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Electronics => Beginners => Topic started by: danderson on February 12, 2014, 07:22:36 am

Title: Is this clock signal ringing normal?
Post by: danderson on February 12, 2014, 07:22:36 am
I'm playing with MCP2515 CAN controllers on a breadboard, hooked up to random ATmegas I had lying around. Unrelated to the CAN functionality, the 2515 provides a clock out signal, so that you can run multiple chips from the 2515's oscillator. Out of curiosity, I looked at the clock output on my scope, with a 16MHz crystal... And I got this signal, with nice clean rises and falls, but lots of ringing in between: http://imgur.com/a/25t9b (http://imgur.com/a/25t9b) .

The oscillation isn't enough to anger digital chips, but it's still 0.3-0.5V of oscillation, which feels like a lot. This is a breadboard setup, so I'm expecting some noise here and there, but this is rock steady once the scope is triggered, not randomized, which makes me think this is actually a "feature" of the 2515's output signal, rather than something introduced by the breadboard.

I'm still very new to peeking at signals with a scope. Is this sort of oscillation on digital signals to be expected? If not, what things should I check to find the source?
Title: Re: Is this clock signal ringing normal?
Post by: codeboy2k on February 12, 2014, 08:31:24 am
That's not ringing.  There is a little bit of overshoot on the leading edge, then following that would normally be any ringing, if you had it.  But there is little to no ringing there.  Ringing would be a initial large amplitude wave then decaying to 0 after a few cycles.

What you have there is about 300mV of ripple noise that is showing up on the clock's output.

It could be one of two things... Either decoupling, or the voltage regulator bulk capacitance is too low.

Start with a decoupling cap on your breadboard near the positive rail pin on the MCP2515.  Put about 0.1u there. If that doesn't help, then add bulk 10uF caps and 100uF caps around the breadboard.

It's possibly something that is on your power supply to the MCP2515 chip, so check that with a scope and check also that your PSU has enough output bulk capacitance.  Maybe you are using a voltage regulator on the breadboard, if so, make sure it has both input and output capacitors.