Raspberry Pi UART0 Configuration
Overview
Configure Raspberry Pi 3 Model B UART0 to enable RTS0 to GPIO17
For my application, I needed RTS0 available to drive the transmit enable pin on an RS-485 adapter. libmodbus does this by switching the RTS line state when it wants to transmit.
If CTS0 is also needed, the file below can be modified by changing the '17 32 33' to '16 17 32 33', and the '0 0 2' to '0 0 0 2'.
Create The Overlay
Copy the following text into a file named 'uart0-full-overlay.dts'
/dts-v1/;
/plugin/;
/{
compatible = "brcm,bcm2708";
fragment@0 {
target = <&uart0>;
__overlay__ {
pinctrl-names = "default";
pinctrl-0 = <&uart0_pins>;
status = "okay";
};
};
fragment@1 {
target = <&gpio>;
__overlay__ {
uart0_pins: uart0_pins {
brcm,pins = <17 32 33>; /* UART0: RTS0 TX0 RX0 */
brcm,function = <7>; /* ALT3 */
brcm,pull = <0 0 2>;
};
};
};
fragment@2 {
target-path = "/aliases";
__overlay__ {
serial0 = "/soc/uart@7e201000";
};
};
};
Compile the overlay:
dtc -@ -I dts -O dtb -o uart0-full.dtbo uart0-full-overlay.dts
Copy the output file 'uart0-full.dtbo' to the /boot/overlay directory:
sudo cp uart0-full.dtbo /boot/overlays/uart0-full.dtbo
Edit /boot/cmdline.txt. Find the console entry that refers to the serial0 device, and remove it, including the baud rate setting. It will look something like
console=serial0,115200
Option 1: Disable Bluetooth Hardware
Update the /boot/config.txt file by adding these two lines to the end:
dtoverlay=pi3-disable-bt dtoverlay=uart0-full
Disable the Bluetooth hardware:
sudo systemctl disable hciuart
Option 2: Move Bluetooth Hardware To The Mini-UART
Update the /boot/config.txt file by adding these two lines to the end:
dtoverlay=pi3-miniuart-bt dtoverlay=uart0-full
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 creates /dev/serial0 and /dev/serial1.
Verification
Run the following command to check the settings of the I/O pins
for i in 16 17 32 33; do sudo raspi-gpio get $i; done
If raspi-gpio is not installed, use the following command to install it:
sudo apt-get -q -q update && sudo apt-get install -y raspi-gpio
The output should look like the following (16 will show as CTS0 if it was enabled)
GPIO 16: level=0 fsel=0 func=INPUT GPIO 17: level=1 fsel=7 alt=3 func=RTS0 GPIO 32: level=1 fsel=7 alt=3 func=TXD0 GPIO 33: level=1 fsel=7 alt=3 func=RXD0
