ST7LITE0X datasheet(33/125 Pages) STMICROELECTRONICS

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8.4 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 79 for further details.

8.4.1 Low Voltage Detector (LVD)

The Low Voltage Detector function (LVD) gener-

ates a static reset when the VDD supply voltage is

below a VIT-(LVD) reference value. This means that

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

keeping the ST7 in reset.

The VIT-(LVD) reference value for a voltage drop is

lower than the VIT+(LVD) reference value for power-

on in order to avoid a parasitic reset when the

MCU starts running and sinks current on the sup-

ply (hysteresis).

The LVD Reset circuitry generates a reset when

VDD is below:

–VIT+(LVD)when VDD is rising

–VIT-(LVD) when VDD is falling

The LVD function is illustrated in Figure 18.

The voltage threshold can be configured by option

byte to be low, medium or high. See section 15.1

on page 114.

Provided the minimum VDD value (guaranteed for

the oscillator frequency) is above VIT-(LVD), 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:

The LVD is an optional function which can be se-

lected by option byte. See section 15.1 on page

114.

It allows the device to be used without any external

RESET circuitry.

If the LVD is disabled, an external circuitry must be

used to ensure a proper power-on reset.

It is recommended to make sure that the VDD sup-

ply voltage rises monotonously when the device is

exiting from Reset, to ensure the application func-

tions properly.

Caution: If an LVD reset occurs after a watchdog

reset has occurred, the LVD will take priority and

will clear the watchdog flag.

Figure 18. Low Voltage Detector vs Reset

VDD

VIT+(LVD)

RESET

VIT-(LVD)

Vhys

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