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thermistor.c
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thermistor.c
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#include <unistd.h>
#include "beaglebone.h"
#include "thermistor.h"
struct conversion_entry
{
unsigned int adc_value;
double celsius;
};
// {AIN, C} for beta=4066
static const struct conversion_entry thermistor_100k[] = {
{ 3617, 0.0 },
{ 3612, 5.0 },
{ 3604, 10.0 },
{ 3595, 15.0 },
{ 3584, 20.0 },
{ 3571, 25.0 },
{ 3555, 30.0 },
{ 3536, 35.0 },
{ 3513, 40.0 },
{ 3486, 45.0 },
{ 3455, 50.0 },
{ 3419, 55.0 },
{ 3377, 60.0 },
{ 3330, 65.0 },
{ 3276, 70.0 },
{ 3216, 75.0 },
{ 3149, 80.0 },
{ 3075, 85.0 },
{ 2994, 90.0 },
{ 2907, 95.0 },
{ 2814, 100.0 },
{ 2715, 105.0 },
{ 2612, 110.0 },
{ 2504, 115.0 },
{ 2393, 120.0 },
{ 2279, 125.0 },
{ 2164, 130.0 },
{ 2049, 135.0 },
{ 1935, 140.0 },
{ 1822, 145.0 },
{ 1711, 150.0 },
{ 1604, 155.0 },
{ 1500, 160.0 },
{ 1400, 165.0 },
{ 1305, 170.0 },
{ 1215, 175.0 },
{ 1130, 180.0 },
{ 1049, 185.0 },
{ 974, 190.0 },
{ 904, 195.0 },
{ 838, 200.0 },
{ 777, 205.0 },
{ 720, 210.0 },
{ 668, 215.0 },
{ 619, 220.0 },
{ 574, 225.0 },
{ 532, 230.0 },
{ 494, 235.0 },
{ 459, 240.0 },
{ 396, 250.0 },
{ 369, 255.0 },
{ 343, 260.0 },
{ 298, 270.0 },
{ 278, 275.0 },
{ 260, 280.0 },
{ 243, 285.0 },
{ 227, 290.0 },
{ 212, 295.0 },
{ 199, 300.0 },
};
/*
* Calibration table derived from calculated transfer function
* with values from the EPCOS B57560G104F thermistor datasheet
* and the BeBoPr Cape schematics.
*/
static const struct conversion_entry epcos_b5760g104f[] = {
{ 3635, -20.0 },
{ 3630, -10.0 },
{ 3622, 0.0 },
{ 3609, 10.0 },
{ 3590, 20.0 },
{ 3561, 30.0 },
{ 3520, 40.0 },
{ 3463, 50.0 },
{ 3340, 60.0 },
{ 3287, 70.0 },
{ 3089, 85.0 },
{ 2832, 100.0 },
{ 2072, 135.0 },
{ 1235, 175.0 },
{ 920, 195.0 },
{ 681, 215.0 },
{ 543, 230.0 },
{ 435, 245.0 },
{ 351, 260.0 },
{ 305, 270.0 },
{ 265, 280.0 },
{ 232, 290.0 },
{ 203, 300.0 },
};
/*
* Calibration table derived from calculated transfer function
* with measured values from a recycled 330kOhm oven thermistor
* and the BeBoPr Cape schematics.
*/
static const struct conversion_entry my_330k_bed_thermistor[] = {
{ 3650, 0.0 }, // forged value to prevent out of range result
{ 3626, 21.0 },
{ 3601, 35.0 },
{ 3597, 37.0 },
{ 3571, 47.0 },
{ 3546, 54.0 },
{ 3534, 57.0 },
{ 3519, 60.0 },
{ 3485, 66.0 },
{ 3452, 71.0 },
{ 3412, 76.0 },
{ 3393, 78.4 },
{ 3375, 80.4 },
{ 3357, 82.1 },
{ 3342, 83.7 },
{ 3328, 85.2 },
// the following values are not measured but extrapolated from an exponential fit
{ 3240, 90.0 },
{ 3157, 95.0 },
{ 3061, 100.0 },
{ 2949, 105.0 },
{ 2822, 110.0 },
{ 2678, 115.0 },
{ 2520, 120.0 },
{ 2348, 125.0 },
{ 2165, 130.0 },
{ 1975, 135.0 },
{ 1782, 140.0 },
{ 1589, 145.0 },
{ 1401, 150.0 },
};
static int convert_using_table( const struct conversion_entry* table, int entries, int adc, double* celsius)
{
// TODO: implement short to GND and short to +12V detection
if (celsius != NULL) {
int ix;
for (ix = 0 ; ix < entries ; ++ix) {
int delta_adc = adc - table[ ix].adc_value;
if (delta_adc > 0) {
if (ix > 0) {
double celsius_ix = table[ ix].celsius;
unsigned int adc_ix = table[ ix].adc_value;
double delta_celsius = celsius_ix - table[ ix - 1].celsius;
unsigned int delta_adc = table[ ix - 1].adc_value - adc_ix;
double rc = delta_celsius / delta_adc;
*celsius = celsius_ix - (adc - adc_ix) * rc;
} else {
/*
* Before first entry, assume open circuit (sensor fault)
* Return very high temperature to turn heaters off
*/
*celsius = 777.0; // lll (low)
}
break;
} else if (delta_adc == 0) {
*celsius = table[ ix].celsius;
break;
} else if (ix == entries) {
/*
* After last entry, assume short circuit (sensor fault)
* Return very high temperature to turn heaters off
*/
*celsius = 444.0; // hhh (high)
}
}
return 0;
}
return -1;
}
int bone_thermistor_100k( int adc, double* celsius)
{
return convert_using_table( thermistor_100k, NR_ITEMS( thermistor_100k), adc, celsius);
}
int bone_epcos_b5760g104f( int adc, double* celsius)
{
return convert_using_table( epcos_b5760g104f, NR_ITEMS( epcos_b5760g104f), adc, celsius);
}
int bone_bed_thermistor_330k( int adc, double* celsius)
{
return convert_using_table( my_330k_bed_thermistor, NR_ITEMS( my_330k_bed_thermistor), adc, celsius);
}