STM802LSLASHM datasheet(16/43 Pages) STMICROELECTRONICS

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Operation

STM690A/692A/703/704/802/805/817/818/819

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Doc ID 10522 Rev 10

Figure 12.

Chip-enable waveform

2.8

Power-fail input/output (NOT available on STM818)

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 12) 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

STM690A/692A/703/704/802/805/817/818/819 Supervisor or before the microprocessor

drops below the minimum operating voltage. This provides several milliseconds of advanced

warning that power is about to fail.

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

(see Figure 13 below and Figure 14). This occurs after VCC drops below 2.4 V (or VSO).

When power returns, PFO is forced high (STM817/819 only), irrespective of VPFI for the

WRITE protect time (trec). At the end of this time, 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 STM703/704/818 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.

AI08803b

RST

VCC

VRST

VBAT

ECON

trec

15µs

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