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UART

UART Overview

UART Overview - VAR-SOM-MX93

The VAR-SOM-MX93 and DART-MX93 expose up to seven LPUART interfaces, some of which are multiplexed with other peripherals.

Serial Port Device Node Device Tree VAR-SOM-MX93 / Symphony Board DART-MX93 / DT8MCustomBoard
UART0 /dev/ttyLP0 lpuart1 Symphony board serial console DT8MCustomboard serial console
UART2 /dev/ttyLP2 lpuart3 Disabled by default, see datasheet Disabled by default, see datasheet
UART3 /dev/ttyLP3 lpuart4 Disabled by default, see datasheet Disabled by default, see datasheet
UART4 /dev/ttyLP4 lpuart5 1.8V Signal level, used on SOM for Bluetooth interface and can be accessible only if Bluetooth is disabled. [1] 1.8V Signal level, used on SOM for Bluetooth interface and can be accessible only if Bluetooth is disabled. [1]
UART5 /dev/ttyLP5 lpuart6 Connected to Symphony board J18.7 and J18.9 Connected to DT8MCustomboard J12.4 and J12.6
UART6 /dev/ttyLP6 lpuart7 Connected to Symphony board J18.3 and J18.5 Connected to DT8MCustomboard J12.11 and J12.13
UART7 /dev/ttyLP7 lpuart8 Disabled by default, see datasheet Disabled by default, see datasheet
  1. ↑ 1.0 1.1 See Disabling Bluetooth / Enabling UART4 (/dev/ttyLP4)

Disabling Bluetooth / Enabling UART4 (/dev/ttyLP4)

UART4/ttyLP4 is used by the Bluetooth on the SOM. To use it on the carrier, Bluetooth must be disabled on the SOM.

First, disable variscite-bt by running:

# systemctl stop variscite-bt; systemctl stop variscite-ot
# systemctl disable variscite-bt; systemctl disable variscite-ot

Then, disable bluetooth in the device tree imx93-var-som.dtsi:

&lpuart5 {
   ...
    bluetooth {
        compatible = "nxp,88w8987-bt";
        status = "disabled";
    };
   ...
};

Testing UART5

The following demonstrates how to test UART5 on the VAR-SOM-MX93 Symphony board. The same process applies to the DT8MCustomboard following the table above.

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

# stty -F /dev/ttyLP5 -echo -onlcr 115200
# cat /dev/ttyLP5 &
# echo hello > /dev/ttyLP5

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

Testing UART6

The following demonstrates how to test UART6 on the VAR-SOM-MX93 / Symphony board. The same process applies to the DT8MCustomboard following the table above.

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

# stty -F /dev/ttyLP6 -echo -onlcr 115200
# cat /dev/ttyLP6 &
# echo hello > /dev/ttyLP6

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

Disabling UART5

To disable UART5 edit arch/arm64/boot/dts/freescale/imx93-var-som-symphony.dts or arch/arm64/boot/dts/freescale/imx93-var-dart-dt8mcustomboard.dts under kernel source directory and modify

&lpuart6 {
        ...
        status = "okay";
};

to

&lpuart6 {
        ...
        status = "disabled";
};

Other UARTs can be disabled in a similar manner by referencing the table above.

Configuring RS485 Half-Duplex

The i.MX93 supports controlling an rs485 transceiver driver enable using RTS_B. For more details, please refer to 63.3.4.4 Transceiver driver enable using RTS_B of the i.MX 93 Applications Processor Reference Manual.

RS485 is enabled in software by:

  1. Enabling the RTS pin in the device tree.
  2. Enabling RS485 in the serial driver.

The example below demonstrates how to do this on the VAR-SOM-MX93 using /dev/ttyLP6 on J18.3 (TX), J18.4 (RX) and J17.8 (RTS).

First, disable the ov5640_mipi0 which uses the RTS pin:

&ov5640_mipi0 {
    status = "disabled";
};

Then, configure the RTS pinmux in pinctrl_uart7:

    pinctrl_uart7: uart7grp {
        fsl,pins = <
            ...
            MX93_PAD_GPIO_IO11__LPUART7_RTS_B       0x31e
            ...
        >;
    };

After booting the updated device tree, use the following python script to test RS485:

import sys
import serial
import serial.rs485
import time

def configure_rs485(port, data):
    try:
        # Open the serial port
        ser = serial.Serial(port, baudrate=9600)

        # Configure RS485
        ser.rs485_mode = serial.rs485.RS485Settings(
            delay_before_tx=0,
            delay_before_rx=0,
            rts_level_for_tx=False,  # RTS is low during transmission
            rts_level_for_rx=True,   # RTS is high during reception
            loopback=False
        )

        # Write data to the port three times with a delay of 10ms between each
        for _ in range(3):
            ser.write(data.encode())
            time.sleep(0.01)  # 10ms delay

        # Close the serial port
        ser.close()

        print("Data sent successfully.")

    except Exception as e:
        print(f"Error: {str(e)}")

if __name__ == "__main__":
    if len(sys.argv) != 3:
        print("Usage: python3 rs485.py <port> <data>")
    else:
        port = sys.argv[1]
        data = sys.argv[2]
        configure_rs485(port, data)

Finally, install pyserial and run the script:

root@imx93-var-som:~# pip3 install pyserial
root@imx93-var-som:~# python3 rs485.py /dev/ttyLP6 "hello"

The following image was captured on a logic analyzer using this example:

Rs485-mx93.png