I'm using a Atmega168 which I'm trying to interface with a Nokia FBus, which operates at 3.3V, 115200b/s, no parity, 1 stop bit. The m/c is running at 14.7456MHz, with divide by 8 prescaler, so runs at 1.8432MHz. The bootloader has been modified so the UART functions at 19200b/s. The main application adjusts the prescaler to divide by 2, giving 7.3728MHz. It then sets up to UART to function at 115200b/s.

At 5V, everything functions fine. I can upload new firmware, run the application, and communicate over the UART at 115200b/s. As soon as I drop Vcc to 3.3V, the UART refuses to function. At this voltage I can use a dapa cable with avrdude and successfully communicate with the m/c. The clock seems to be functioning, but nothing else. I've checked the brownout fuse bits, and it should be ok down to 2.9V.

Is there anything fundamental the might stop the UART functioning at Vcc=3.3V? The digital scope seems to show it as completely dead at 3.3V, not a squeek.