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Datasheet: AN609 (Maxim Integrated Products)

Tiny Ic Debounces Pushbutton Switch

 

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Maxim Integrated Products
www.maxim-ic.com/an609 Page 1 of 2
SUPERVISORY/VOLTAGE MONITOR CIRCUITS
Application Note 609: Jun 01, 2001
Tiny IC Debounces Pushbutton Switch
When a push-button is used to manually reset a P, ringing from the switch may cause
a false reset output signal when the switch is press and de-pressed. Maxim's
microprocessor supervisors have a debounced manual reset input to help ignore this
ringing.
Though they're not complicated, schemes for debouncing a pushbutton switch usually entail
using several logic gates. It's easy to include such circuits in an ASIC. Adding a debouncer as a
last-minute design change, however, can be inconvenient. In such cases, the circuit in the figure
below can come in handy. The circuit, using only a 4-pin SO-package IC, squares up and
debounces a pushbutton signal. IC
1
is a reset chip with a reset output that goes high when its
supply voltage drops below 2.65V or when its manual-reset input (MR-bar) goes low. MR-bar
usually connects to a system-reset input, and reset connects to a C, but the connections shown
enable the device to debounce any signal.
The internal one-shot provides an instant response to the first falling edge on the input and then
delays any further response until 140msec after the last rising edge. The MAX812 shown
inverts the pushbutton input; a similar device (MAX811) is noninverting. The ICs also
incorporate a power-on reset function that asserts the reset output when V
CC
falls below a
preset threshold and also asserts the output for 140msec after each application of power.
Selecting an R-suffix part sets this threshold at its lowest value (2.63V), preventing false
outputs in the debouncer except when the supply voltage fails.
www.maxim-ic.com/an609 Page 2 of 2
Using almost no real estate, this circuit squares up and debounces a pushbutton
signal and extends it to at least 140msec.
A similar version of this article appeared in the April 1, 2001 issue of
EDN.
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