Capacitor between reset and DTR causing a problem

I have a question related to issues I'm having with an open source project I'm working on. The project is the next version of the Camera Axe for those interested. http://www.CameraAxe.com

Images: Bad Oscilloscope Image: http://www.glacialwanderer.com/_blog/blog2011/10_Oct/b_bad.jpg Bad Oscilloscope Image2 (with 5V rail and micro reset pin): http://www.glacialwanderer.com/_blog/blog2011/10_Oct/IMG_2558.jpg Good Oscilloscope Image http://www.glacialwanderer.com/_blog/blog2011/10_Oct/v_good.jpg Schematic: http://www.glacialwanderer.com/_blog/blog2011/10_Oct/ca_schematic.png

I have 5 test units and two of them don't startup correctly. If it has been sitting for more than 3 seconds I need to turn it on, then turn it off, and then turn it on again to make it work. If I just turn it on the micro doesn't initialize correctly. Three of the oscilloscope images above are the output voltages from a good unit, the bad unit, and then the bad unit with the reset voltage. The problem is that reset pin dropping to 0V after going up to 1+ V. I don't understand why that is happening, but if I remove C1 (cap between DTR and reset pin for micro) the problem goes away. I can't just do that though or I won't be able to load new programs. I did some looking an it seems that all the arduino boards use a similar cap so I'm not sure why it is causing me problems. Ideas?

You might want to try a simple 'experimental' fix that was recently uncovered having to do with the reset circuit on typical arduino boards. That is to wire a diode (a 1N4001 or equivelent is fine) from the reset pin to +5vdc, with the cathode end wire to the +5vdc.

Lefty

I actually uncovered that bug in a separate project. That was caused by a voltage spike caused by an ill placed capacitor. My circuit doesn't have that issue nor do my voltage graphs show that issue.

That said I appreciate the suggestion and I'm looking for something like that being my problem. It's just that one isn't my issue.

The two 'bad' scope pictures, why the notch in the second and not the first?

You have quite a complex +5vdc power source circuit, anyway to bypass most of it and just test with a simple regulated +5vdc external power source?

Lefty

The reason for the notch in one and not in the other is the mystery I'm trying to solve. I have 5 boards all with identical PCBs and the same components. One shows the problem often. One shows the problem once in awhile. The other 3 don't show the problem.

I did try replacing the power circuit with external power and that fixes the problem, but I still need to figure out what part of the circuit is causing the intermittent problem.

I just realized I read Lefty's question wrong. The reason for the notch in the second bad image is because the first bad picture is just the 5V rail. In the second bad image I am showing the 5V rail and the reset pin. It is the reset pin that has the notch in it.