STM704 datasheet(17/42 Pages) STMICROELECTRONICS

17 / 42 page

STM690, STM704, STM795, STM802, STM804, STM805, STM806

Operation

Doc ID 10519 Rev 9

17/42

Figure 12.

Chip enable waveform (STM795)

2.8

Power-fail input/output (NOT available on STM795)

The Power-Fail Input (PFI) is compared to an internal reference voltage (independent from

the VRST comparator). If PFI is less than the power-fail threshold (VPFI), the Power-Fail

Output (PFO) will go low. This function is intended for use as an undervoltage detector to

signal a failing power supply. Typically PFI is connected through an external voltage divider

(see Figure 10) to either the unregulated DC input (if it is available) or the regulated output

of the VCC regulator. The voltage divider can be set up such that the voltage at PFI falls

below VPFI several milliseconds before the regulated VCC input to the STM690/704/795/802/

804/805/806 or the microprocessor drops below the minimum operating voltage.

During battery backup, the power-fail comparator is turned off and PFO goes (or remains)

low (see Figure 13). This occurs after VCC drops below VSW (2.4 V). When power returns,

the power-fail comparator is enabled and PFO follows PFI. If the comparator is unused, PFI

should be connected to VSS and PFO left unconnected. PFO may be connected to MR on

the STM704/806 so that a low voltage on PFI will generate a reset output.

2.9

Applications information

These supervisor circuits are not short-circuit protected. Shorting VOUT to ground -

excluding power-up transients such as charging a decoupling capacitor - destroys the

device. Decouple both VCC and VBAT pins to ground by placing 0.1 µF capacitors as close to

the device as possible.

AI08855c

RST

VCC

VRST

VBAT

ECON

trec

½ trec

10 µs

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