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:

out of sync

Or arduino:

arduino error

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:

rts reset

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

invert pins

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?

  • \$\begingroup\$ what kinds of oscilloscope is that? \$\endgroup\$
    – Pete W
    Commented Sep 30, 2021 at 17:19
  • \$\begingroup\$ also why capacitive coupling of RTS? \$\endgroup\$
    – Pete W
    Commented Sep 30, 2021 at 17:23
  • \$\begingroup\$ @PeteW I always used capacitive coupling with DTR pin when using FT232, so I though it would make sense, and it works. (except from the bootloader thing of course). Do you think this is the mistake? \$\endgroup\$ Commented Oct 1, 2021 at 7:06
  • 1
    \$\begingroup\$ @PeteW Looks like an "old" LeCroy oscilloscope :) \$\endgroup\$ Commented Oct 7, 2021 at 9:53
  • 1
    \$\begingroup\$ @PeteW LeCroy 9314AL Quad channel 400MHz (Last calibrated 26-July-2000 hehe) \$\endgroup\$ Commented Oct 7, 2021 at 11:23

1 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.


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