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SPI and CAN BUS Signal Integrity

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sanwal209:
Hi,

We have a board on which MPC5774P MCU is connected to external flash via SPI and have 3 CAN BUS. SPI and CAN BUSES are working fine but we want to make sure in field it will not go absolutely nuts due to some noise.

What could be the possible tests to make sure SPI and CAN BUS have good signal integrity. If i check for signal integrity then what is accepted level of error. I am totally confused here.

jbb:
Well done for being proactive about this!

Generally, I would be more worried about the SPI bus. SPI was really designed to go on a circuit board. CAN was designed to work over wires inside a car (a difficult environment).

I suggest the following as the ‘basics’:

* Review your software to estimate what happens if data gets corrupted. Or a wire falls off :-|.
* Check you have ESD protection on your SPI and CAN cable ports. (Note: CAN driver chips may have it built in.)
* Measure the waveforms at both ends of the SPI with a scope. You want to see smooth rises and falls, without overshoot, undershoot or cross talk.
* repeat test with minimum and maximum supply voltage.
* Put the units in an oven and run them for some hours at the maximum specified ambient temperature.
* Put the units in a freezer ...
Following that, it’s time to do some EMC lab work:

* Do some ESD testing.
* Do EMC immunity testing (I.e. hit the unit with external RF sources to see what happens.)
You’ll need to determine which standards you test to - which is where I always feel a bit uncomformtable because there are so many to choose from...

Brutte:
You should put a stress on a bus.

With short SPI the problem is the impedance of the lines. Increase R*C on the lines for tests. Pick values that would double the natural RC constant. If it passes the tests with additional load then it would most likely work flawlessly without it.

With CAN - well a termination could be a problem, also abuse (shorts, overvoltages etc).

sanwal209:
Thanks @jbb and @Brutte, I really appreciate it.

What if there is an under or over shoot on line how can i tackle it? I didn't checked SPI because of the busy schedule of software guys but i did check CAN BUS and there is high frequency ringing on it. How can i remove it?

I have attached CAN Filter which we have on board and snapshot of the waveform.

Another question which confuses me is that for example if we have some noise on either SPI or CAN, how can i know which is acceptable value of noise?

tautech:
Probe the supply pin of the MCU and the main power rails too to check if supply decoupling is sufficient to suppress noise that might be coupled through to the bus.
You've got enough channels to line up all the traces to check if it's a problem or not.
Use AC coupling for these checks.  ;)

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