IBM EM78P259N/260N manual Power-on Considerations, Programmable WDT Time-out Period

EM78P259N/260N

8-Bit Microprocessor with OTP ROM

6.12Power-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 (18ms5 or 4.5ms6). 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.

Vdd
EM78P259N	R
EM78P260N	D
/RESET
	Rin
	C

Fig. 6-21 External Power-on Reset Circuit

5VDD=5V, WDT time-out period = 16.5ms ± 30%. VDD=3V, WDT time-out period = 18ms ± 30%.

6VDD=5V, WDT time-out period = 4.2ms ± 30%. VDD=3V, WDT time-out period = 4.5ms ± 30%.

Product Specification (V1.2) 05.18.2007 • 65

(This specification is subject to change without further notice)