What's the minimum acceptable reset circuitry?

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kk6gm
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Posted by kk6gm: Wed. Mar 20, 2013 - 06:53 PM
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I've always used an external resistor and capacitor on the AVR reset pin, but now I have a very small board that would benefit from losing one or both of those components, if possible. So, what is the minimum reset circuit that meets good engineering practice?

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AVR Microcontrollers, megaAVR and tinyAVR
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Gold Markclawson
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Posted by clawson: Wed. Mar 20, 2013 - 06:56 PM
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Section 3 of AVR042 suggests the resistor is more important than the capacitor:

AVR042 wrote:
To protect the RESET line further from noise, it is an advantage to connect a capacitor from the RESET pin to ground. This is not directly required since the AVR internally have a low-pass filter to eliminate spikes and noise that could cause reset. Applying an extra capacitor is thus an additional protection.

There is, of course, an internal pull-up so even the resistor isn't an absolute requirement.

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ka7ehk
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Posted by ka7ehk: Wed. Mar 20, 2013 - 07:00 PM
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Agree. If reset is not going off-board, then the cap can be dropped.

The resistor simply makes the pull up a bit more "stiff" so that it is a little more difficult for noise to cause a reset. If there are no "antennas" to couple external signals in, then it, also, can be eliminated.

That said, if it is going to be used in a high-RF environment, then keep the cap.

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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alank2
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Posted by alank2: Wed. Mar 20, 2013 - 07:16 PM
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Hi,

I've never put the cap, but I've got some designs that use just the resistor and others that use the resistor and diode. Haven't had any issue with either of them.

When I was playing around with various AVR's when I first got started, some of them would be stable without a pullup resistor, but others would not, so the minimum I would do is the resistor...

Good luck,

Alan

Autoprogrammer:

https://www.avrfreaks.net/forum/wts-standalone-pdiisp-auto-programmers

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mikericetga
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Posted by mikericetga: Wed. Mar 20, 2013 - 10:15 PM
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PLEASE note that this is not true for all cpu's... typically, the external reset line IS the Power On Reset line.

But AFAIK, the AVR external reset circuit is entirely separate from POR... at least for all I have worked with.

If you do not need to 'expose' the reset line, dont worry about any extra components.

Just tie it to Vcc, via a (normally open) solder jumper. Its 'free' copper.

'Burn' your application, drop a blob on the jumper..
only Armageddons' EMP could cause it to reset unexpectedly.

Just be sure to prominently mark the jumper with a warning that it ties directly to Vcc, in case I want to fiddle with it later :)

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ka7ehk
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Posted by ka7ehk: Wed. Mar 20, 2013 - 10:35 PM
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Big question is whether or not this reset line is tied to a programming connector (or pads).

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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