[SOLVED] J-Link VCOM max baudrate.

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  • [SOLVED] J-Link VCOM max baudrate.


    I've recently encountered some problems with J-Link VCOM using higher baudrates. The custom board I use needs 1250000 baud which is rather unusual but nevertheless, another ordinary USB/Serial converter seems to not have any problems with it. Every time I'm trying to open the connection using specified baudrate(1250000) J-Link disconnects from Windows' device manager. It happens no matter what serial terminal I'm using(tested on Putty, SmarTTY, TeraTerm, and so on). I wonder if there are any hardware-related constraints to J-Link's max baudrate (from my understanding they shouldn't exist as the USB CDC class doesn't have such) or maybe it can be fixed via a simple software update? Using lower baudrates(115200, etc.) works without any problems.

    JLink EDU
    Serial number: 260108435
    Version: 10.1
    It has the newest available firmware installed.

    Any help would be much appreciated.

    Mateusz Piesta
  • Hello,

    Thank you for your inquiry.
    The answer is, it depends.
    Technically the J-Link Plus should also be able to handle that baud rate, but there are limiting factors that can further decrease the maximum value:

    We already have on our todo to create an overview with different scenarios and measurements so it becomes clearer which J-Link can handle what VCOM baudrates.

    For now we can clearly say that a J-Link Ultra+/J-Link Pro support 10 MBaud without any issues.

    Alternatively we recommend to use SEGGER RTT instead of UART for I/O:
    It is free, magnitudes faster than UART, works with any J-Link and no extra pins are needed.

    Would that be an option for you?

    Best regards,
    Please read the forum rules before posting: Forum Rules

    Keep in mind, this is not a support forum. Its main purpose is user to user interaction.
    Our engineers will try to answer your questions between their projects if possible but this can be delayed by longer periods of time.
    Should you be entitled to support you can contact us via our support system: segger.com/ticket/

    Or you can contact us via e-mail.
  • Hi Nino,

    Thank you very much for the fast reply.

    I've already been using RTT for some time in the various projects but unfortunately, this one needs UART. Such unusual baudrate was chosen because of the clock configuration rather than for pure throughput. I wonder if it's possible to unblock higher baudrates using different software? I highly doubt I will be sending more than a few kb/s of data via UART. The thing that bothers me is why J-Link keeps disconnecting? Any ideas? I will try to do the same thing on a Linux machine and let you know if it behaves the same.

    Mateusz Piesta
  • There may be a misunderstanding here.
    You wrote: 1250000 baud => 1,250,000 baud => 1.25 MBaud
    The J-Link EDU V10 does not have any hardware UART on the VCOM pins, so it does things in software.
    This means it has a certain limit of what is doable. Max. baudrate for the V10 is about 180 kBaud.
    There is no way of unlocking higher baudrates for this model.
    As Nino wrote, the current ULTRA+ / PRO models support up to 10 MBaud.

    And before this comes up as a topic:
    Yes, we are aware of the FTDI USB to serial adapters that exist. Keep in mind that these adapters are specifically build to fulfill exactly 1 job: USB 2 UART.
    The can't do anything else.
    J-Link was designed as a debug probe and this feature was added later on, into an existing design, to make it a more universal solution.
    So of course a more universal solution may not be able to keep up in all fields with special solutions designed for exactly that field.

    The disconnect you are seeing is another story. It is probably a bug in the firmware, causing it to crash when selecting a too high baudrate, J-Link is unable to manage.
    We will give that a quick try and fix the bug so that J-Link correctly limits the baudrate and does not crash, assuming that we will be able to reproduce the issue.