STM706TM6F datasheet(12/33 Pages) STMICROELECTRONICS

12 / 33 page

Operation

STM706T/S/R, STM706P, STM708T/S/R

12/33

Doc ID 10518 Rev 10

3.5

Power-fail input/output

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 9) 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 STM70x or the micro-

processor drops below the minimum operating voltage.

If the comparator is unused, PFI should be connected to VSS and PFO left unconnected.

PFO may be connected to MR on the STM70x so that a low voltage on PFI will generate

a reset output.

3.6

Ensuring a valid reset output down to VCC = 0 V

When VCC falls below 1 V, the state of the RST output can no longer be guaranteed, and

becomes essentially an open circuit. If a high value pulldown resistor is added to the RST

pin, the output will be held low during this condition. A resistor value of approximately

100 k

Ω will be large enough to not load the output under operating conditions, but still

sufficient to pull RST to ground during this low voltage condition (see Figure 10).

Figure 10.

Reset output valid to ground circuit

AI08844

STM70x

RST

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