Skip to content

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 Sonata V1.1 connector
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 is accessible only when Bluetooth is disabled. [1] 1.8V Signal level, used on SOM for Bluetooth interface and is accessible only when 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 J6.4 / J6.6
UART6 /dev/ttyLP6 lpuart7 Connected to Symphony board J18.3 and J18.5 Connected to DT8MCustomboard J12.11 and J12.13 J6.11 / J6.13
UART7 /dev/ttyLP7 lpuart8 Disabled by default, see datasheet Disabled by default, see datasheet —

A dash in the Sonata column means this table does not specify a corresponding connection.

  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 board, 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 or imx93-var-dart.dtsi:

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

Testing UART5

Connect the UART5 loopback pins for your carrier, then run the commands below:

SignalDT8MCustomBoardSonata-Board
LoopbackJ12.4 / J12.6J6.4 / J6.6
 
stty -F /dev/ttyLP5 -echo -onlcr 115200
cat /dev/ttyLP5 &
echo hello > /dev/ttyLP5

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

Testing UART6

Connect the UART6 loopback pins for your carrier, then run the commands below:

SignalDT8MCustomBoardSonata-Board
LoopbackJ12.11 / J12.13J6.11 / J6.13
 
stty -F /dev/ttyLP6 -echo -onlcr 115200
cat /dev/ttyLP6 &
echo hello > /dev/ttyLP6

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

Disabling UART6

To disable UART6 edit arch/arm64/boot/dts/freescale/imx93-var-dart-dt8mcustomboard.dts or arch/arm64/boot/dts/freescale/imx93-var-dart-dt8mcustomboard.dts in the 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:

pip3 install pyserial
python3 rs485.py /dev/ttyLP6 "hello"

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

Rs485-mx93.png