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[DM8148] Intermittent Barcode Issue

Hi all

I am not sure whether this is the right place to bring up this issue. Still i am posting this so that people could share their experience and throw some light on possible cause for this intermittent issue.

In our custom board using DM8148, some of the setups (2 out of 14) has an intermittent bar-code issue. The bar code reader is a module with UART interface for transferring data from the module to the DM8148 on successful bar-code reading. The module is controlled by two control signals - (i) Power Enable and (ii) Trigger. Power enable signal brings out the module from power down mode back to normal operating mode and Trigger is issued 300ms after Power Enable is activated. The path taken by signals are Main PCBA-->Cable Harness-->Another Sub- PCBA-->FFC-->Barcode Module. 

The issue we face is that of intermittent bar-code reading occasionally. It is very random and happens once in few weeks - the systems are running tests every work day. Power cycling does not help most of the times. The interesting part is whenever the issue props up and i hook up the probes to the two control lines, the things starts working and then no matter what we try, it continues to work after the probes are removed. hence we are unsure how to reproduce this issue.

We have followed all guidelines of the bar-code manufacture with respect to the interfacing and also with the control signal issuance for the module

Since these are not exactly high speed interfaces, what is causing the issue? The signals 'look' clean on normal operation. Would adding a capacitor of around 22pF to ground solve this intermittent issue? (I am assuming the probe is presenting a capacitve load to the lines). Even if that is the case, what could be the possible root cause?

Suggestions and inputs welcome! Thanks :)

Regards,

KS

  • Hi KS,

    I can provide you the below pointers:

    http://processors.wiki.ti.com/index.php/Barcode_and_QR_Code_with_Beaglebone

    http://www.ti.com/solution/barcode_scanner

    http://e2e.ti.com/support/embedded/linux/f/354/t/160879.aspx

    I will also check with the UART team, if they have something to share.

    Best regards,
    Pavel

  • If the issue goes away (or even changes in any way, better or worse) when you connect oscilloscope probes to a signal then the issue is signal integrity.

    What scope probe settings did you use? (high speed probes, 10X, low capacitance etc...) My guess is that you are using probes with a high capacitance hence filtering some high frequency noise/ringing.

    Just because the signal data rate is low does not mean that the edges are not fast. Fast edges will cause ringing if the signal wires/traces are not correctly impedance matched. This in turn can/will cause glitches in the detecion circuits.

    I suggest using high speed, low capacitance probes to look at the signals for any ringing, then tune the signals with series resistors and/or parallel capacitors to remove the ringing but still maintain fast enough switching.

    BR,

    Steve

  • So happy to learn this info about bar code, now I'm just doing simple work about bar code with .net application, what you've mentioned never meet, but maybe sometime it will help me. Thank you all so much.

  • fen den said:

    So happy to learn this info about bar code, now I'm just doing simple work about bar code with .net application, what you've mentioned never meet, but maybe sometime it will help me. Thank you all so much.

    Hi, fen den.

    Have you ever tried it? I am a green hand on this barcode scanning issue. Any suggestion will be appreciated.

    Thanks in advance.

    Best regards,

    Arron

  • HI

         I have met the issue before. I change the barcode reader,and the  running speed  is fast. but u must pay attention to the interfacing.

        hope it  helps.

        BR

                  leo