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1 UART Overview

On ' all 4 UARTs are enabled by default.
UART1 is connected to J30 header on Symphony-Board (located on the back of the board).
UART2 is connected to Bluetooth module.
UART3 is connected to J18 header.
UART4 is connected to debug console.
On SoMs without WIFI/BT module UART2 is also connected to J18 header.
See the carrier board datasheet for the exact pinout.
Only UART2 has RTS and CTS lines.

2 UART naming under Linux

The Linux devices corresponding to UART1 - UART4 are /dev/ttymxc0 - /dev/ttymxc3 respectively.

3 Testing UART3 on

Short J18.3 and J18.5 pins and run the following commands:

stty -F /dev/ttymxc2 -echo -onlcr 115200
cat /dev/ttymxc2 &
echo hello > /dev/ttymxc2

For each invocation of echo command the "hello" string should appear on the terminal.

4 Disabling UART2

To disable UART2 on ' edit arch/arm64/boot/dts/ under kernel source directory and modify

&uart2 {
        status = "okay";


&uart2 {
        status = "disabled";

Other UARTs can be disabled in the same manner.

5 Configuring RS485 Half-Duplex

Each UART can be configured for RS485 Half-Duplex mode by using a GPIO pin to drive the receive and transmit enable inputs. This can be configured in the device tree by making the following changes to the uart node and replacing X, Y & Z with the proper values:

&uartX {                                                          /* Add RS485 properties to uartX */
	pinctrl-names = "default";
	pinctrl-0 = <&pinctrl_uartX>, <&pinctrl_uartX_rs485>;     /* Add RS485 GPIO pinctrl */
	rts-gpios = <&gpioY Z GPIO_ACTIVE_LOW>;                   /* Add rts-gpios property */
	linux,rs485-enabled-at-boot-time;                         /* Enable RS485 at boot time to skip using TIOCSRS485 ioctl */
	status = "okay";

Next, configure the RS485 GPIO pin by adding pinctrl_uartX_rs485 to iomuxc. Replace X, GPIO_PIN_FUNCTION, and GPIO_PIN_SETTINGS with the proper values:

&iomuxc {
	pinctrl_uartX_rs485: uartXrs485 {
		fsl,pins = <

Note: For more information about configuring pins, please see i.MX Device Tree Pinmux Settings Guide

After making these changes, RS485 mode will be enabled by default and can be verified from the console by running the commands below. Replace N with the proper value, which is typically X-1 relative to the device tree node uartX:

# stty -F /dev/ttymxcN -echo -onlcr 115200
# echo hello > /dev/ttymxcN

The below logic analyzer capture shows the RS485 RX/TX enable line toggling when writing to the UART:


Please refer to the Linux device tree bindings for more RS485 configuration options.