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SPI Signal Overshoot (to WIFI chip)

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Discussion in 'Radio and Wireless' started by Rajinder, May 21, 2020.

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  1. May 21, 2020 #1
    Rajinder

    Rajinder

    437
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    Jan 30, 2016
    Hi all,
    I have been monitoring some SPI signals for a WiFi IC. The device is powered from 3.3V. The CLK frequency is 4.5MHz

    However, I am seeing overshoot and undershoot on the CLK, MISO etc
    For example on the CLK the overshoot can get to 3.9V. The undershoot is about -0.65V. This could cause potential issues by exceeding the voltage levels for the WiFi I/O pins.

    I have two questions
    1. I cant see how it is possible to see ringing which is greater than the supply voltage of 3.3V?
    2. Its more than likely caused by inductance. I have tried a shorter gnd probe, it makes a difference to the CLK i.e. no ringing but the MOSI and MISO are the same.

    Any help would be appreciated.
     
  2. May 21, 2020 #2
    Hunter64

    Hunter64

    41
    13
    Nov 20, 2018
    Another WiFi IC without any information.

    A typenumber maybey?
     
  3. May 21, 2020 #3
    Rajinder

    Rajinder

    437
    7
    Jan 30, 2016
    WFM2000 from SiLabs.
     
  4. May 22, 2020 #4
    Nanren888

    Nanren888

    312
    71
    Nov 8, 2015
    At high frequency everything looks like a transmission line. The waveforms can easily go above the rail, if not clipped to it by something. As you say "overshoot". If I drive a transmission line to a voltage v, it can overshoot, regardless of whether V is the rail or not.
    .
    Generally, short lines. Also sometimes it is all in or caused by the probe itself.
     
  5. May 23, 2020 #5
    Rajinder

    Rajinder

    437
    7
    Jan 30, 2016
    Hi
    I shortened the GND clip on the probe, it was set to x10. After this, the clock signal had no issues, however the mosi and miso still had overshoot/undershoot.
    The only thing that I can think of is inductance. I can add series damping resistors in each line, around 39R?
    Are there any other solutions?
    Thanks in advance
     
  6. May 24, 2020 #6
    WHONOES

    WHONOES

    940
    241
    May 20, 2017
    If you have a decent scope lead, it should have a method of compensation. This normally comprises a small variable capacitor within the probe body. There will then be a small hole in the body of the probe through which you can pass a non conductive driver (normally provided with the lead). Your scope should have an output probably labelled "Probe Compensation" or something similar. This provides a square wave signal of a couple of volts or so at a frequency of 1KHz (can vary from manufacturer to manufacturer) with a fast rise time. You should connect your scope lead to these terminals and then adjust the probe so that you have a perfect square wave with no over or under shoot.
    Then you can carry on with your testing and see if you get the same results.
     
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  7. May 24, 2020 #7
    bertus

    bertus Moderator

    1,013
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    Nov 8, 2019
    Hello,

    What scope are you using with wich probes?
    As @WHONOES said, is the probe compensation adjusted correctly?
    When you use a 1:10 or 1:100 probe, the probe compensation is a must.
    Read this page for more info on the probe compensation:
    https://www.electronics-notes.com/articles/test-methods/oscilloscope/scope-probe-compensation.php
    You might also want to read the XYZs of Oscilloscopes primer:
    https://www.tek.com/document/primer/xyzs-oscilloscopes-primer
    Just press the download file button on the page and store it on your PC.

    Bertus
     
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  8. May 26, 2020 #8
    Rajinder

    Rajinder

    437
    7
    Jan 30, 2016
    Hi,
    I have tried this. The square wave at probe comp looks fine, with no overshoot of the signal. I will try and upload a screenshot.
    The scope is a MSOX3024A KEYSIGHT.
    Thanks for your help
     
  9. May 26, 2020 #9
    Harald Kapp

    Harald Kapp Moderator Moderator

    10,363
    2,260
    Nov 17, 2011
    The thread title is misleading: The overshoot is on the SPI signals, not on the WIFI signal.
     
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  10. May 26, 2020 #10
    bertus

    bertus Moderator

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    Nov 8, 2019
    Rajinder likes this.
  11. May 26, 2020 #11
    Rajinder

    Rajinder

    437
    7
    Jan 30, 2016
    Sorry, yes SPI signal.
     
  12. May 26, 2020 #12
    Rajinder

    Rajinder

    437
    7
    Jan 30, 2016
  13. May 27, 2020 #13
    Harald Kapp

    Harald Kapp Moderator Moderator

    10,363
    2,260
    Nov 17, 2011
    I adapted the thread title accordingly.
     
  14. May 27, 2020 #14
    Rajinder

    Rajinder

    437
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    Jan 30, 2016
    No problem.
     
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