IC80C51
IC80C31
Power-on Reset
Vcc
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 IC80C51/31, the external resistor can be
removed because the RST pin has an internal pulldown.
The capicator value can then be reduced to 1µF (see
Figure 18).
+
Vcc
IC80C51/31
RST
1.0 F
-
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.
GND
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.
Figure 18. Power-On Reset Circuit
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.
Integrated Circuit Solution Inc.
MC001-0B
31
ICSI [ INTEGRATED CIRCUIT SOLUTION INC ]
