ST72F321 datasheet(28/189 Pages) STMICROELECTRONICS

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ST72321

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SYSTEM INTEGRITY MANAGEMENT (Cont’d)

6.4.2 Auxiliary Voltage Detector (AVD)

The Voltage Detector function (AVD) is based on

an analog comparison between a VIT-(AVD) and

VIT+(AVD) reference value and the VDD main sup-

ply or the external EVD pin voltage level (VEVD).

The VIT- reference value for falling voltage is lower

than the VIT+ reference value for rising voltage in

order to avoid parasitic detection (hysteresis).

The output of the AVD comparator is directly read-

able by the application software through a real

time status bit (AVDF) in the SICSR register. This

bit is read only.

Caution: The AVD function is active only if the

LVD is enabled through the option byte.

6.4.2.1 Monitoring the VDD Main Supply

This mode is selected by clearing the AVDS bit in

the SICSR register.

The AVD voltage threshold value is relative to the

selected LVD threshold configured by option byte

(see section 14.1 on page 175).

If the AVD interrupt is enabled, an interrupt is gen-

erated when the voltage crosses the VIT+(AVD) or

VIT-(AVD) threshold (AVDF bit toggles).

In the case of a drop in voltage, the AVD interrupt

acts as an early warning, allowing software to shut

down safely before the LVD resets the microcon-

troller. See Figure 16.

The interrupt on the rising edge is used to inform

the application that the VDD warning state is over.

If the voltage rise time trv is less than 256 or 4096

CPU cycles (depending on the reset delay select-

ed by option byte), no AVD interrupt will be gener-

ated when VIT+(AVD) is reached.

If trv is greater than 256 or 4096 cycles then:

– If the AVD interrupt is enabled before the

VIT+(AVD) threshold is reached, then 2 AVD inter-

rupts will be received: the first when the AVDIE

bit is set, and the second when the threshold is

reached.

– If the AVD interrupt is enabled after the VIT+(AVD)

threshold is reached then only one AVD interrupt

will occur.

Figure 16. Using the AVD to Monitor VDD (AVDS bit=0)

VDD

VIT+(AVD)

VIT-(AVD)

AVDF bit

RESET VALUE

IF AVDIE bit = 1

Vhyst

AVD INTERRUPT

REQUEST

INTERRUPT PROCESS

VIT+(LVD)

VIT-(LVD)

LVD RESET

Early Warning Interrupt

(Power has dropped, MCU not

not yet in reset)

trv VOLTAGE RISE TIME

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