[SI-LIST] Re: SDRAM timing
- From: Dan Bostan <dbostan@xxxxxxxxx>
- To: brian.p.moran@xxxxxxxxx, peter.zhu@xxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Mon, 29 Nov 2004 12:08:37 -0800 (PST)
If I may add my two cents, this solution is fine if
the controller is standard.
Some FPGAs (IP) vendors use retiming ( a 4x or 2x
clock) to register the data.
In this situation, DQ must meet certain tsu and th,
which a function of the clock.
/dan
--- "Moran, Brian P" <brian.p.moran@xxxxxxxxx> wrote:
> Peter,
>
> There are of course numerous timing paths on a DDR2
> interface. The
> primary paths associated with the data bus are DQ to
> DQS setup and hold
> margins, and the DQS to CLK setup and hold margins.
> The DQ to DQS timing
> is often discussed in terms of DQ to DQS skew, and
> is measured within
> each byte lane independently. To maximize these
> margins it is necessary
> to match each of the DQ signals to there associated
> DQS strobe, on a
> byte lane by byte lane basis. (In some cases there
> may be a fixed offset
> specified between DQ and DQS in order to center the
> setup and hold
> margins.) The overall length of the byte lane has
> only a second order
> impact on these calculations. The length range over
> which each byte lane
> may be routed as a group is usually fairly wide.=20
>
>
> The amount of length range over which the byte lanes
> may vary is
> controlled by the DQS to CLK path, for the most
> part. Here you have to
> worry about traditional setup and hold with respect
> to the clock. In
> this case the longest DQS signal will generally
> control setup margin and
> the shortest DQS will control hold margin.
>
> If you look at Intel guidelines for example, there
> is a spec for DQ to
> DQS length matching within a byte lane, which is
> generally on the order
> of DQ length =3D (DQS Length - Offset) +/-20 mils.
> Then there is a
> guideline for DQS vs CLK length matching which may
> be on the order of
> CLK - 2.0" </=3D DQS </=3D CLK + 1.0". This is just
> an example and you =
> need
> to consult the guidelines provided by your
> controller vendor, but this
> is how the guidelines are generally structured.
>
>
> Brian P. Moran=20
> SIE Engineer=20
> Intel Corporation=20
> brian.p.moran@xxxxxxxxx=20
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of peter zhu
> Sent: Friday, November 26, 2004 3:04 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] SDRAM timing
>
> Hi, all:
>
> I oftem see designer reel the PCB trace for
> equivalent length in SDRAM
> data bus. I am very puzzled about it all the time I
> think, for SDRAM
> data bus, there is on need for equivalent trace
> length in SDRAM data
> bus. The timing is determined by the longest data
> bus. For other SDRAM
> data bus, the short, the better, and no need for
> equivalent. Do you
> think so?
>
> Also where can I find some detailed guide for SDRAM
> and DDR tracing?
>
> Thanks in advanced
>
> Peter
>
>
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