MIC809 Datasheet, PDF (5/8 Pages) Micrel Semiconductor – Microprocessor Reset Circuits

MIC809/MIC810 Microprocessor Reset Circuits
Circuit Description
Microprocessor Reset
The RESET pin is asserted whenever VCC falls below
the reset threshold voltage. The reset pin remains
8++
8JDH
8JDH
asserted for a period of 240ms after VCC has risen
J
J
above the reset threshold voltage. The reset function
ensures the microprocessor is properly reset and powers 4-5-6
up into a known condition after a power failure. RESET
will remain valid with VCC as low as 1.4V.
Figure 2. Reset Timing Diagram
VCC Transients
The MIC809/MIC810 are relatively immune to negative-
going VCC glitches below the reset threshold. Typically,
a negative-going transient 125mV below the reset
threshold with a duration of 50µs (25µs for
MIC8__R/S/T) or less will not cause an unwanted reset.
Interfacing to Bidirectional Reset Pins
The MIC809/MIC810 can interface with µPs with
bidirectional reset pins by connecting a 4.7KΩ resistor in
series with the MIC809/MIC810 output and the µP reset
pin.
VCC
VCC
MIC809
RESET
VCC
µP
RESET
100K
Figure 3. RESET Valid to VCC = OV
RESET Valid to 0V
A resistor can be added from the RESET pin to ground
to ensure the RESET output remains low with VCC
down to 0V. A 100KΩ resistor connected from RESET
to ground is recommended. The size of the resistor
should be large enough to not load the RESET output
and small enough to pull-down any stray leakage
currents.
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