AN975 Datasheet, PDF (2/11 Pages) STMicroelectronics – As part of a process of continuous improvement

UPGRADING FROM ST625X/6XB TO ST625X/6XC
1 SILICON CHANGES
1.1 NEW FEATURES
1.1.1 LOW VOLTAGE DETECTOR (LVD)
ST625X/6XC now features an on-chip Low Voltage Detector which is enabled by setting a bit
in the option bytes (refer to the Option Bytes section in the device data sheet).
The LVD allows the device to be used without any external RESET circuitry. In this case, the
RESET pin should be left unconnected. When the LVD is not activated, an external circuit is
mandatory to ensure correct Power On Reset operation. For more details, please refer to the
application note AN669.
The LVD generates a static Reset when the supply voltage is below a reference value. This
means that it secures the power-up as well as the power-down keeping the ST6 in reset.
The V_IT- reference value for a voltage drop is lower than the V_IT+ 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+ when V_DD is rising
– V_IT- when V_DD is falling
The LVD function is illustrated in Figure 1.
If the LVD is enabled, the MCU can be in only one of two states:
– Over the input threshold voltage, it is running under full software control
– Below the input threshold voltage, it is in static safe reset
In these conditions, secure operation is guaranteed without the need for external reset hard-
ware.
During a Low Voltage Detector Reset, the RESET pin is held low, thus permitting the MCU to
reset other devices.
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