Regarding the USB connection on UNO

From some site information, I found this:

The Uno differs from all preceding boards in that it does not use the FTDI USB-to-serial driver chip. Instead, it features the Atmega16U2 (Atmega8U2 up to version R2) programmed as a USB-to-serial converter.

Does anyone know why the usb chip was changed? And, is there any problem with patching the USB connection to the outside world back to an FTDI chip on the UNO board?

Thank you!

The idea was that people could reprogram them to make the Uno act as keyboard or mouse. Someone has firmware to do something like that, but most people just ignored that, and use it as a serial converter with the default hardware. So it's just like an FTDI adapter, only not as good at being a USB serial adapter. That, and I hear Atmel gave them a sweetheart price.

You can use an FTDI adapter to communicate over serial - There are 1k resistors between TX and RX on the 16u2 and the '328p, so a directly connected FTDI or other serial adapter would override the 16u2's transmit.

Unfortunately, you can't do the autoreset without modifying the board, because the 0.1uf cap from reset to the 16u2's "DTR" line is still there, and will interfere with the action of your own capacitor from the FTDI's DTR to reset.

Thank you for the information!

I think you are referring to the cap C5, yes? It appears that pin 13 might be normally high. Then when the USB is plugged in, pin 3 will pull low to reset the 328 by way of C5. Is that how the interaction happens?

Is there some information on how the Auto-reset operates? Perhaps the USB port is toggled on-off-on to force a reset on the 328.

Yes it is C5. The capacitor turns the DTR signal, which goes from high to low while a serial connection is esablished, into a momentary pulse low. You can see what it looks like on a scope in this article from Nick Gammon: Gammon Forum : Electronics : Microprocessors : How to make an Arduino-compatible minimal board

N6XB:
is there any problem with patching the USB connection to the outside world back to an FTDI chip on the UNO board?

Does this mean you no longer want to use the ATmega16U2 because it is broken on your board?