SPI Master MISO Noise Issue

[george]

I'm trying to connect to a ST LSM6DS3 via SPI on the Nordic chip. I have used SPI on Nordic with other peripherals but couldn't seem to get it to work with this ST chip. Eventually, after debugging it from a firmware stand-point I decided to scope it out and see if the SPI signals were corresponding to what I was getting in firmware. What I discovered is the two situations:

1. when I probe MISO when it is disconnected to the Nordic chip the response from the ST chip is correct. Nice clean crisp signal

2. When I probe it with MISO connected to the Nordic chip I get some sort of noise.

I included the two captures below and was hoping to get an idea why the SPI interface is behaving like this. Does anyone know why it is doing this and how to make it stop? Do I have some internal pull-up/pull-down's configured incorrectly? I am a bit out of ideas... :/

ST Datasheet: www.st.com/.../DM00133076.pdf

[Ben]

I know this is an old post but others, like myself, may see it as extremely useful as it is still relevant. Not going too deep into detail I had the same symptoms as displayed on your oscilloscope but different chips. It came down to unbalanced lines. Even if you have a low clock frequency the fast ns clock edges need to be respected. I found a 470 ohm resistor in series with the SPI CLK line on the Master side. Removing that resistor and implementing the solution recommended for unbalanced lines from Lattice (I could only implement half the solution from the Slave side) I was able to extend the length of my ribbon cable from 10cm to 160cm with no comms errors:

www.latticesemi.com%2F~%2Fmedia%2FLatticeSemi%2FDocuments%2FWhitePapers%2FHM%2FImprovingNoiseImmunityforSerialInterface.pdf%3Fdocument_id%3D50728&usg=AFQjCNG00KXfdgaufV_zq0V5o_YFqEE8OQ