FT230XS is not in sync when trying to burn bootloader

Question

I want to use FT230XS IC to translate UART to USB, as a cheaper alternative to FT232RL, which I use for years now.

The MCU I use is an Atmega328p-au (so the same configuration as an arduino), and I also have an USB isolator between the PC and the rest of the PCB, like this:

^{simulate this circuit – Schematic created using CircuitLab}

What works: Every time I re-plug the USB or reset the com port the MCU resets, and I can transmit and receive data from the MCU.

What does not work: The issue is, when I try to burn code using the bootloader (either by using Arduino IDE or Microchip Studio), it says "out of sync". BUT, I can see the RTS line resetting as normal via the oscilloscope when I initiate a bootloader burn. So it actually resets, but not fast/slow enough?

Here is the "out of sync" error using AVRdude:

Or arduino:

I can see that the MCU is responding, so it actually tries, but its really out of sync. I also see on the oscilloscope, the RTS pin resets the MCU as it should be:

I tried Inverting the pins: RTS,CTS,DTR,DSR,DCD,RI (one at the time) but no luck:

Also a picture of the PCB while probing it with the oscilloscope:

So I suspect that there is something wrong either with the connection between FT230XS and MCU (Like I should use the CTS pin), or a sync error that can be caused by longer wire from PC to the PCB, (but I used a short wire and didn't help), or the USB Isolator might delaying the signal(?).

This is the first time I use the FT230XS. Has anyone use it before and managed to upload a bootloader to any MCU (preferrably an atmega)? If yes, what was the connection between the FT230XS an the MCU?

Answer 1

It Eventually fixed by itself the next day (in the middle of the next day). I am pretty sure there was something dirty on the PCB (flux leftovers probably).

And it works just fine -for a week now- after that day.

I actually used a IC oven to solder that PCB, using liquid solder that was expired. Maybe this caused some issues, since I do not use liquid solder to solder these components, I just tried it on that PCB.