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SPI Signal Overshoot (to WIFI chip)

Rajinder

Jan 30, 2016
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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.
 

Hunter64

Nov 20, 2018
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Another WiFi IC without any information.

A typenumber maybey?
 

Nanren888

Nov 8, 2015
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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.
 

Rajinder

Jan 30, 2016
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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
 

WHONOES

May 20, 2017
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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.
 

bertus

Moderator
Nov 8, 2019
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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
 

Rajinder

Jan 30, 2016
568
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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.
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
 

Harald Kapp

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Nov 17, 2011
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The thread title is misleading: The overshoot is on the SPI signals, not on the WIFI signal.
 
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