Series resistor on reset pin

Hello all --
As I strategize how to layout a stripboard for a small robot (my child's robot), my readings included Atmel's "AVR042-Hardware Design Considerations." With some follow-up research on this forum, and looking at the Arduino Uno R3 schematic as a reference example, I am a bit confused on which options are best suited to my application. I will be using the 1284P-PU chip, with peripheral motor driver and motors with magnetic/Hall effect shaft encoders, obstruction sensors, &c., on a roughly 3" x 4" (75x112mm) stripboard. The robot has a wheelbase of 85mm, kinda small. My modest understanding of these matters would call this a "noisy environment."

What is new to me is the inclusion of a series resistor (330R) on the reset switch to GND, and a couple other things, described by Atmel in AVR042. This series resistor is not found in the Uno R3 schematic (best as I can read). There is found in that schematic though a diode in parallel with the 10kR pull-up resistor. This diode was added in the Uno R3. Also, no external capacitor in the schematic, from reset pin to GND, as recommended in AVR042.

My aforementioned confusion arises primarily from what I have found in topic responses on these forums, and tutorials on which I have heavily relied to get me to this point.

What are the considerations given to the varied responses I have found on these forums? And, secondly, given my application, what is recommended?

Many thanks in advance for your kind responses. Mark

I would look at Nick Gammon's bread board example. I have tried his stuff and it works well. See here.

There is also information here on burning a bootloader.

The RESET external capacitor to ground, was added for some additional noise reduction. But it is not required, so most designs leave it off. The series resistor for the switch is needed only if you have this capacitor installed.

Thanks Chill --

I am familiar with Mr. Gammon's tutorials, and am a frequent visitor. In the support of my endeavours, he has become one of my heros. The bootloader link, well, with my novice experience I am having to rely upon the hard work of others to obtain that elusive 1284P 20MHz Optiboot. I am following a couple threads in re this, but have been occupied elsewhere recently.

Aloha, Mark

Hiduino --

I was of the understanding that a capacitor internal to the chip was shorted and rapidly discharged on depression of an external reset switch. This situation is ameliorated with addition of a series resistor, that is how I read it. Sometimes it takes several tries at an App Note to fully grok the meaning.

Mahalo, Mark

I do a lot of '1284P designs, and find the 10K pullup, parallel diode, and 0.1uF series DTR cap to be all that is needed. Minimal '1284P example:

Aloha Bob (correct?) --

Yes, I have been following your posts in re 1284's and browsed your schematics. You give one a high bar to aspire to.

Would you recommend the appropriate diode. Atmel cites a Zener diode, an item which is not in my supplies box. I know, "how does one get by without a Zener here an' there?" Just not something I have gotten to. Much to learn.


I don't think the diode selected is too critical. That board used a B120B-13-F.

I have used 1N4148 thru hole part on boards:

Atmel cites two different diodes. One for clamping which ties to Vcc. The other one, again optional, is a Zener diode to ground. Used to limit the high voltage to RESET.

The clamping diode should be a fast switching type, like a 1N4148.

Kewl --

I have a few 1N4148's, perfect. I tried those on some small solar PV panels, but switched over to 5819's for some reason.

Although I am using gendered power connectors, "reverse voltage protection circuit" is on my list of research topics. I will be learning more about diodes soon.

Me ka mahalo!