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I know why we need a pull-up resistor on the reset lines, but what is the reason for a capacitor parallel to the reset button?

I saw it on Texas Instrument's schematic for MSP430F5529 launchpad.

Reset line for MSP430F5529

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It is for "debouncing" of the pushbutton. Virtually all pushbuttons, when pressed, do not make a single clean contact, but instead make several repeated contacts within a period of 10-50 ms. This would cause the microcontroller to begin to reset several times on each pushbutton press. The capacitor suppresses this.

For more information on debouncing, see this article.

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  • \$\begingroup\$ I actually have another push button in my circuit that is connected to an interrupt pin. I was planning to use a de-bouncer IC, can I get away with a capacitor instead? \$\endgroup\$ – dvdmn Feb 21 '16 at 19:32
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    \$\begingroup\$ I suggest you read the article first. A single capacitor may not be the best, but adding a couple extra components may do the trick. \$\endgroup\$ – tcrosley Feb 21 '16 at 19:33
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    \$\begingroup\$ I don't think debouncing is the reason. Guaranteeing a minimum reset pulse with most probably is. \$\endgroup\$ – Wouter van Ooijen Feb 21 '16 at 20:09
  • \$\begingroup\$ @WoutervanOoijen I disagree. Did you read the datasheet? The MSP430F552 only needs RST to be low for 2 µs. Any button press will be longer than that. If you didn't have the cap it would reset for each bounce. \$\endgroup\$ – tcrosley Feb 21 '16 at 20:20
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    \$\begingroup\$ I didn't read the DS, did now. That Treset is weell-hidden :) But I don't see how you can be sure that a bounce-up pulse can't be shorter than 2us? Which reverts your anwer to a good one :) Another thing I couldn't find is the start-up requirement: theer seems to be an area where the chip could operate out-of-spec (< 1.8V), which is prevented (when the rise is not too slowe) by the RC time. \$\endgroup\$ – Wouter van Ooijen Feb 21 '16 at 20:36
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For a controlled signal on the line. Many MCUs have reset timing that must be enforced and the capacitor is a cheap alternative to e.g. a supervisor IC.

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The capacitor provides certain reset time, which is normally required to ensure all registers are cleared. This implementation is not very clean, because on power off the capacitor will remain charged, applying voltage on reset pin. Which nowadays is not a problem, the clamping diode will discharge it. But once it could damage the input.

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