DDR2-Vref Noise

HiIn ddr2, VREF noise causes strobe-to-data skew. Since DQ and DQS areconnected to same Vref, then how does Vref noise cause skew between thesetwo signals?Please clarify so that I can understand.-- Best Regards,Balamanikandan.K
kbmanick 7 years 3 months 3 days

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Answered bykbmanick 7 years 3 months
Thank You sir.Yes I understood.If we use a differential DQS, then the noise event on Vref will affect theabsolute voltage that a signal (DQ) has to reach to achieve a valid high orlow level. So, the set up and hold time gets affected.If we use a same reference voltage for DQS (i.e. Single ended DQS) and DQ,then the skew can be eliminated (But the advantages of differential DQSwill be lost)Am I right sir?On Mon, Aug 5, 2013 at 4:04 PM, Hermann Ruckerbauer  wrote:> DQS is (usually) differential and referenced to the Crossing, not to> Vref ...>>> Hermann>> EKH - EyeKnowHow> Hermann Ruckerbauer> www.EyeKnowHow.de> Hermann.Ruckerbauer@xxxxxxxxxxxxx> Itzlinger Strasse 21a> 94469 Deggendorf> Tel.:   +49 (0)991 / 29 69 29 05> Mobile: +49 (0)176  / 787 787 77> Fax:    +49 (0)3212 / 121 9008>> schrieb Balamanikandan K:> > Hi> > In ddr2, VREF noise causes strobe-to-data skew. Since DQ and DQS are> > connected to same Vref, then how does Vref noise cause skew between these> > two signals?> >> > Please clarify so that I can understand.> >>> 
Answered bykwiljugrein 7 years 3 months
No. Let's say noise lowers the Vref slightly at some point in time. Thatwould speed up a rising edge (crosses Vref earlier), but delay a fallingedge (crosses Vref later). Now imagine what happens to the setup time ifDQ changes from high to low (delayed), and DQS is rising (faster).Do the other combinations of falling/rising DQ/DQS and noiseraising/lowering Vref, you see that this simply closes your timing-window.Knut GeorgOn 08.08.2013 12:55, Balamanikandan K wrote:> Thank You sir.>> Yes I understood.>>> If we use a differential DQS, then the noise event on Vref will affect the> absolute voltage that a signal (DQ) has to reach to achieve a valid high or> low level. So, the set up and hold time gets affected.>>> If we use a same reference voltage for DQS (i.e. Single ended DQS) and DQ,> then the skew can be eliminated (But the advantages of differential DQS> will be lost)>>> Am I right sir?>>> On Mon, Aug 5, 2013 at 4:04 PM, Hermann Ruckerbauer <> Hermann.Ruckerbauer@xxxxxxxxxxxxx> wrote:>>> DQS is (usually) differential and referenced to the Crossing, not to>> Vref ...>>>>>> Hermann>>>> EKH - EyeKnowHow>> Hermann Ruckerbauer>> www.EyeKnowHow.de>> Hermann.Ruckerbauer@xxxxxxxxxxxxx>> Itzlinger Strasse 21a>> 94469 Deggendorf>> Tel.:   +49 (0)991 / 29 69 29 05>> Mobile: +49 (0)176  / 787 787 77>> Fax:    +49 (0)3212 / 121 9008>>>> schrieb Balamanikandan K:>>> Hi>>> In ddr2, VREF noise causes strobe-to-data skew. Since DQ and DQS are>>> connected to same Vref, then how does Vref noise cause skew between these>>> two signals?>>>>>> Please clarify so that I can understand.>>>>> 
Answered byHermann.Ruckerbauer 7 years 3 months 3 days
DQS is (usually) differential and referenced to the Crossing, not toVref ...HermannEKH - EyeKnowHowHermann Ruckerbauerwww.EyeKnowHow.deHermann.Ruckerbauer@xxxxxxxxxxxxxItzlinger Strasse 21a94469 DeggendorfTel.:   +49 (0)991 / 29 69 29 05Mobile: +49 (0)176  / 787 787 77Fax:    +49 (0)3212 / 121 9008schrieb Balamanikandan K:> Hi> In ddr2, VREF noise causes strobe-to-data skew. Since DQ and DQS are> connected to same Vref, then how does Vref noise cause skew between these> two signals?>> Please clarify so that I can understand.>