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UART

UART Overview

UART Overview

The DART-MX91 exposes up to seven LPUART interfaces, some of which are multiplexed with other peripherals.

Serial Port Device Node Device Tree DART-MX91 on DT8MCustomBoard/Sonata-Board
UART0 /dev/ttyLP0 lpuart1 DT8MCustomBoard/Sonata-Board serial console
UART2 /dev/ttyLP2 lpuart3 Disabled by default, see datasheet
UART3 /dev/ttyLP3 lpuart4 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]
UART5 /dev/ttyLP5 lpuart6 Connected to J12.4 and J12.6 on DT8MCustomboard/to J6.4 and J6.6 on Sonata-Board
UART6 /dev/ttyLP6 lpuart7 Connected to J12.11 and J12.13 on DT8MCustomboard only
UART7 /dev/ttyLP7 lpuart8 Disabled by default, see datasheet
  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 imx91-var-dart.dtsi:

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

Testing UART5

The following demonstrates how to test UART5 on the DART-MX91 and Sonata-Board.

Short J6.4 and J6.6 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 DART-MX91 and DT8MCustomBoard only.

Short J12.11 and J12.13 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/imx91-var-dart-dt8mcustomboard.dtsi 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.MX91 supports controlling an rs485 transceiver's driver enable signal via RTS_B signal. For more details, please refer to 47.3.4.4 Transceiver driver enable using RTS_B of the i.MX 91 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 DART-MX91 and Sonata-Board using /dev/ttyLP4 on J6.1 (TX), J6.5 (RX) and J6.7 (RTS_B).

The signals can be accessible only if Bluetooth is disabled. See Disabling Bluetooth / Enabling UART4 (/dev/ttyLP4)

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@imx91-var-som:~# pip3 install pyserial
root@imx91-var-som:~# python3 rs485.py /dev/ttyLP4 "hello"

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

Rs485-mx93.png