On 19 April 2016 at 23:16, Nick Hill <n...@nickhill.co.uk> wrote:
> #define THERMISTOR_BETA 3380
> #define THERMISTOR_R0 10000
> #define THERMISTOR_T0 (298.15)
> #define Rinf (THERMISTOR_R0*pow(2.718,(-1*(THERMISTOR_BETA/THERMISTOR_T0))))
>
> With Cosmic compiler, any reference to Rinf pulls a large chunk of
> math library into the executable.
Incidentally, this can't be taken as a usage scenario where floating
point libraries can otherwise be excluded completely from the runtime
since the Rinf variable is used in conjunction with a runtime variable
in a natural logarithm expression in order to derive temperature.
The working function looks like this.Taking ADC output then providing
temperature in centigrade. RES_GND defines whether the potential
divider resistor (COMP_RESISTOR) is connected to Vcc or GND. I have
tested this in circuit with an actual thermistor and I find the output
is a close match to measured temperature.
float calculate_temp(short ADC_reading){
short R1;
float temperature;
#if(RES_GND)
R1=((float)(COMP_RESISTOR/(float)(ADC_reading/(float)ADC_MAX_SCALE))-(float)COMP_RESISTOR);
#else
R1=((float)COMP_RESISTOR/((float)(ADC_MAX_SCALE-(float)ADC_reading)/(float)ADC_MAX_SCALE))-(float)COMP_RESISTOR;
#endif
temperature=(float)THERMISTOR_BETA/log((float)R1/(float)Rinf);
return (temperature-273.15);
}
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