IS80C51 Datasheet, PDF (30/43 Pages) Integrated Silicon Solution, Inc – CMOS SINGLE CHIP 8-BIT MICROCONTROLLER

IS80C51
IS80C31
Power-on Reset
An automatic reset can be obtained when VCC goes
through a 10µF capacitor and GND through an 8.2K
resistor, providing the VCC rise time does not exceed 1 msec
and the oscillator start-up time does not exceed 10 msec.
For the IS80C51/31, the external resistor can be removed
because the RST pin has an internal pulldown.
The capicator value can then be reduced to 1 µF (see
Figure 19).
When power is turned on, the circuit holds the RST pin high
for an amount of time that depends on the value of the
capacitor and the rate at which it charges. To ensure a
good reset, the RST pin must be high long enough to allow
the oscillator time to start-up (normally a few msec) plus
two machine cycles.
Note that the port pins will be in a random state until the
oscillator has start and the internal reset algorithm has
written 1s to them.
With this circuit, reducing VCC quickly to 0 causes the RST
pin voltage to momentarily fall below 0V. However, this
voltage is internally limited, and will not harm the device.
ISSI ^®
Vcc
1.0 µF ⁺
Vcc
IS80C51/31
RST
GND
Figure 19. Power-On Reset Circuit
30
Integrated Silicon Solution, Inc. — 1-800-379-4774
MC003-1D
11/19/98