nRF54LM20B UART Communication

RE: nRF54LM20B UART Communication

Vidar Berg — Mon, 01 Jun 2026 09:32:32 GMT

Hi Vinay,

Thanks for the update. Unfortunately, changing the logic voltage to 5v is not a solution. The IO voltage of the nRF follows the VDD supply voltage, which adjustable from 1.8v to 3.3v on the DK. When setting the serial bridge IO voltage to 5 V, you will exceed the maximum IO voltage rating of the chip with good margin, and this may cause irreversible damage to the chip: https://docs.nordicsemi.com/bundle/ps_nrf54LM20A/page/chapters/abs_max_ratings/doc/abs_max_ratings.html

Best regards,

Vidar

RE: nRF54LM20B UART Communication

VinayKotadamakki — Mon, 01 Jun 2026 09:18:21 GMT

Hi Vidar,

Thank you for your response.

I missed one setting TTL to USB converter. I should have changed logic to 5v. I was trying 3.3v. No I am able to see the output.

Thank you for solving the issue.

RE: nRF54LM20B UART Communication

Vidar Berg — Mon, 01 Jun 2026 09:02:49 GMT

Hi Vinay,

[quote userid="122190" url="~/f/nordic-q-a/128278/nrf54lm20b-uart-communication/567177"]a logic analyzer to RX,TX and Grnd and tried to see the changes in RX, TX lines. [/quote]

So the logic trace isn't showing anything when the external serial bridge is sending data either? Keep in mind that this sample only prints the boot banner and the info messages once at startup. After that, it will only echo any received data.

[quote userid="122190" url="~/f/nordic-q-a/128278/nrf54lm20b-uart-communication/567177"]Please let me know Console output you have shown is from COM ports detected when you connect USB. Or external UART-USB Converter.[/quote]

I am using the on-board bridge as said in my previous response. See board picture for the connections I made.

RE: nRF54LM20B UART Communication

VinayKotadamakki — Mon, 01 Jun 2026 08:50:08 GMT

Hi Vidar,

I have used hex file shared by you and flashed it to nRF54LM20 DK using Programmer. And connected a logic analyzer to RX,TX and Grnd and tried to see the changes in RX, TX lines.
But could not able to detect any changes in line. Its only high flat line.

Please let me know Console output you have shown is from COM ports detected when you connect USB. Or external UART-USB Converter.

Thanks and regards,

Vinay

RE: nRF54LM20B UART Communication

Vidar Berg — Mon, 01 Jun 2026 06:49:43 GMT

Hi Vinay,

P1.5 and P1.6 are not assigned to any specific functionality on the DK, so you can use them freely. As for why it is not working, it is difficult to say without more debugging information. Have you verified that your serial bridge is working and that the connections are correct (for example, could TX and RX be swapped)?

I quickly tested the echo_bot sample with the following DT overlay:

&pinctrl {
    uart20_default: uart20_default {
        group1 {
            psels =
                <NRF_PSEL(UART_TX, 1, 5)>,
                <NRF_PSEL(UART_RX, 1, 6)>;
        };
        /delete-node/ group2;
    };

    uart20_sleep: uart20_sleep {
        group1 {
            psels = <NRF_PSEL(UART_RX, 1, 6)>;
            low-power-enable;
        };
        group2 {
            psels = <NRF_PSEL(UART_TX, 1, 5)>;
            low-power-enable;
            bias-pull-up;
        };
    };
};

I used jumper wires to connect P1.16 to P1.5 and P1.17 to P1.6 in order to use the on-board serial bridge.

Console output:

Hex file from my build:

devzone.nordicsemi.com/.../echo_5F00_bot.hex

Best regards,

Vidar

RE: nRF54LM20B UART Communication

VinayKotadamakki — Sat, 30 May 2026 17:11:32 GMT

Hi Vidar,

Thank you for your reply.

Yes because we are using nRF54LM20 DK because we are going to build a custom board using nRF54LM20B. So we have used other pins for different usage. Ok I have tried this and still no output.

&pinctrl {
    uart20_default: uart20_default {
        group1 {
            psels = <
                NRF_PSEL(UART_TX, 1, 5)
                NRF_PSEL(UART_RX, 1, 6)
            >;
        };
    };

    uart20_sleep: uart20_sleep {
        group1 {
            psels = <
                NRF_PSEL(UART_TX, 1, 5)
                NRF_PSEL(UART_RX, 1, 6)
            >;
            low-power-enable;
        };
    };
};

&uart20 {
    compatible = "nordic,nrf-uarte";
    status = "okay";
    current-speed = <115200>;

    pinctrl-0 = <&uart20_default>;
    pinctrl-1 = <&uart20_sleep>;
    pinctrl-names = "default", "sleep";
};

Please let me know why I am facing this issue.

Thanks and regards,

Vinay

RE: nRF54LM20B UART Communication

Vidar Berg — Fri, 29 May 2026 13:16:47 GMT

Hi Vinay,

I see you are still using P1.02 for UART_TX, which as I mentioned in my previous reply, isn't connected to the pin header by default:

Is there a reason for why you cannot use the default pin assigments with the on-board Jlink serial bridge?

Regards,

Vidar

RE: nRF54LM20B UART Communication

VinayKotadamakki — Fri, 29 May 2026 12:08:09 GMT

Hi Vidar,
Thank you for your info.

Sorry. Now I have changed the pinmux for UART20. Still I am not able to see any prints in my console.

Pinmux:

/ {
    aliases {
        uart0 = &uart20;
    };
};

&pinctrl {

    uart20_default: uart20_default {
        group1 {
            psels = <
                NRF_PSEL(UART_TX, 1, 2)
                NRF_PSEL(UART_RX, 1, 3)
                NRF_PSEL(UART_RTS, 1, 4)
                NRF_PSEL(UART_CTS, 1, 5)
            >;
            bias-pull-up;   // RX pull-up applied here
        };
    };

    uart20_sleep: uart20_sleep {
        group1 {
            psels = <
                NRF_PSEL(UART_TX, 1, 2)
                NRF_PSEL(UART_RX, 1, 3)
                NRF_PSEL(UART_RTS, 1, 4)
                NRF_PSEL(UART_CTS, 1, 5)
            >;
            low-power-enable;
        };
    };
};

&uart20 {
    status = "okay";

    current-speed = <115200>;

    pinctrl-0 = <&uart20_default>;
    pinctrl-1 = <&uart20_sleep>;
    pinctrl-names = "default", "sleep";
};

// / {
//     chosen {
//         zephyr,console = &uart21;
//         zephyr,shell-uart = &uart21;
//     };
// };

Application Sample using: Echo bot

External UART-USB is being used for checking the UART logs.

Please let me know the issue.

Thanks and regards,

Vinay

RE: nRF54LM20B UART Communication

Vidar Berg — Fri, 29 May 2026 11:21:32 GMT

Please try to include this kind of relevant information when posting a question. If you check the out the link I shared in my post here: https://devzone.nordicsemi.com/f/nordic-q-a/128250/nrf54lm20-i2c-communication-issue/566941, you can see that P1.02 is connected to the NFC antenna by default.

RE: nRF54LM20B UART Communication

VinayKotadamakki — Fri, 29 May 2026 11:09:48 GMT

Hi Vidar,

Yes we have external UART-USB Bridge which we have connected to PC and checking the output. But we are not able to see any logs printed on the console.

Please need your guidance on this.

Thanks and regards,

Vinay

RE: nRF54LM20B UART Communication

Vidar Berg — Thu, 28 May 2026 08:24:13 GMT

Hi,

Do you have an external UART-USB bridge connected to these pins to bridge the serial communication to your PC? If not, you need to use the default pin assignments to use the on-board J-Link as your serial bridge, see https://docs.nordicsemi.com/bundle/ug_nrf54lm20_dk/page/UG/nRF54LM20_DK/program_debug/virtual_serial_ports.html

Best regards,

Vidar