/* Copyright (c) 2025 Rodrigo Arias Mallo <rodarima@gmail.com>
 * SPDX-License-Identifier: GPL-3.0-or-later */

#include <math.h>

/* Steinhart-Hart Thermistor Coefficients, used to convert resistance into
 * temperature.
 *
 * The current NTC sensor has 102kOhm at 24C but I don't know the specific
 * model, so the coefficients are computed for this NTC sensor instead:
 * https://www.tme.eu/Document/f9d2f5e38227fc1c7d979e546ff51768/NTCM-100K-B3950.pdf
 *
 * The table seems to match what I would expect. Their R2 resistor is 6.8 kOhm,
 * which yields a cuttof temperature of around 98.5 C at exactly half voltage
 * (where the ADC would have more precision).
 *
 * In any case, we can calibrate the original NTC sensor by taking three
 * temperature points. See:
 * https://www.thinksrs.com/downloads/programs/therm%20calc/ntccalibrator/ntccalculator.html
 */
#define C1	0.7740577674e-3
#define C2	2.073449619e-4
#define C3	1.263502259e-7

/* Return the temperature in celsisus */
float
ntc_temp(float R)
{
	/* Computing the log is slow, we may want to build a table */
	float logR = log(R);
	float T = (1.0 / (C1 + C2*logR + C3*logR*logR*logR));
	float Tc = T - 273.15;
	return Tc;
}

/* Return resistance in Ohms */
float
ntc_resistance(int Vo)
{
	float R1 = 10.0e3; /* Resistor for voltage divider */
	float R2 = R1 * (1023.0 / (float)Vo - 1.0);

	return R2;
}