User Manual

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23.4 UARTs and Device Tree
● Various UART Device Tree Overlay definitions can be found in the kernel github tree. The two most
useful overlays are disable-bt and miniuart-bt .
● disable-bt disables the Bluetooth device and restores UART0/ttyAMA0 to GPIOs 14 and 15. It is also
necessary to disable the system service that initialises the modem so it doesn't use the UART: sudo systemctl
disable hciuart .
● miniuart-bt switches the Raspberry Pi 3 and Raspberry Pi Zero W Bluetooth function to use the mini
UART (ttyS0), and restores UART0/ttyAMA0 to GPIOs 14 and 15. Note that this may reduce the maximum
usable baudrate (see mini UART limitations below). It is also necessary to edit
/lib/systemd/system/hciuart.service and replace ttyAMA0 with ttyS0, unless you have a system with udev rules
that create /dev/serial0 and /dev/serial1. In this case, use /dev/serial1 instead because it will always be correct.
If cmdline.txt uses the alias serial0 to refer to the user-accessible port, the firmware will replace it with the
appropriate port whether or not this overlay is used.
●There are other UART-specific overlays in the folder. Refer to /boot/overlays/README for details on
Device Tree Overlays, or run dtoverlay -h overlay-name for descriptions and usage information.
● For full instructions on how to use Device Tree Overlays see this page. In brief, add a line to the
config.txt file to enable Device Tree Overlays. Note that the -overlay.dts part of the filename is removed.
...
dtoverlay=disable-bt
...
23.5 Relevant Differences Between PL011 and Mini UART
23.3 UART Output on GPIO Pins
●By default, the UART transmit and receive pins are on GPIO 14 and GPIO 15 respectively, which are
pins 8 and 10 on the GPIO header.