1. Have a thermistor, data whose dataset is available not anything else from company except B value. Attached is data.
2. Trying to make a equation for it and implement for 8 bit MCU, usin IAR IDE.
3. First I made this code, steinhart thermistor equation. calculated values of A,B,C from this site: https://www.thinksrs.com/downloa... . Put three values at -20C, 25C & 100C and get A,B,C
uint8_t get_ntc_temperature_4(float32_t resistance , float32_t *tempearture_c)
{
uint8_t err = 0U;
float32_t x;
float32_t x2;
float32_t x3;
if( (resistance <= 956040.0f) && (resistance >= 6683.0f) )
{
x = logf(resistance);
x2 = x * x;
x3 = x2 * x;
x = 0.000641295533f + (0.0002233488434f * x) + (0.0000000926090998f * x3);
x = 1.0f / x;
*tempearture_c = x - 273.15f;
}
else
{
err = 1U;
}
return err;
}
4. Then made equation using curve fit tool and below is code:
uint8_t get_ntc_temperature_2(float32_t resistance , float32_t *tempearture_c)
{
uint8_t err = 0U;
float32_t c0;
float32_t c1;
float32_t c2;
if( (resistance <= 956040.0f) && (resistance >= 482170.0f) )
{
c0 = 14.5029518320319131f;
c1 = -0.000057344817030149406f;
c2 = 0.000000000021788855747503695f;
}
else if(resistance >= 250140.0f)
{
c0 = 29.3136847463533279f;
c1 = -0.000119496884518122221f;
c2 = 0.0000000000876747221220605721f;
}
else if(resistance >= 130230.0f)
{
c0 = 45.3985586636038647f;
c1 = -0.000249401966454676711f;
c2 = 0.000000000352768556658954028f;
}
else if(resistance >= 71230.0f)
{
c0 = 62.1448935476119572f;
c1 = -0.000506560472110208432f;
c2 = 0.00000000135019658772898851f;
}
else if(resistance >= 39200.0f)
{
c0 = 79.7753263268828902f;
c1 = -0.00100556530593662709f;
c2 = 0.00000000491385322365418885f;
}
else if(resistance >= 21830.0f)
{
c0 = 98.8979251125107339f;
c1 = -0.00198872984475097995f;
c2 = 0.0000000176669521860767602f;
}
else if(resistance >= 12750.0f)
{
c0 = 118.809289117525665f;
c1 = -0.00381338043851388829f;
c2 = 0.0000000598289053823529209f;
}
else if(resistance >= 7327.0f)
{
c0 = 140.313484336487619f;
c1 = -0.00722784306211046376f;
c2 = 0.000000196440993753396293f;
}
else if(resistance >= 6683.0f)
{
c0 = 133.215333415477943f;
c1 = -0.00480709971651089099f;
c2 = 0.0f;
}
else
{
err = 1U;
}
if(0U == err)
{
*tempearture_c = c0 + (c1 * resistance) + (c2 * resistance * resistance);
}
return err;
}
5. Size build on full size optimization on IAR for 8 bit MCU:
a) No function call:
5 138 bytes of readonly code memory
139 bytes of readonly data memory
357 bytes of readwrite data memory
b) With polynomial equation fit:
5 593 bytes of readonly code memory
139 bytes of readonly data memory
357 bytes of readwrite data memory
c) With natural log:
5 820 bytes of readonly code memory
139 bytes of readonly data memory
359 bytes of readwrite data memory
6. Noticed that with polynomial fit extra space is 455 bytes & with natural log its 682 bytes.
7. Error with polynomial fit is +0.069 to -0.098C.
Error with natural log is slightly more though.
8. Any other better method to implement or curve fit this data? Max end function accuracy I am looking is +-0.099C.
PS: In function call, I got coefficients generated in float64_t in curve fit toolbox, put f in the end though.
