Longest path length from SDRAM controller to DRAM?

You can get a rough idea by the propagation time. 2 feet of cable will have a one-way propagation time in the order of 2 nanosecs, and a characteristic impedance in the order of 100 ohms. So ringing will be in the order of 250 MHz. The cures are proper matching and timing.

Someone with strong current hardware skills is sure to answer this better, but I think the length through ribbon cable is basically 0 inches. The timing is very fast and will be very susceptible to cross talk. Many current SDRAM designs even use trace length balancing to insure all the signals are available at the same time.

You will most likely need another solution, possibly an RF shield can over the other circuitry.

Scott

So, two feet of ribbon cable. Take a velocity factor of 0.66 for grins, which gives you an effective 3 foot free-space length. Then consider that the signals have to go there and get back.

Hmm. That's 6ns, or just about 1/166MHz. And that's ignoring any nasty impedance bumps you may have in your connector and cable, not to mention cross-talk.

What was your clock rate again?

Did you check for a minimum clock rate on the data sheet? The last time I was around SDRAM there was a minimum not-to-be-exceeded clock rate, and it wasn't all that much lower than the maximum not-to-be-exceeded.

A stupid question.

Are you exposing the SDRAM to radiation or are you measuring EMI from the SDRAM?

That's a bit harsh, although the poor chap seems unable to spell "because".

Either way the OP could probably get reasonable results by wrapping his controller up in aluminium foil or, more professionally, a metal enclosure with proper braided earthing. However, I believe they mentioned that they were expecting the device to fail so presumably they are testing for EMI succeptibility.

My apologies to the original poster and the group. My meaning was "I have a stupid question".

Forgive me for being a physicist but: what kind of radiation would that be?

What kind of "isolation"?

Ultimately, you never do. That's why people do stability checks, a.k.a. calibration runs, where they measure the effect of the experimental environment on the measurement devices themselves.

I'll second the guess already voiced here before: zero. SDRAM signals aren't designed to travel on pretty much anything else than short(!), scrupulously designed traces on multi-layer PCBs.

Well I'm willing to provide ANY clock rate. I'm trying 20 Mhz because that seemed like a reasonable clock speed to work with. The data sheet didn't specify a minimum clock speed but I would assume it would have to be at least fast enough to provide a refresh rate (~100kHz).

I'm exposing it to radiation. And I knew what you meant by "A stupid question" when I read it, but thanks for clarifying anyway.

When I mean isolated, I mean, "away from". The DRAM must be 19 inches away from the irradiation source.

That's what I originally thought too, but people I asked (who don't necessarily have experience in the subject) said it seems reasonable to assume it's okay. We ended up using CAT5e cable for the cabling...

That seems reasonable. I'm using a 20 or 50Mhz clock but I can certainly slow it down or speed it up fairly easily. The cabling is made with twisted pair although they're not perfectly the same length. There's also a short length of it (to be reduced) that's using ribbon cable.

I guess I'll know soon enough if it works or not since we pressed ahead with building a test fixture. I'll let everyone know how it works out...

Thanks,

Eric

Well we've been playing with the test fixture for a while now. It doesn't appear to be working. Not sure if it's due to skew although we're compensating that with a PLL. We're also getting some voltage spikes.

We're going to reduce the wire lengths and see if that helps. If so, then we'll need to redesign our test. If not, I don't know what to do.

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Actually we manage to get it working. There's still some jitter and ringing but we hope to be able to clean that up with some tweaking. Thanks for the help. So if anyone ever needs to run DRAM over 20 inches of wire, use twisted pair and run it at a 10 Mhz clock... we have had some success over a 50Mhz clock...

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Without termination we got it to run perfectly (passed some basic memory scans) on 4 inches of twisted pair at 66.66667 Mhz. It fails somewhere between 66.67Mhz and 75Mhz but it wasn't important to find exactly when it failed.

On the longer cable, ~24 inches, it passes the memory scan at 20 Mhz without any type of termination.