[SI-LIST] Re: routing of DDR clock signals
- From: steve weir <weirsp@xxxxxxxxxx>
- To: "Henrik Gild" <hencox@xxxxxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Mon, 16 Aug 2004 06:51:36 -0700
At 02:58 PM 8/16/2004 +0200, Henrik Gild wrote:
>Hi!
>
>>Henrik, if you are new at this, you will probably get the best results by=
=20
>>treating the pair as a loosely coupled differential pair. That means=20
>>providing greater than normal horizontal spacing between the CK and CK#=20
>>than normal signals. Then the differential impedance will be close to 2X=
=20
>>the normal mode impedance of each line.
>
>Should the differential impedance of the CK+CK# pair actually be 100ohm or=
=20
>50ohm? To reach for example 50ohm differential impedance, should I set the=
=20
>individual width of CK and CK# so they equal 25ohm normal mode impedance?
Individually to ground ( normal mode ) close to 50 ohms each which will=20
result in close to 100ohms differential IFF loosely coupled. If they are=20
tightly coupled the differential impedance will be less than 2X the normal=
=20
mode.
>If I use 6 mil signals, what distance between the CK and the CK# signals=20
>would you recommend?
18 - 24 mils on centers should be loose enough.
>I'm connecting this SO-DIMM to a Spartan3 FPGA. Do I need to switch to=20
>another signal standard on the pins of the FPGA acting as the CK and CK#=20
>signals (like LVDS) because they are differential on the board, or will=20
>normal SSTL2 suite well anyway?
>Many questions... :)
No, you must use the 2.5V totem pole drive specified for DDR.
133MHz DDR is pushing Spartan3 pretty hard. This will require considerable=
=20
care, and/or -5 parts.
Xilinx gives away a DDR design for Virtex II / PRO. I have not seen it=20
used with Spartan3.
>>Additionally, timing aberrations caused by crosstalk can be reduced by=20
>>affording 5H spacing between either CK or CK# and the nearest outside=20
>>neighbor signal.
>
>Do you include other DDR signals as well in the "neighbor signal" group?
Yes, all signals. If you can afford 30 mils on either side of the pair, do=
it.
>I'm not sure what you mean by 5H, is it five times the normal distance=20
>between tracks?
5 X height over plane.
>> Remember that you are matching electrical length. The easiest way to=20
>> do that is to keep the physical routes reasonably close together and=20
>> then match physical length.
>
>Thanks a lot for your answers!
>/Henrik
You are welcome.
>_________________________________________________________________
>Chat: Ha en fest p=E5 Habbo Hotel=20
>http://habbohotel.msn.se/habbo/sv/channelizer Checka in h=E4r!
>
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