Operation

STM705, STM706, STM707, STM708, STM813L

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Doc ID 10520 Rev 9

3.4

Watchdog output (STM705/706/813L)

exceeds the reset threshold. WDO may be used to generate a reset pulse by connecting it

to the MR input.

3.5

Power-fail input/output

The power-fail input (PFI) is compared to an internal reference voltage (independent from

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

or the microprocessor drops below the minimum operating voltage.

PFO may be connected to MR on the STM703/704/818 so that a low voltage on PFI will

generate a reset output.

3.6

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

AI08835

STMXXX

RST

R1