EM78156E
8-Bit Microcontroller with MASK ROM
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
EM78156E
Rin
C
Fig. 14 External Power-Up Reset Circuit
4.11 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.17 and Fig.18 show how to build a residue-voltage
protection circuit.
Vdd
Vdd
33K
EM78156E
/RESET
Q1
10K
40K
1N4684
Fig. 15 Circuit 1 for the Residue Voltage Protection
26 •
Product Specification (V1.3) 07.29.2004
(This specification is subject to change without further notice)
ELAN [ ELAN MICROELECTRONICS CORP ]
