Thermistor Lookup Table Generator

This python script is used to generate a ready-to-compile C source and header file containing a thermistor temperature calculation algorithm, using a lookup table method. It is intended for use by small resource-constrained microcontrollers.

LICENSE: MIT

How to Use

1. create and edit json file with your table parameters (see example.json)
2. run the generator script
3. copy the generated source files (.c and .h) to your project
4. call the function from your program when you need to compute the temperature from ADC value

Assumed Circuit

            Thermistor
    Vref ----/\/\/\/----+
                        |
                        +----> to ADC input
                        |
    GND  ----/\/\/\/----+
            Rpulldown

JSON File

Field	Description
board	name of your board or project (cosmetic)
thermistor	thermistor part number (cosmetic)
Tstart	lowest temperature of table
Tstop	highest temperature of table (exclusive)
Tstep	temperature step size
Tnominal	thermistor nominal temperature (from data sheet)
Rnominal	thermistor nominal resistance (from data sheet)
Rpulldown	value of the pull-down resistor
beta	thermistor B value (from data sheet)
counts	max counts of the ADC

NOTES:

1. All temperatures are in C.
2. Cosmetic fields are shown in generated comments but have no other function.
3. The size of the table is adjusted by tweaking Tstart, Tstop, and Tstep. You can adjust the tradeoff between how well the curve fits and how much memory is needed for the table.
4. For values outside the range of the lookup table, the algorithm interpolates linearly, using the end segments. The error of the calculated temperature will increase the further the actual measurement is outside the table range.

Generating the Files

Assumes python3.

./generator myboard.json

Using the Files

Add the source files to your project. Include the header where needed. Call the function when needed to compute temperature from ADC value.

API

API	Definition
prototype	int16_t adc_to_temp(uint16_t adc);
parm adc	unsigned 16-bit ADC value for thermistor circuit voltage
returns	signed 16-bit temperature in C

Usage

#include <stdint.h>
#include "thermistor_table.h"
...

...
uint16_t adcval = read_adc(TEMP_SENSOR_CHANNEL);
int16_t temp_in_C = adc_to_temp(adcval);
...

Testing

You can do a quick check of the lookup table using the simple test.c program. Edit the source of test.c to cover the range you want to check. Then compile and run with:

gcc -o test test.c thermistor_table.c
./test

It will print a table. You can examine the output and verify the ADC to temperature curve is correct.