EM78P458/459
OTP ROM
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. 22 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.
VDD
/RESET
R
D
EM78P458
EM78P459
Rin
C
Fig. 22 External Power on Reset Circuit
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. 23 and Fig. 24 show how to build a residue-voltage protection circuit
VDD
VDD
EM78P458
33K
EM78P459
Q1
10K
/RESET
100K
1N4684
Fig. 23
Circuit 1 for the Residue Voltage Protection
This specification is subject to change without prior notice.
49
06.25.2004 (V1.4)
ELAN [ ELAN MICROELECTRONICS CORP ]
