
{1495, 2630}, {1630, 2730}, {1772, 2830}, {1922, 2930},
{2080, 3030}, {2245, 3130}, {2417, 3230}, {2597, 3330},
{2785, 3430}, {2980, 3530}, {3182, 3630}, {3392, 3730},
{3607, 3830}};
ret = -1;
for (i = 0; i < 12; ++i) {
/* If we have a matching resistance, nothing to calculate. */
if (res == lt[i].res) {
ret = lt[i].temp;
break;
} else if (res == lt[i + 1].res) {
ret = lt[i + 1].temp;
break;
/* If no matching resistance is found, calculate temp from subrange. */
} else if (res > lt[i].res && res < lt[i + 1].res) {
ret = lt[i].temp + get_temp_subrange(lt[i].res, lt[i + 1].res, res);
break;
}
}
return ret;
}
/* Returns the last 0-10 part of the temperature, in 0.1°K. */
int get_temp_subrange(short a, short b, int res)
{
return (res - a) * 100 / (b - a);
}
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