EM78P5840N/41N/42N
8-Bit Microcontrollers
7.12 Power-on Considerations
Any microcontroller is not guaranteed to function properly before the power supply
stabilizes at its steady states. The EM78P5840N/41N/42N power-on reset voltage
ranges from 1.6V ~ 2.0V. Depending on user’s application, VDD must drop to below
1.6V and remains OFF for 10µs before power can be switched on again. This will reset
the EM78P5840N/41N/42N and allows it to work normally. The extra external reset
circuit can work well if VDD can rise at a very fast speed (50ms or less). However, in
most cases where critical applications are involved and due to unstable power on
conditions, extra external devices are required to deal with the power-up concerns.
7.13 External Power-on Reset Circuit
By setting the code option “P71S” to 1, the /RESET pin is selected. The following figure
shows how an external RC produces a reset pulse. The pulse width should be kept
long enough for VDD to reach minimum operating voltage. The Diode D acts as a short
circuit during power down. The Capacitor C will discharged rapidly and fully.
VDD
D
R
C
/RESET
Figure 7-12a External Power-on Reset Circuit 1
The POR reset voltage varies depending on the actual temperature or process
variations. For some applications, a constant reset voltage is important. The following
figure shows an example circuit that supports an adjusted reset voltage. By adjusting
R41 and R46, POR reset voltage will be a constant (Vpor) and the potential voltage on
the /RESET pin will drop to “0” when the VDD drops to below Vpor. The graph in Figure
7-12c shows the relation between VDD and Vpor. When R41=3.9MΩ and R46=910KΩ,
/RESET will remain at “0” if VDD is below 2.24V, and is reactivated after VDD goes
above 2.1V (see Figure 7-12c).
46 •
Product Specification (V1.0) 04.25.2006
(This specification is subject to change without further notice)
ELAN [ ELAN MICROELECTRONICS CORP ]
