ST72340 datasheet(33/191 Pages) STMICROELECTRONICS

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ST72340, ST72344, ST72345

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7.5 SYSTEM INTEGRITY MANAGEMENT (SI)

The System Integrity Management block contains

the Low Voltage Detector (LVD) and Auxiliary Volt-

age Detector (AVD) functions. It is managed by

the SICSR register.

Note: A reset can also be triggered following the

detection of an illegal opcode or prebyte code. Re-

fer to Section 12.2.1 on page 149 for further de-

tails.

7.5.1 Low Voltage Detector (LVD)

The Low Voltage Detector function (LVD) gener-

ates a static reset when the VDD supply voltage is

below a VIT- reference value. This means that it

secures the power-up as well as the power-down

keeping the ST7 in reset.

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 run-

ning 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 18.

The LVD is an optional function which can be se-

lected by option byte.

Note: LVD Threshold Configuration

The voltage threshold can be configured by option

byte to be low, medium or high. The configuration

should be chosen depending on the fOSC and VDD

parameters in the application. When correctly con-

figured, the LVD ensures safe power-on and pow-

er-off conditions for the microcontroller without us-

ing any external components.

To determine which LVD thresholds to use:

– Define the minimum operating voltage for the ap-

plication VAPP(min)

– Refer to the Electrical Characteristics section to

get the minimum operating voltage for the MCU

at the application frequency VDD(min) .

– Select the LVD threshold that ensures that the

internal RESET is released at VAPP(min) and ac-

tivated at VDD(MCUmin)

During a Low Voltage Detector Reset, the RESET

pin is held low, thus permitting the MCU to reset

other devices.

Figure 18. Low Voltage Detector vs Reset

VDD

VIT+(LVD)

RESET

VIT-((LVD)

Vhys

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