Hey! I've been fighting to get this to work on selfmade PCB for quite some time, now I have to ask for some assistance.
So I have basically the same circuit.
Ftdi RTS -> 100nf -> (pullup) avr reset, so it's supposed to be the exact same it was on first arduinos, besides that my ftdi230xs does have 3.3v level signals.
Proplem is that autoreset only works if I have scope attached to the reset pin or atmel Ice on the ISP header.
As soon as there is no scope or ice the autoreset stops working.
Devices(scope,programmer) connected to the reset pin seems to add something since it enables the reset to happen, but what? Maybe one could point me to right direction.
Edit: avr is powered from 5v via computer PSU, ftdi powered from USB, and have internal 3.3v regulator.(I/O are 3.3v level, but 5v tolerant)
Uart communication itself via USB is working all the times, without or with the scope/programmer attached.
What I've found out so far(mostly blindly trying different things)
- Reset pullup needs to be +20k or autoreset never works, I have gone up to 68k with no difference to that.
- Bootloader(optiboot with modification to work with 328pb uart1) is 100% working, tested up to 1000000 baud with success.
- 10nf.....100nf cap did not do difference.