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EMC DESIGN GUIDE FOR ST MICROCONTROLLERS

The VIT- reference value for a voltage drop is lower than the VIT+ reference value for power-on

in order to avoid a parasitic reset when the MCU starts running and sinks current on the supply

(hysteresis).

The LVD Reset circuitry generates a reset when VDD is below:

–VIT+ when VDD is rising

–VIT- when VDD is falling

The LVD function is illustrated in Figure 11..

The voltage threshold can be configured by option byte to be low, medium or high.

Provided the minimum VDD value (guaranteed for the oscillator frequency) is above VIT-, the

MCU can only be in two modes:

– under full software control

– in static safe reset

In these conditions, secure operation is always ensured for the application without the need

for external reset hardware.

During a Low Voltage Detector Reset, the RESET pin is held low, thus permitting the MCU to

reset other devices.

Notes:

The LVD allows the device to be used without any external RESET circuitry.

The LVD is an optional function which can be selected by option byte. Refer to product spec-

ification.

Figure 11. Low Voltage Detector vs Reset

VDD

VIT+

RESET

VIT-

Vhys

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