There is an abnormal spike when analyzing SPI line

[Sean Wang]

I use Analog Discovery 2 to analyze SPI line and there is an abnormal spike during SS is set to low. How do I elimenate it?

[attila]

Hi @Sean Wang

This is crosstalk between signal wires or lack of ground connection.
- As James pointed out, make sure you have proper ground connection between the devices.
- Twist the signal wires with a ground wire, specially the CS and CLK (or use shielded wires).
To reduce ringing use termination like series resistor.

Similar to the HS adapter:
https://digilent.com/shop/digital-discovery-high-speed-adapter-and-logic-probes/

 

[attila]

Hi @Sean Wang

Ideally all signal signal wires should be shielded and have termination.
The twisted wire is ideal for differential signals but twisting single ended signals with gnd and connecting this on both sides to a gnd plane close to the signal source/destination provides a kind of shielding.
For SPI it may be suffice to twist CS with gnd and have ground connection only on one side, on which it is simpler to do. Similar twisting may be needed also for CLK. Here a series resistor on source side may also be used to reduce ringing, if such problem is noticed.

[JColvin]

Hi @Sean Wang,

What system do you have driving the SPI bus? Do you have the grounds connected between the Analog Discovery 2

You can get another point of data regarding the source of these spikes by viewing the chip select line with the oscilloscope inputs on the Analog Discovery 2 to observe the voltage changes on your signal lines.

Thanks,
JColvin

[Sean Wang]

I use CY8CKIT-059 as master and esp8266 as slave. Master can transmit data to slave via SPI correctly and there are the spikes no matter ground of Analog Discovery 2 is connected to the system.

[Sean Wang]

1.Should I connect all Analog Discovery 2's ground to my development board? or one ground to be connected is ok?

2.I already sperate all wires connection, but the spike still appears.

3.Twist the signal wires with a ground wire. Did you mean this one? If so, can I do it with dupont line or it has to use special wire? And which side should I use twist wire? The Master side or oscilloscope side or both?

By the way, I observe chip select line from oscilloscope and there is the noise on the ground(one ground on Analog Discovery 2 already connected to my device and all wires are separated).

 

Edited March 21, 2023 by Sean Wang