UART

UART Overview

On VAR-SOM-MX8 UART1 - UART3 and UART5 are enabled, UART4 is disabled as it shares pins with the second Ethernet controller.
UART1 is used as a debug console, UART2 is connected to Bluetooth module, UART3 and UART5 are connected to J18 header on carrier board.
On SoMs without WIFI/BT module UART2 is connected to J18 header.
Inspect the carrier board datasheet or schematics for the exact pinout.
Only UART2 has RTS and CTS lines.

On SPEAR-MX8 UART1 - UART3 are enabled.
UART1 is used as a debug console, UART2 is connected to Bluetooth module, UART3 is connected to J26 header on SPEAR-MX8 carrier board.
On SPEAR-MX8 SoMs without WIFI/BT module UART2 is connected to J20 header.
Inspect the carrier board datasheet or schematics for the exact pinout.

UART naming under Linux

The Linux devices corresponding to UART1 - UART5 are /dev/ttyLP0 - /dev/ttyLP4 respectively.

Testing UART3 on VAR-SOM-MX8

Short J18.7 and J18.9 pins and run the following commands:

stty -F /dev/ttyLP2 -echo -onlcr 115200
cat /dev/ttyLP2 &
echo hello > /dev/ttyLP2

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

Testing UART3 on SPEAR-MX8

Short J26.17 and J26.19 pins and run the following commands:

stty -F /dev/ttyLP2 -echo -onlcr 115200
cat /dev/ttyLP2 &
echo hello > /dev/ttyLP2

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

Disabling UART3

To disable UART3 on VAR-SOM-MX8 edit arch/arm64/boot/dts/freescale/fsl-imx8qxp-var-som-common.dtsi under kernel source directory and modify

&lpuart2 {
        ...
        status = "okay";
};

to

&lpuart2 {
        ...
        status = "disabled";
};

On SPEAR-MX8 modify arch/arm64/boot/dts/freescale/fsl-imx8qxp-var-spear-common.dtsi in the same manner.