> > The DRAM needs to be isolated from our control unit > > What kind of "isolation"? >When I mean isolated, I mean, "away from". The DRAM must be 19 inches away from the irradiation source.> > 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.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...
Longest path length from SDRAM controller to DRAM?
Started by ●October 16, 2007
Reply by ●October 25, 20072007-10-25
Reply by ●October 25, 20072007-10-25
On Oct 16, 5:48 pm, CBFalconer <cbfalco...@yahoo.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? > > 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 fromhttp://www.teranews.comThat 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
Reply by ●November 6, 20072007-11-06
On Oct 25, 6:43 pm, sendt...@gmail.com wrote:> On Oct 16, 5:48 pm, CBFalconer <cbfalco...@yahoo.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? > > > 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 fromhttp://www.teranews.com > > 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- Hide quoted text - > > - Show quoted text -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.
Reply by ●November 13, 20072007-11-13
On Nov 6, 3:13 pm, sendt...@gmail.com wrote:> On Oct 25, 6:43 pm, sendt...@gmail.com wrote: > > > > > > > On Oct 16, 5:48 pm, CBFalconer <cbfalco...@yahoo.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? > > > > 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 fromhttp://www.teranews.com > > > 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- Hide quoted text - > > > - Show quoted text - > > 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.- Hide quoted text - > > - Show quoted text -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...
Reply by ●November 15, 20072007-11-15
On Nov 13, 3:20 pm, sendt...@gmail.com wrote:> On Nov 6, 3:13 pm, sendt...@gmail.com wrote: > > > > > On Oct 25, 6:43 pm, sendt...@gmail.com wrote: > > > > On Oct 16, 5:48 pm, CBFalconer <cbfalco...@yahoo.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? > > > > > 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 fromhttp://www.teranews.com > > > > 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- Hide quoted text - > > > > - Show quoted text - > > > 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.- Hide quoted text - > > > - Show quoted text - > > 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...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.
