| /* |
| * Copyright (c) 2023 Google LLC |
| * |
| * SPDX-License-Identifier: Apache-2.0 |
| */ |
| |
| #include <limits.h> |
| #include <stdlib.h> |
| #include <zephyr/devicetree.h> |
| #include "ntc_thermistor.h" |
| |
| /** |
| * fixp_linear_interpolate() - interpolates a value from two known points |
| * |
| * @x0: x value of point 0 |
| * @y0: y value of point 0 |
| * @x1: x value of point 1 |
| * @y1: y value of point 1 |
| * @x: the linear interpolant |
| */ |
| static int ntc_fixp_linear_interpolate(int x0, int y0, int x1, int y1, int x) |
| { |
| if (y0 == y1 || x == x0) { |
| return y0; |
| } |
| if (x1 == x0 || x == x1) { |
| return y1; |
| } |
| |
| return y0 + ((y1 - y0) * (x - x0) / (x1 - x0)); |
| } |
| |
| /** |
| * ntc_lookup_comp() - Finds indicies where ohm falls between |
| * |
| * @ohm: key value search is looking for |
| * @i_low: return Lower interval index value |
| * @i_high: return Higher interval index value |
| */ |
| static void ntc_lookup_comp(const struct ntc_type *type, unsigned int ohm, int *i_low, int *i_high) |
| { |
| int low = 0; |
| int high = type->n_comp - 1; |
| |
| if (ohm > type->comp[low].ohm) { |
| high = low; |
| } else if (ohm < type->comp[high].ohm) { |
| low = high; |
| } |
| |
| while (high - low > 1) { |
| int mid = (low + high) / 2; |
| |
| if (ohm > type->comp[mid].ohm) { |
| high = mid; |
| } else { |
| low = mid; |
| } |
| } |
| |
| *i_low = low; |
| *i_high = high; |
| } |
| |
| /** |
| * ntc_get_ohm_of_thermistor() - Calculate the resistance read from NTC Thermistor |
| * |
| * @cfg: NTC Thermistor configuration |
| * @sample_mv: Measured voltage in mV |
| */ |
| uint32_t ntc_get_ohm_of_thermistor(const struct ntc_config *cfg, int sample_mv) |
| { |
| int pullup_mv = cfg->pullup_uv / 1000; |
| uint32_t ohm; |
| |
| if (sample_mv <= 0) { |
| return cfg->connected_positive ? INT_MAX : 0; |
| } |
| |
| if (sample_mv >= pullup_mv) { |
| return cfg->connected_positive ? 0 : INT_MAX; |
| } |
| |
| if (cfg->connected_positive) { |
| ohm = cfg->pulldown_ohm * (pullup_mv - sample_mv) / sample_mv; |
| } else { |
| ohm = cfg->pullup_ohm * sample_mv / (pullup_mv - sample_mv); |
| } |
| |
| return ohm; |
| } |
| |
| int32_t ntc_get_temp_mc(const struct ntc_type *type, unsigned int ohm) |
| { |
| int low, high; |
| int temp; |
| |
| ntc_lookup_comp(type, ohm, &low, &high); |
| /* |
| * First multiplying the table temperatures with 1000 to get to |
| * millicentigrades (which is what we want) and then interpolating |
| * will give the best precision. |
| */ |
| temp = ntc_fixp_linear_interpolate(type->comp[low].ohm, type->comp[low].temp_c * 1000, |
| type->comp[high].ohm, type->comp[high].temp_c * 1000, |
| ohm); |
| return temp; |
| } |