SPI Signal Overshoot (to WIFI chip)

[Rajinder]

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]

Another WiFi IC without any information.

A typenumber maybey?

 

[Rajinder]

WFM2000 from SiLabs.

 

[Nanren888]

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]

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]

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]

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]

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]

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

 

[bertus]

Hello,

Do the signals look a bit like the ones on this stack exchange page:
https://electronics.stackexchange.c...hoot-and-ringing-does-that-affect-reliability

Bertus

 

[Rajinder]

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

Sorry, yes SPI signal.

 

[Rajinder]

Hello,

Do the signals look a bit like the ones on this stack exchange page:
https://electronics.stackexchange.c...hoot-and-ringing-does-that-affect-reliability

Bertus

Yes they do

 

[Harald Kapp]

Sorry, yes SPI signal.

I adapted the thread title accordingly.

 

[Rajinder]

I adapted the thread title accordingly.

No problem.