Reset Pullup

Forum 2005-2010 (read only) Hardware Interfacing
1

I am currently underway to create my very first PCB with eagle. Since I want to have very small results I already switched to SMD parts. However one issue is bogging me:

Either I have to go for two layers or I have to ommit the pullup for the reset pin. However for my idea I do not need all IO pins. So I wonder if it would be a reasonable idea to connect reset to one of the IO pins and activate its pullup resistor.

Any comments on this idea? Is this reasonable or are there any hidden pitfalls with this approach? I do NOT intend to pull the pin low in order to trigger a reset. I already read the application note that says that is is not a good idea. However no application note states something about using IO pullups to connect with reset.

Udo

2

Here's my $0.02 worth

When the ATmega does a Reset, all the pins become inputs and I believe all pull-ups are disabled.

So, when the Reset button is pressed, the pull-up will disappear, so no particular reason for Reset to go high, so no reason to come out of Reset.

I'll try to read the datasheet to double check

3

... Since I want to have very small results I already switched to SMD parts. However one issue is bogging me:

Either I have to go for two layers or I have to ommit the pullup for the reset pin.

I don't understand why connecting a resistor to ground and Reset alone would force a change to 2 layer. Is it the size?
Is it a mass-produced product, or a one/few-off?
If it's the later, you could do something a bit out-of-the-box, like a small through hole component.

4

... all the pins become inputs and I believe all pull-ups are disabled.

... with one exception - the internal pullup on the reset pin itself.

If you consider skipping the external pullup you can read up on the issue here:

Another possibility is to disable reset altogeher through the RSTDISBL
fuse.

5

BenF, thanks, I didn't know that.

6

@Benf: thanks a lot. I should have read AVR 042 more carefully. I will just drop the resistor and connect reset with some unneeded IO Pins. Makes my whole layout a lot simpler :slight_smile:

@Gbulmer: of course it is the size. That's the whole point why I had to switch to SMD. TTH would have been a lot easier. I could have used a simpler beginner project with eagle, but then again: you grow with your challenges :wink: