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ST7LITEUS2, ST7LITEUS5
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7.4
System integrity management (SI)
The System Integrity Management block contains the low voltage detector (LVD) and
Auxiliary Voltage Detector (AVD) functions. It is managed by the SICSR register.
Note:
A reset can also be triggered following the detection of an illegal opcode or prebyte code.
Refer to
Section 11.2.1: Illegal opcode reset
for further details.
7.4.1
Low voltage detector (LVD)
The low voltage detector function (LVD) generates a static reset when the V
DD
supply
voltage is below a V
IT-(LVD)
reference value. This means that it secures the power-up as well
as the power-down keeping the ST7 in reset.
The V
IT-(LVD)
reference value for a voltage drop is lower than the V
IT+(LVD)
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 V
DD
is below:
●
V
IT+(LVD)
when V
DD
is rising
●
V
IT-(LVD)
when V
DD
is falling
The LVD function is illustrated in
The voltage threshold can be configured by option byte to be low, medium or high. See
Provided the minimum V
DD
value (guaranteed for the oscillator frequency) is above V
IT-(LVD)
,
the MCU can only be in two modes:
●
Under full software control
●
In static safe reset
In these conditions, secure operation is always ensured for the application without the need
for external reset hardware.
During a low voltage detector reset, the RESET pin is held low, thus permitting the MCU to
reset other devices.
Note:
Use of LVD with capacitive power supply: with this type of power supply, if power cuts occur
in the application, it is recommended to pull V
DD
down to 0V to ensure optimum restart
conditions. Refer to circuit example in
and note 4.
The LVD is an optional function which can be selected by option byte. See
. It allows the device to be used without any external RESET circuitry. If the LVD is
disabled, an external circuitry must be used to ensure a proper Power-on reset.
It is recommended to make sure that the V
DD
supply voltage rises monotonously when the
device is exiting from Reset, to ensure the application functions properly.
Make sure the right combination of LVD and AVD thresholds is used as LVD and AVD levels
are not correlated. Refer to
and
for more details.
Caution:
If an LVD reset occurs after a watchdog reset has occurred, the LVD will take priority and will
clear the watchdog flag.
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