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Supply, reset and clock management ST72321xx-Auto
44/243 Doc ID 13829 Rev 1
Figure 14. Low voltage detector versus reset
6.6.2 Auxiliary voltage detector (AVD)
The auxiliary voltage detector function (AVD) is based on an analog comparison between a
V
IT-(AVD)
and V
IT+(AVD)
reference value and the V
DD
main supply or the external EVD pin
voltage level (V
EVD
). The V
IT-
reference value for falling voltage is lower than the V
IT+
reference value for rising voltage in order to avoid parasitic detection (hysteresis).
The output of the AVD comparator can be read directly 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.
Monitoring the V
DD
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 21.1.1: Flash configuration on page 223).
If the AVD interrupt is enabled, an interrupt is generated when the voltage crosses the
V
IT+(AVD)
or V
IT-(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 microcontroller. See Figure 15.
The interrupt on the rising edge is used to inform the application that the V
DD
warning state
is over.
If the voltage rise time t
rv
is less than 256 or 4096 CPU cycles (depending on the reset delay
selected by option byte), no AVD interrupt will be generated when V
IT+(AVD)
is reached.
If t
rv
is greater than 256 or 4096 cycles
two AVD interrupts will be received if the AVD interrupt is enabled before the V
IT+(AVD)
threshold is reached: the first when the AVDIE bit is set, and the second when the
threshold is reached.
only one AVD interrupt will occur if the AVD interrupt is enabled after the V
IT+(AVD)
threshold is reached.
V
DD
V
IT+
RESET
V
IT-
V
hys
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