confusion about Uno schematic and reset-en

I remain completely confused about the whole auto reset/reset-en portion of the Uno schematic, in spite of lots of searching the forum on the subject.

In closely observing my Uno r2 , I can see that the solder blob/trace marked reset-en is cut through(it was purchased used and it came that way). My Uno r2 resets anytime the serial monitor is opened.

Now in looking at the Uno schematic,d.aWw

in particular the partial schematic snippet that I have attached, I fail to see how making or breaking the connection marked 1 and 2 would have any effect on the overall circuit. It is in parallel with another unbroken trace.

What am I missing, it is driving me crazy?


billcat: My Uno r2 resets anytime the serial monitor is opened.

What am I missing,

That's normal behaviour.

The Uno is designed to reset every time you open the serial port device.

billcat: it is driving me crazy?


If you look at the artwork, you’ll see the connecting trace is actually routed thru the solder jumper.
Shown seperately on the schematic for clarity tho.
If you buzz the jumper, you may find the pins are not actually separated.


And to further explain the auto-reset 'feature' of the arduino I will add some context. The arduino IDE uses the serial com DTR signal to generate a short pulse that is routed by the signal trace to cause the 328P to reset and start the bootloader which then waits a short time to see if the IDE wants to upload a new sketch, and if not then just runs the sketch that was last loaded into the flash memory. However a side effect of this function is that on most operating systems the opening of a com port (which is what happens when you open the serial monitor) also cause the same board reset that you are seeing, the IDE can't do anything to prevent this behaviour. However as you see the trace art work allows you to 'cut' the signal going to the 328P to stop auto-reset function but then you would not be able to load a sketch unless you either use the reset switch at the correct time when needing to upload a sketch, or install a jumper clip across the cut trace and add the shorting jumper when you wish to upload using auto-reset or remove the jumper when wishing to prevent auto-resets.

thanks for the info......but...,

In simply trying to understand the physical electrical connections of the board, I am less concerned with whether or not the auto reset function is enabled at the moment. I simply cannot see how cutting the trace between the pins would affect the circuit in any way.

@Crossroads....if I were to buzz the pins I would not expect to find them separated, regardless of if I had cut the trace(that I now see in your artwork, of which I was unaware) in between them or left it intact.

Again, have a look at my posted circuit. Even if the trace is cut, there is an alternate parallel path between points A and B.

The alternate path is what is cut; the trace is routed right over, ovelapping, the solder pads.

What you see in the circuit schematic, represents the default trace that appears directly between the pads.

It's just a peculiar way of representing it on the schematic.

Ok, so I think you guys are saying there is actually only one trace there, like in the altered schematic pic I have attached??

If so, I believe I would say that the provided schematic is just plain wrong rather than calling it peculiar, but that’s just me. :stuck_out_tongue:

Anyway, thanks for clearing it up, was bugging me horribly.

reset en.jpg

I suppose one could make an eagle symbol for a solder pad with the trace as part of it.
Then you need one for default of without trace, and one with a default of with trace.
Or just do the 2 in parallel.

I'm diseble auto-reset arduino UNO r3