What's the minimum acceptable reset circuitry?

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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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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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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

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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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

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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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Big question is whether or not this reset line is tied to a programming connector (or pads).

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net