I have a custom atMega328 board that's essentially an Arduino Nano with a few circuits doing some custom interfacing. Nothing super crazy. However I'm running into an issue with the RESET line, which should be automatically toggled when a serial connection is made by the DTR line from an FT231XS USB to Serial converter. My setup is the same exact reset circuit most of the Arduino boards use - a 1K resistor to VCC and a series 0.1uF capacitor to convert the logic low to a short pulse. I added a removable jumper so that I can disable serial reset once the final sketch is loaded to prevent unwanted resets.
(13 Sept 2021 - Updated circuit to reflect actual reset circuit)
After burning the bootloader succesfully using a standard ISP header, I attempted to load a simple sketch to make sure USB reprogramming would work properly. However, avrdude was unable to connect to the board and gave the generic stk500_getsync(): not in sync
error that's typically seen with hardware connection issues.
So, I set up a scope probe to try and capture the reset line and see what was going wrong. However, once I connected the scope probe, the reset line seemed to work fine and the sketch uploaded without issues. Disconnecting the scope probe caused the issue to come back. Thinking it could be a capacitance issue, I lightly put my finger on the reset line, and the upload worked fine again.
So, there's obviously an capacitance issue in the reset circuit. My question is - why is this happening and how do I permanently fix it? I'm following almost the exact same reset setup that most Arduino boards use, but for some reason mine only works when I add some capacitance via a scope probe or my finger.
Update 13 Sept 2021
Here are two scope captures from the working reset pulses. One with a 1X probe and one with a 10x probe. In both cases the reset is successful, so the probe loading the circuit is enough to make things work.
I'm assuming the fact that the line isn't going fully low (FT231 VOL is 0.4V) is just an artifact of the DC blocking capacitor.
For completeness' sake, here's the full schematics for both the FT231XS and the AtMega328P: