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CALIBRATING THE RC OSCILLATOR OF THE ST7FLITE0 MCU USING THE MAINS

2 POWER SUPPLY AND TIMEBASE DELIVERY CIRCUIT

The following figures show circuits which will provide 5V DC to the ST7LITE0 and protect the

input capture from overcurrent. If no power supply is needed, the only component to keep is

the resistor on the LTIC input, which is mainly to protect from overcurrent.

2.1 BASIC CIRCUIT

This circuit contains a capacitive power supply which converts the 220V/50Hz of the mains, as

well as the 110V/60Hz of the US mains, into 5V DC.

Warning: be aware that this kind of power supply can’t be used if there are big current varia-

tions.

It also inputs 220V/50Hz to the Lite Timer Input Capture pin (LTIC/PA0) protected by resistor

R2.

The incoming alternating signal on the LTIC input pin is 220V/50Hz. Because of the clamping

diode on the input of the ST7FLITE0, the input signal can be considered as a 0-5V square

signal.

Figure 5. Power supply and timebase delivery circuit diagram

The maximum current available in the microcontroller depends on the C2 value. Table 1 gives

the maximum average current versus the capacitor value. The average current follows the

equation below:

GND

LTIC

ST7Lite0

zener

5.6V

C1

220uF/16V

1N4148

C2

220nF/400V

R1 47R/0.5W

R2 470k/0.5W

C2

(220V/50Hz)

Mains

Imax

Vmax 2 fC

⋅⋅ ⋅

=