1 /*
2 * Copyright (c) 2023 Google LLC
3 *
4 * SPDX-License-Identifier: Apache-2.0
5 */
6
7 #include <limits.h>
8 #include "ntc_thermistor.h"
9
10 /**
11 * fixp_linear_interpolate() - interpolates a value from two known points
12 *
13 * @x0: x value of point 0
14 * @y0: y value of point 0
15 * @x1: x value of point 1
16 * @y1: y value of point 1
17 * @x: the linear interpolant
18 */
ntc_fixp_linear_interpolate(int x0,int y0,int x1,int y1,int x)19 static int ntc_fixp_linear_interpolate(int x0, int y0, int x1, int y1, int x)
20 {
21 if (y0 == y1 || x == x0) {
22 return y0;
23 }
24 if (x1 == x0 || x == x1) {
25 return y1;
26 }
27
28 return y0 + ((y1 - y0) * (x - x0) / (x1 - x0));
29 }
30
31 /**
32 * Finds indices where ohm falls between.
33 *
34 * @param ohm key value search is looking for
35 * @param i_low return Lower interval index value
36 * @param i_high return Higher interval index value
37 */
ntc_lookup_comp(const struct ntc_type * type,unsigned int ohm,int * i_low,int * i_high)38 static void ntc_lookup_comp(const struct ntc_type *type, unsigned int ohm, int *i_low, int *i_high)
39 {
40 int low = 0;
41 int high = type->n_comp - 1;
42
43 if (ohm > type->comp[low].ohm) {
44 high = low;
45 } else if (ohm < type->comp[high].ohm) {
46 low = high;
47 }
48
49 while (high - low > 1) {
50 int mid = (low + high) / 2;
51
52 if (ohm > type->comp[mid].ohm) {
53 high = mid;
54 } else {
55 low = mid;
56 }
57 }
58
59 *i_low = low;
60 *i_high = high;
61 }
62
ntc_get_ohm_of_thermistor(const struct ntc_config * cfg,int sample_value,int sample_value_max)63 uint32_t ntc_get_ohm_of_thermistor(const struct ntc_config *cfg, int sample_value,
64 int sample_value_max)
65 {
66 uint32_t ohm;
67
68 if (sample_value <= 0) {
69 return cfg->connected_positive ? INT_MAX : 0;
70 }
71
72 if (sample_value >= sample_value_max) {
73 return cfg->connected_positive ? 0 : INT_MAX;
74 }
75
76 if (cfg->connected_positive) {
77 ohm = cfg->pulldown_ohm * (sample_value_max - sample_value) / sample_value;
78 } else {
79 ohm = cfg->pullup_ohm * sample_value / (sample_value_max - sample_value);
80 }
81
82 return ohm;
83 }
84
ntc_get_temp_mc(const struct ntc_type * type,unsigned int ohm)85 int32_t ntc_get_temp_mc(const struct ntc_type *type, unsigned int ohm)
86 {
87 int low, high;
88 int temp;
89
90 ntc_lookup_comp(type, ohm, &low, &high);
91 /*
92 * First multiplying the table temperatures with 1000 to get to
93 * millicentigrades (which is what we want) and then interpolating
94 * will give the best precision.
95 */
96 temp = ntc_fixp_linear_interpolate(type->comp[low].ohm, type->comp[low].temp_c * 1000,
97 type->comp[high].ohm, type->comp[high].temp_c * 1000,
98 ohm);
99 return temp;
100 }
101
