[SI-LIST] Re: What is the difference between tDQSS, tDSS and tDSH
- From: Hermann Ruckerbauer <hermann.ruckerbauer@xxxxxxxxxxxxx>
- To: Istvan Nagy <buenos@xxxxxxxxxxx>
- Date: Sun, 28 Mar 2010 21:50:42 +0200
Hi Istvan,
t_DQSS is an input parameter to align DQS to clock input. As you
mentioned, this is basically to make sure that the DRAM is activating
it's receiver at the right time to catch the preamble of the DQS.
tDSS and tDSH are just the normal Setup/Hold times that every receiver
needs. In this case it is the relation of the DQ to the corresponding
timing signal, the DQS, so they are pure input timings.
In the DRAM spec there are several cases that two different timings need
to be fullfilled, but sometimes it is not possible that all combinations
are valid. I would need to check the exact timings, but I guess this is
one of this cases.
Setup time is always minimum required input setup time, and as you
mentioned "at a given point on the board": All timings are defined at
the ball of the DRAM.
Regards
Hermann
EKH - EyeKnowHow
Hermann Ruckerbauer
www.EyeKnowHow.de
Hermann.Ruckerbauer@xxxxxxxxxxxxx
Veilchenstrasse 1
94554 Moos
Tel.: +49 (0)9938 / 902 083
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schrieb Istvan Nagy:
> Hi
>
> The JESD79-xx Jedec standards specify three timing parameters for DDRx
> memory write clock-strobe timing:
> tDQSS, tDSS and tDSH.
> I understand the t_DQSS, that it makes shure that the strobe will arrive to
> the memory chip within a window relative to the clock, so we will latch the
> right data bit at the right time.
> but what is the purpose of the other two parameters?
> they seem to have a tighter value, so if they specify the same write
> clk-strobe matching, then the DQSS specification does not make sense, since
> the other two parameters already specified the same thing with a narrover
> limit.
> anyway, are the DSS and DSH parameters input requirements or output
> guaranteed timings?
> the jedec specs dont really make a difference between input requirements and
> output guaranteed timings in the explanations of the timing parameters. for
> example they say "setup time", but it can mean "input setup requirement" or
> "output worst-case guaranteed setup time", or just the actual setup time of
> a signal on a given point on the board.
>
> regards,
> Istvan Nagy
> CCT
>
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