AN1709 datasheet(19/38 Pages) STMICROELECTRONICS

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EMC DESIGN GUIDE FOR ST MICROCONTROLLERS

Figure 12. Max. Operating frequency vs. supply voltage.

The minimum value of each supply range is limited by the LVD threshold which can be config-

ured at one of three levels (low, medium or high) depending on the application.

3.1.2 Auxiliary Voltage Detector (AVD)

Like the LVD, this feature improves EMS performance by ensuring that the microcontroller be-

haves safely when the power supply is disturbed by external noise.

The AVD feature can be used only if the LVD is activated. AVD has also different levels

(around 200mV above LVD levels), enabling a early warning before the reset caused by the

LVD. Then, when AVD threshold is crossed, an interrupt is generated, requesting for example

some user action or preparing the application to shut down in the interrupt routine until the

power supply returns to the correct level for the device (refer to the product datasheet).

Example: If fCPU is between 8 MHZ and 16 MHZ the min. working level is 3.5 volt.

The Voltage Detector function (AVD) is based on an analog comparison between a VIT- and

VIT+ reference value and the VDD main supply. The VIT- reference value for falling voltage is

lower than the VIT+ reference value for rising voltage in order to avoid parasitic detection (hys-

teresis).

The output of the AVD comparator is directly readable by the application software through a

real time status bit (VDF). This bit is read only.

The AVD voltage threshold value is relative to the selected LVD threshold configured by op-

tion byte (see Microcontroller spec).

Vsupply

Freq.

8Mhz

16Mhz

LVD high

LVD med.

LVD low

Absolute working

window

Safe 16Mhz

area

3.8V Reset

issued

Safe 16MHz

area.

3.5V Reset

Safe 8Mhz

area

issued

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