Datasheet

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 V
RST
comparator). If PFI is less than the power-fail threshold (V
PFI
), 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 V
CC
regulator. The voltage divider can be set up such that the voltage at PFI falls
below V
PFI
several milliseconds before the regulated V
CC
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 V
CC
drops below V
SW
(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 V
SS
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 V
OUT
to ground -
excluding power-up transients such as charging a decoupling capacitor - destroys the
device. Decouple both V
CC
and V
BAT
pins to ground by placing 0.1 µF capacitors as close to
the device as possible.
AI08855c
RST
E
V
CC
V
RST
V
BAT
E
CON
t
rec
t
rec
½ t
rec
½ tr
ec
10 µs