If you’ve started to play with the brand new Raspberry Pi 3 you might have discovered, like Ladyada did on this live stream, that the serial UART pins don’t behave exactly like they did in the past. Brian Dorey has a great blog post that talks about one of the Pi 3 changes to use the hardware UART of the chip to talk to the Bluetooth module. A consequence of this change is that the UART pins on the GPIO header, like you might use to connect to a GPS device, a serial cable, etc., now use a ‘mini-UART’ that’s tied to the CPU speed. This means the GPIO UART might not have a stable baud rate as it changes with the CPU speed.
Brian’s blog post mentions a workaround from the Pi foundation to disable Bluetooth, however he also found a great way to remap the GPIO pins to use the hardware UART and the Bluetooth module to use the mini-UART. There might still be issues with Bluetooth using this option, but it’s great to have in case you need a very stable UART on the GPIO pins. Check out Brian’s post for all the details.