ST7L15 datasheet(28/138 Pages) STMICROELECTRONICS

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ST7L15, ST7L19

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SUPPLY, RESET AND CLOCK MANAGEMENT (cont’d)

7.6 SYSTEM INTEGRITY MANAGEMENT (SI)

The System Integrity Management block contains

the Low Voltage Detector (LVD) function. 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 95 for further details.

7.6.1 Low Voltage Detector (LVD)

The Low Voltage Detector (LVD) function 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 to avoid a parasitic reset when the MCU starts

running and sinks current on the supply (hystere-

sis).

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 16.

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 allows the device to be used without any

external RESET circuitry.

The LVD is an optional function which can be se-

lected by the option byte.

Use of LVD with capacitive power supply: With this

type of power supply, if power cuts occur in the ap-

plication, it is recommended to pull VDD down to

0V to ensure optimum restart conditions. Refer to

the circuit example in Figure 95 on page 119 and

note 4.

For the application to function correctly, it is rec-

ommended to make sure that the VDD supply volt-

age rises monotonously when the device is exiting

from RESET.

Figure 16. Low Voltage Detector vs Reset

VDD

VIT+(LVD)

RESET

VIT-(LVD)

Vhys

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