ST72334J datasheet(25/125 Pages) STMICROELECTRONICS

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ST72334J/N, ST72314J/N, ST72124J

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4.1 LOW VOLTAGE DETECTOR (LVD)

To allow the integration of power management

features in the application, the Low Voltage Detec-

tor function (LVD) generates a static reset when

the VDD supply voltage is below a VLVDf reference

value. This means that it secures the power-up as

well as the power-down keeping the ST7 in reset.

The VLVDf reference value for a voltage drop is

lower than the VLVDr 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:

–VLVDr when VDD is rising

–VLVDf when VDD is falling

The LVD function is illustrated in the Figure 13.

Provided the minimum VDD value (guaranteed for

the oscillator frequency) is below VLVDf, the MCU

can only be in two modes:

– under full software control

– in static safe reset

In these conditions, secure operation is always en-

sured for the application without the need for ex-

ternal reset hardware.

During a Low Voltage Detector Reset, the RESET

pin is held low, thus permitting the MCU to reset

other devices.

Notes:

1) the LVD allows the device to be used without any exter-

nal RESET circuitry.

2) three different reference levels are selectable through

the OPTIO N BYTE according to the application require-

ment.

LVD application note

Application software can detect a reset caused by

the LVD by reading the LVDRF bit in the CRSR

This bit is set by hardware when a LVD reset is

generated and cleared by software (writing zero).

Figure 13. Low Voltage Detector vs Reset

VDD

VLVDr

RESET

VLVDf

HYSTE RESIS

VLVDhyst

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