Clock ringing

What's the driver chip?

John

Too much to run unterminated for modern logic parts.

Probably not. Here's why: Modern logic, and that includes modern uC, drive stuff at transition times in the low single-digit nanoseconds. This ain't grandpa's old 7400 TTL stuff no more.

So what to do now?

Tim's suggestion of slowing the transition times is one solution but you'd need a chip that can do this. Except for a few data line drivers most won't, at leat not down to where it needs to be for 10" line lengths.

AC-termination is one option: Calculate the trace impedance for each line, can be done from the Gerber and PCB fab data. At the end of each line, hang a a series combination of a resistor and a 33pF cap. The value of the resistor should equal the impedance you calculated which .. hopefully .. won't be too low.

Technically you should drive each line separately using a buffer, for example using a simple 8-bit bus driver. Those are cheap. The reason is that if the impedance is low and you have many lines this overwhelms the usual digital chip output. For example, a port pin of a MSP430 drives loads with roughly 30ohms of Rdson in its output devices. If you had five 100ohm clock traces in parallel that would be like hitching a 5-ton travel trailer to a Volkswagen Beetle.

P.S.: If the stubs are 1" or less don't worry about them right now. But if there's inputs with lots of load or capacitance hanging off of them, that would be not so cool.

I was thinking the same thing. It ain't the clock rate, it is the rise and fall times, and how many headaches could be alleviated by one little 'ol resistor.

The lone resistor isn't going to cut it because he drives several clock lines in parallel. So if each was 100ohms and you'd use only a resistor the five lines would be 20ohms. Then you could fry scrambled eggs on the driver chip :-)

what if that lone resistor is in series at the source, slowing down the edge just enough ..

-Lasse

Then you can end up with ringing going up and down the lines. The impedance presented to the source is a pretty bizarre one in this case.

But at this stage of the game the OP should just try what works best, AC-termination, series resistor, resonant this that and the other thing, and so on. I guess folks are already breathing down his back asking when they can ship it :-)

he

he did say it works just fine, it just looks bad on the scope

maybe there isn't really an issue, could just be the scope and probe playing with him, wouldn't be the first time smoething like that happend ;)

-Lasse

He wrote, quote "The peak of the initial transient of the ring is close to 1/3 of the clock so it is rather significant".

If that isn't a measurement error I'd certainly second that statement.

Sure, but I assume he has also measured the clock at the source and saw a nice clean square. Then his scope hook-up would be ok, provided the probe is switched to 10x.

It seems it was the length of the ground clip on the probe. When I shortened it the clocks cleaned up nicely. No ringing at all from what I could tell.

How does an extra long ground wire on a probe cause ringing? In my case the ground points should be virtually identical(low current and big ground planes). Does this have to do with the 10x probe mentioned?

The ground lead is an inductor and the scope cannot be considered grounded for RF purposes. This puts the ground lead inductance in series with your measured signal.

Hint for next time: Always measure the clock at the source as well. If it's ringing the same as at the far end then there is very likely a problem with the measurement setup.

Also, make sure you have a scope of 200MHz bandwidth or more, depending on how fast your transitions are. With a scope of too little bandwidth it can happen that everything looks ok on screen but in reality it isn't.

Yep. I guess maybe some type of AC or active termination (???), but if he is going to tack that much crap on, he might as well figure it out and respin the board to include the termination he wanted to avoid.

Ah, after I posted, I saw that indeed it was his scope set up. That can get ya'.