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EM78P259N/260N
8-Bit Microprocessor with OTP ROM
Product Specification (V1.2) 05.18.2007
• 65
(This specification is subject to change without further notice)
6.12 Power-on Considerations
Any microcontroller is not warranted to start operating properly before the power supply
stabilizes in its steady state. The EM78P259N/260N POR voltage range is 1.9V ~ 2.1V.
Under customer application, when power is switched OFF, Vdd must drop below 1.9V
and remains at OFF state for 10μs before power can be switched ON again.
Subsequently, the EM78P259N/260N will reset and work normally. The extra external
reset circuit will work well if Vdd rises fast enough (50ms or less). However, under
critical applications, extra devices are still required to assist in solving power-on
problems.
6.12.1 Programmable WDT Time-out Period
The Option word (WDTPS) is used to define the WDT time-out period (18ms
5
or
4.5ms
6
). Theoretically, the range is from 4.5ms or 18ms. For most crystal or ceramic
resonators, the lower the operation frequency is, the longer is the required set-up time.
6.12.2 External Power-on Reset Circuit
The circuits shown in the following figure implement an external RC to produce a reset
pulse. The pulse width (time constant) should be kept long enough to allow Vdd to
reach the minimum operating voltage. This circuit is used when the power supply has a
slow power rise time. Because the current leakage from the /RESET pin is about ±5μA,
it is recommended that R should not be greater than 40K. This way, the voltage at Pin
/RESET is held below 0.2V. The diode (D) acts as a short circuit at power-down. The
“C” capacitor is discharged rapidly and fully. Rin, the current-limited resistor, prevents
high current discharge or ESD (electrostatic discharge) from flowing into Pin /RESET.
EM78P259N
EM78P260N
/RESET
Vdd
D
R
Rin
C
Fig. 6-21 External Power-on Reset Circuit
5
VDD=5V, WDT time-out period = 16.5ms ± 30%.
VDD=3V, WDT time-out period = 18ms ± 30%.
6
VDD=5V, WDT time-out period = 4.2ms ± 30%.
VDD=3V, WDT time-out period = 4.5ms ± 30%.