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CALIBRATING THE RC OSCILLATOR OF THE ST7FLITE0 MCU USING THE MAINS

Figure 2. Using the Timer Input Capture to Measure the Mains Frequency

To measure the frequency, the software uses the Lite Timer input capture (LTIC) so that on

each edge of the mains the value of the free running counter is stored as shown in Figure 2.

Then the microcontroller calculates the elapsed time between the two edges of the mains.

This time is given by the following equation:

where nbover represents the number of counter overflows during the measurement, capture 1

and capture 2 are the values captured on the free running counter when an edge occurs on

the mains and F9h is the overflow value of the free running counter.

If the RC oscillator frequency is equal to 1 MHz, the result time will be 138h for European

standard (220V/50Hz) or 104h for US standard mains (110V/60Hz), so these are the refer-

ence values.

This measurement result is compared to the reference value and, depending on the result of

the comparison, the microcontroller adds to or subtracts from the current RCCR value.

Free-running

Counter

Mains

Capture 1

Capture 2

Overflow

0h

F9h

time

nbover

F

–

+

×

=