Longest path length from SDRAM controller to DRAM?

I'm trying to design a test setup to do radiation experiments on SDR
DRAM
for my thesis. The DRAM needs to be isolated from our control unit b/c
I'm
worried about contaminating our results by having our control
circuitry
in there too - b/c then how do we know if it's the DRAM or
"something"
else that fails.

So my question is, what's the longest length I can run a ribbon cable
(or other type of connection) and not having issues? Can I bring the
clock down? We are using 20-24 inches of cabling. We decided maybe to
use CAT6 cable?

Thanks,
Eric

sendthis@gmail.com wrote:
> 
> I'm trying to design a test setup to do radiation experiments on
> SDR DRAM for my thesis. The DRAM needs to be isolated from our
> control unit b/c I'm worried about contaminating our results by
> having our control circuitry in there too - b/c then how do we
> know if it's the DRAM or "something" else that fails.
> 
> So my question is, what's the longest length I can run a ribbon
> cable (or other type of connection) and not having issues? Can
> I bring the clock down? We are using 20-24 inches of cabling. We
> decided maybe to use CAT6 cable?

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.

-- 
 Chuck F (cbfalconer at maineline dot net)
   Available for consulting/temporary embedded and systems.
   <http://cbfalconer.home.att.net>



-- 
Posted via a free Usenet account from http://www.teranews.com

<sendthis@gmail.com> wrote in message 
news:1192568512.220140.258800@i13g2000prf.googlegroups.com...
> I'm trying to design a test setup to do radiation experiments on SDR
> DRAM
> for my thesis. The DRAM needs to be isolated from our control unit b/c
> I'm
> worried about contaminating our results by having our control
> circuitry
> in there too - b/c then how do we know if it's the DRAM or
> "something"
> else that fails.
>
> So my question is, what's the longest length I can run a ribbon cable
> (or other type of connection) and not having issues? Can I bring the
> clock down? We are using 20-24 inches of cabling. We decided maybe to
> use CAT6 cable?
>
> Thanks,
> Eric
>
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 

sendthis@gmail.com wrote:
> I'm trying to design a test setup to do radiation experiments on SDR
> DRAM
> for my thesis. The DRAM needs to be isolated from our control unit b/c
> I'm
> worried about contaminating our results by having our control
> circuitry
> in there too - b/c then how do we know if it's the DRAM or
> "something"
> else that fails.
> 
> So my question is, what's the longest length I can run a ribbon cable
> (or other type of connection) and not having issues? Can I bring the
> clock down? We are using 20-24 inches of cabling. We decided maybe to
> use CAT6 cable?
> 
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.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

sendthis@gmail.com wrote:
> I'm trying to design a test setup to do radiation experiments on SDR
> DRAM
> for my thesis. The DRAM needs to be isolated from our control unit b/c
> I'm
> worried about contaminating our results by having our control
> circuitry
> in there too - b/c then how do we know if it's the DRAM or
> "something"
> else that fails.
> 
> So my question is, what's the longest length I can run a ribbon cable
> (or other type of connection) and not having issues? Can I bring the
> clock down? We are using 20-24 inches of cabling. We decided maybe to
> use CAT6 cable?

A stupid question.

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

"Jim Stewart" <jstewart@jkmicro.com> wrote in message 
news:D9OdnUGssp1Zp4janZ2dnUVZ_r_inZ2d@omsoft.com...
> sendthis@gmail.com wrote:
>> I'm trying to design a test setup to do radiation experiments on SDR
>> DRAM
>> for my thesis. The DRAM needs to be isolated from our control unit 
>> b/c
>> I'm
>> worried about contaminating our results by having our control
>> circuitry
>> in there too - b/c then how do we know if it's the DRAM or
>> "something"
>> else that fails.
>>
>> So my question is, what's the longest length I can run a ribbon cable
>> (or other type of connection) and not having issues? Can I bring the
>> clock down? We are using 20-24 inches of cabling. We decided maybe to
>> use CAT6 cable?
>
> A stupid question.

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

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

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. 

Tom Lucas wrote:
> "Jim Stewart" <jstewart@jkmicro.com> wrote in message 
> news:D9OdnUGssp1Zp4janZ2dnUVZ_r_inZ2d@omsoft.com...
>> sendthis@gmail.com wrote:
>>> I'm trying to design a test setup to do radiation experiments on SDR
>>> DRAM
>>> for my thesis. The DRAM needs to be isolated from our control unit 
>>> b/c
>>> I'm
>>> worried about contaminating our results by having our control
>>> circuitry
>>> in there too - b/c then how do we know if it's the DRAM or
>>> "something"
>>> else that fails.
>>>
>>> So my question is, what's the longest length I can run a ribbon cable
>>> (or other type of connection) and not having issues? Can I bring the
>>> clock down? We are using 20-24 inches of cabling. We decided maybe to
>>> use CAT6 cable?
>> A stupid question.
> 
> That's a bit harsh, although the poor chap seems unable to spell 
> "because".

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

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

sendthis@gmail.com wrote:

> I'm trying to design a test setup to do radiation experiments on SDR 
> DRAM for my thesis.

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

 > The DRAM needs to be isolated from our control unit

What kind of "isolation"?

> - b/c then how do we know if it's the DRAM or
> "something" else that fails.

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.

> So my question is, what's the longest length I can run a ribbon cable
> (or other type of connection) and not having issues? 

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.

> What was your clock rate again?
>

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).

On Oct 16, 6:27 pm, Jim Stewart <jstew...@jkmicro.com> wrote:
> sendt...@gmail.com wrote:
> > I'm trying to design a test setup to do radiation experiments on SDR
> > DRAM
> > for my thesis. The DRAM needs to be isolated from our control unit b/c
> > I'm
> > worried about contaminating our results by having our control
> > circuitry
> > in there too - b/c then how do we know if it's the DRAM or
> > "something"
> > else that fails.
>
> > So my question is, what's the longest length I can run a ribbon cable
> > (or other type of connection) and not having issues? Can I bring the
> > clock down? We are using 20-24 inches of cabling. We decided maybe to
> > use CAT6 cable?
>
> A stupid question.
>
> Are you exposing the SDRAM to radiation
> or are you measuring EMI from the SDRAM?

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.