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SPI Signal Overshoot (to WIFI chip)

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

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  1. Rajinder

    Rajinder

    426
    7
    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. Hunter64

    Hunter64

    39
    12
    Nov 20, 2018
    Another WiFi IC without any information.

    A typenumber maybey?
     
  3. Rajinder

    Rajinder

    426
    7
    Jan 30, 2016
    WFM2000 from SiLabs.
     
  4. Nanren888

    Nanren888

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    43
    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. Rajinder

    Rajinder

    426
    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. WHONOES

    WHONOES

    856
    206
    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.
     
    Rajinder likes this.
  7. bertus

    bertus Moderator

    541
    179
    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. Rajinder

    Rajinder

    426
    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. Harald Kapp

    Harald Kapp Moderator Moderator

    9,887
    2,096
    Nov 17, 2011
    The thread title is misleading: The overshoot is on the SPI signals, not on the WIFI signal.
     
    Rajinder likes this.
  10. bertus

    bertus Moderator

    541
    179
    Nov 8, 2019
    Rajinder likes this.
  11. Rajinder

    Rajinder

    426
    7
    Jan 30, 2016
    Sorry, yes SPI signal.
     
  12. Rajinder

    Rajinder

    426
    7
    Jan 30, 2016
  13. Harald Kapp

    Harald Kapp Moderator Moderator

    9,887
    2,096
    Nov 17, 2011
    I adapted the thread title accordingly.
     
  14. Rajinder

    Rajinder

    426
    7
    Jan 30, 2016
    No problem.
     
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