RC on reset pin, necessary or not?

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Since I started working with micros I've always added an RC on the reset pin.  This was always called for back in the day.  But now I'm thinking I missed a memo and that RC is not necessary for AVRs (e.g. Mega328, Tiny84 as examples).  I just looked up an Arduino schematic and they don't have a capacitor on reset, just a 10k R to Vcc.  So, is that what I should be doing, R (which I understand the need for) but no C?

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The only time that I have used a C on the reset pin is if I need to hold the chip in reset for a little longer than "normal" to allow other devices on the same power supply to stabilise. YMMV.

Ross McKenzie, Melbourne Australia

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It is supposedly a good idea if your AVR will live in a high-noise environment. Maybe you are gonna make an arc-welder, so a cap won't hurt. 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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kk6gm wrote:
So, is that what I should be doing, R (which I understand the need for) but no C?
Yes for debugWIRE, no for EMI and EOS (ESD/EFT/lightning)

If reset is close to a ground plane then no capacitor.

If reset is on a connector then at least a capacitor though more complete ESD protection is recommended (R-TVS-R though R-C-R may be good enough [reduce edge speed, current division])

 

Connection of RESET Pin on AVR Devices | AVR® Microcontroller Hardware Design Considerations

[end of fourth paragraph]

However, such extra capacitor cannot be used when DebugWIRE or PDI is used.

Reset Pin Protection | AVR040: EMC Design Considerations

[end of second paragraph]

In addition, a pull-up resistor (10kΩ typical) and a small filter capacitor (4.7nF) should be connected as shown in the figure below.

wrt reset, an AVRISP mkII has significant drive whereas an Atmel-ICE doesn't.

Reset Line | AVRISP mkII

 


The Art of Electronics 3rd Edition | by Horowitz and Hill

Download a sample chapter

[page 16, middle of right column]

12.1.5 Input protection 804

 

"Dare to be naïve." - Buckminster Fuller

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Since I started working with micros I've always added an RC on the reset pin.

+1 however the cap on the pin will stop DW from working. So the board I use for debug has no cap.

John Samperi

Ampertronics Pty. Ltd.

https://www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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The old uC needed a reset IC to prevent malfunctions when the power supply started slowly.
RC was a simple alternative.
AVR has a built-in POR. There is also BOD.
Now C is just a designer's tranquilizer.

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avrcandies wrote:

It is supposedly a good idea if your AVR will live in a high-noise environment. Maybe you are gonna make an arc-welder, so a cap won't hurt. 

 

Funny that you say that.  I worked on this machine: https://youtu.be/tpzZaTTCCoc?t=214