ST72344K2 datasheet(49/247 Pages) STMICROELECTRONICS

49 / 247 page

ST72344xx, ST2345xx

Supply, reset and clock management

Doc ID 12321 Rev 6

49/247

7.6.1

Low-voltage detector (LVD)

The low-voltage detector function (LVD) generates 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 running 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 selected 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 configured, the LVD ensures safe power-on

and power-off conditions for the microcontroller without using any external components.

To determine which LVD thresholds to use:

●

Define the minimum operating voltage for the application VAPP(min).

●

Refer to Section 13: Electrical characteristics 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 activated 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

7.6.2

Auxiliary-voltage detector (AVD)

The AVD is used to provide the application with an early warning of a drop in voltage. If

enabled, an interrupt can be generated allowing software to shut down safely before the LVD

resets the microcontroller. See Figure 19.

Note:

The AVD function is active only if the LVD is enabled through the option byte (see

Section 15.1: Option bytes). The activation level of the AVD is fixed at around 0.5 mV above

the selected LVD threshold.

VDD

VIT+(LVD)

RESET

VIT-((LVD)

Vhys

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