AN2017 datasheet(2/10 Pages) STMICROELECTRONICS

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DIFFERENT WAYS TO RESET ST7 MICROCONTROLLER

1 INTERNAL LOW VOLTAGE DETECTOR (LVD) RESET

The LVD function continuously monitors the supply voltage and generates a reset when the

VDD is below:

– VIT+(LVD), Reset Release voltage when VDD is rising

– VIT-(LVD), Reset Generation voltage when VDD is falling

This means it secures power-up as well as power-down keeping ST7 in reset. The LVD func-

tion is illustrated in Figure 1.

The difference between VIT+(LVD) and VIT-(LVD) is called LVD hysteresis. The VIT-(LVD) refer-

ence value for a voltage drop is lower than the VIT+(LVD) reference value for power-on in order

to avoid parasitic reset when the MCU starts running and sinks current on supply.

Figure 1. LVD Vs Reset

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

is an optional function which can be selected through option byte and it allows the device to be

used without any External Reset circuitry. During a LVD reset, the RESET pin is held low, thus

permitting the MCU to reset other devices.

After the device is powered on the supply voltage starts increasing. when the supply voltage

reaches around 1.1V, the internal pull-up on the RESET pin is enabled and the core is initial-

ized. After the VDD reaches 1.8V, the option bytes are enabled, LVD is enabled by the option

bytes (if selected) and the device is in Run mode. The reset will be released at the reset re-

lease threshold: high, medium or low depending upon the selection through option bytes.

VHysteresis

VIT+(LVD

VIT-(LVD)

RESET

VDD

AN2017 Datasheet(PDF) 2 Page - STMicroelectronics