AN1858

AN1858
Maxim > Design Support > Technical Documents > Application Notes > Microprocessor Supervisor Circuits > APP 1858
Keywords: uP supervisor, microprocessor supervisors, reset, power on reset, por, debounced, manual
reset
APPLICATION NOTE 1858
Tiny IC Debounces Pushbutton Switch
Jun 01, 2001
Abstract: When a push-button is used to manually reset a microprocessor (µP), 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.
IC1 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 µC, 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 VCC 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.
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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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Application Note 1858: http://www.maximintegrated.com/an1858
APPLICATION NOTE 1858, AN1858, AN 1858, APP1858, Appnote1858, Appnote 1858
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