Power-On Considerations; External Power On Reset Circuit; Residue-Voltage Protection - Elan EM78P458 User Manual

4.13 Power-on Considerations

Any microcontroller is not warranted to start proper operation before the power supply stabilizes in
steady state.
EM78P458/459 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 EM78P458/459 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.

1. External Power on Reset Circuit

The circuit shown in Fig IV.13.1-1 implements an external RC to produce a reset pulse. The pulse
width (time constant) should be kept long enough to allow Vdd to reach minimum operation voltage.
This circuit is used when the power supply has a slow rise time. Because the current leakage from the
/RESET pin is about ±5µA, it is recommended that R should not be great than 40 K. In this way, the
voltage at Pin /RESET is held below 0.2V. The diode (D) acts as a short circuit at power-down. The
capacitor, C, is discharged rapidly and fully. Rin, the current-limited resistor, prevents high current
discharge or ESD (electrostatic discharge) from flowing into Pin /RESET.

2. Residue-Voltage Protection

When battery is replaced, device power (Vdd) is taken off but residue-voltage remains. The
residue-voltage may trips below Vdd minimum, but not to zero. This condition may cause a poor
power on reset. Fig. 25 and Fig. 26 show how to build a residue-voltage protection circuit
This specification is subject to change without prior notice.
VDD
/RESET
EM78P458
EM78P459
Fig. 24 External Power on Reset Circuit
R
Rin
C
50
EM78P458/459
OTP ROM
D
07.01.2003 (V1.3)