DocID024134 Rev 2

15/17

AN4232

Application examples

16

When non-inverting COMP4 crosses the DAC1 level, the formula to compute the

capacitance value is:

where:

–

TriggerTime is the time when the capture event occurred

–

RES is the charge resistance. Its value is 82.5 K 1%

–

Capacitance is the capacitance value of the humidity sensor

Thus, the TriggerTime = RES * Capacitance, the Capacitance = TriggerTime/RES by the

consequent Capacitance = (ICReadValue/SystemCoreClock)/RES

where:

–

ICReadValue is the captured TIM4 counter

–

SystemCoreClock is the frequency at which the device (specifically TIM3 and

TIM4) is running.

Once the capacitance has been computed, the humidity value is given by:

where:

–

X = Capacitance/Capacitance55RH

–

Capacitance55RH is the capacitance at 55%.