EM78P447S
OTP ROM
4.9 Power On Considerations
Any microcontroller is not guaranteed to start and operate properly before the power supply stays at its steady state.
EM78P447S POR voltage range is 1.2V~1.8V. Under customer application, when power is OFF, Vdd must drop to
below 1.2V and remains OFF for 10us before power can be switched ON again. This way, the EM78P447S will
reset and work normally. The extra external reset circuit will work well if Vdd can rise at very fast speed (50 ms or
less). However, under most cases where critical applications are involved, extra devices are required to assist in
solving the power-up problems.
4.10 External Power On Reset Circuit
The circuit shown in Fig.16 implements an external RC to produce the reset pulse. The pulse width (time constant)
should be kept long enough for Vdd to reached minimum operation voltage. This circuit is used when the power
supply has slow rise time. Because the current leakage from the /RESET pin is about ±5µA, it is recommended that
R should not be greater than 40 K. In this way, the /RESET pin voltage is held below 0.2V. The diode (D) acts as a
short circuit at the moment of power down. The capacitor C will discharge rapidly and fully. Rin, the current-limited
resistor, will prevent high current or ESD (electrostatic discharge) from flowing to pin /RESET.
Vdd
R
/RESET
D
EM78P447N
Rin
C
Fig. 13 External Power-Up Reset Circuit
This specification is subject to change without prior notice.
28
06.25.2003 (V1.1)
ELAN [ ELAN MICROELECTRONICS CORP ]
