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CALIBRATING THE RC OSCILLATOR OF THE ST7FLITE0 MCU USING THE MAINS

In the case above, C2 is equal to 220nF so the available current is limited to 4.9 mA in the

case of a European mains. To have the same current levels in the case of the US mains

(110V/60Hz), C2 must be multiplied by two. A 440nF capacitor will limit the current to 4.9 mA.

For the US standard, R2 must be divided by two in order not to limit the current too much on

the LTIC input. A 220k resistor is enough in this case.

Table 1. Maximum MCU Current

2.2 HARDWARE PROTECTION

To prevent bad measurements due to noisy mains, a filter can be added between the mains

and the input capture of the LITE0. The following figure shows one example of a filter. This

filter is a pass band centered on the mains frequency in order to reject all frequency which

could be understood by the microcontroller as a mains edge.

Be aware that this is just a second order filter and that this may not be enough if the mains is

really noisy. Any kind of filter can be added on the LTIC.

Figure 6. Band pass filter

The pass band filter above must be tuned to mains frequency. The value of the resistors for

this filter is given in the table below.

Table 2. resistors values

CAPACITOR C2

MAXIMUM CURRENT

220nF

4.9mA

330nF

7.3mA

470nF

10.4mA

680nF

15mA

1uF

22.1mA

resistors

50Hz/220V

60Hz/110V

R1

6.8K/0.5W

5.6K/0.5W

R2

6.8K/0.5W

5.6K/0.5W

R3

470K/0.5W

220K/0.5W

C1

R1

R2

R3

C2

C2

LTIC

mains

470nF/400V

470nF/400V

470nF/250V