Purpose

Document Raspberry information related to integrating the PiloT HAT

Development kit used

• RPi4
• Raspbian based on Bullseye

Physical UART(s) / serial ports

http://raspberrypi.org/documentation/configuration/uart.md

RPi Hardware UARTs

Check current Raspian configuration

The following is an example where ttyAMA0 is connected to

• 40 Way header primary UART GPIO 14 (transmit) and 15 (receive)
• UART0
• UART PL011

pi@raspberrypi:~ $ find /dev/* -type l -ls | grep "serial"
      246      0 lrwxrwxrwx   1 root     root            7 May 16 21:17 /dev/serial0 -> ttyAMA0
      221      0 lrwxrwxrwx   1 root     root            5 May 16 21:17 /dev/serial1 -> ttyS0

If the user wishes to use the PiloT physical UART interface (as opposed to a USB serial port) it is recommended a PL011 is connnected to the PiloT so that a fully featured UART is available to interface with the cellular module.

Summary of steps to disable BT and console

• disable-bt – see below
• Enable serial port
• Disable the Linux Serial Console – see below

RPi UART information

The following sections were copied from the RPi official documentation

There are two types of UART hardware available on the Raspberry Pi - PL011 and mini UART.

The PL011 is a capable, broadly 16550-compatible UART.

The mini UART has a reduced feature set. With limited baud settings etc

Primary UART

On the Raspberry Pi, one UART is selected to be present on GPIO 14 (transmit) and 15 (receive) - this is the primary UART. By default, this will also be the UART on which a Linux console may be present. Note that GPIO 14 is pin 8 on the GPIO header, while GPIO 15 is pin 10.

Secondary UART

The secondary UART is not normally present on the GPIO connector. By default, the secondary UART is connected to the Bluetooth side of the combined wireless LAN/Bluetooth controller, on models which contain this controller.

Raspberry Pi Zero, 1, 2 and 3

The Raspberry Pi Zero, 1, 2, and 3 each contain two UARTs as follows:

Raspberry Pi 4 and 400

The Raspberry Pi 4B and 400 have an additional four PL011s, which are disabled by default:

Primary and Secondary UART

The following table summarises the default Raspbian assignment of the first two UARTs:

Disabling the Linux Serial Console

By default, the primary UART is assigned to the Linux console. If you wish to use the primary UART for other purposes, you must reconfigure Raspberry Pi OS. This can be done by using raspi-config:

Start raspi-config: sudo raspi-config.

Select option 3 - Interface Options.

Select option P6 - Serial Port.

At the prompt Would you like a login shell to be accessible over serial? answer 'No'

At the prompt Would you like the serial port hardware to be enabled? answer 'Yes'

Exit raspi-config and reboot the Raspberry Pi for changes to take effect.

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 makes the first PL011 (UART0) the primary UART. You add a line to the config.txt file to apply a Device Tree overlay. Note that the -overlay.dts part of the filename is removed. For example:

dtoverlay=disable-bt

You must also disable the system service that initialises the Bluetooth modem, so it does not connect to the UART, using

sudo systemctl disable hciuart.