[SI-LIST] Re: DDR3 VTT Bypassing

  • From: steve weir <weirsi@xxxxxxxxxx>
  • To: Stefan Milnor <stefan.milnor@xxxxxxxxxxxxxx>
  • Date: Sat, 08 Sep 2012 16:37:21 -0700

Data signals are supposed to run against GND.  A/C are supposed to run 
against VDD.

Steve
On 9/8/2012 7:19 AM, Stefan Milnor wrote:
> But they are not against VDD, they are against GND. Hence my question; are we 
> getting conflicting advice from the vendor (route signals against GND but 
> bypass VTT to VDD)
>
> On Sep 7, 2012, at 10:55 PM, "steve weir" <weirsi@xxxxxxxxxx> wrote:
>
>> d) makes sense if you are also routing according to the recommendations,
>> which runs A/C against VDD in the PCB.
>>
>> Steve
>> On 9/7/2012 10:17 PM, Stefan Milnor wrote:
>>> Experts -
>>>
>>>
>>> For a DDR3 interface, for an SOC from a vendor that you all have heard
>>> of, we are being advised to:
>>>
>>>
>>>
>>> a)      Reference all DDR3 signals to GND
>>>
>>> b)      Terminate certain signals at the far end, to VTT (0.75V)
>>>
>>> c)       Make sure the VTT supply can sink and source current
>>>
>>> d)      Bypass the VTT supply with caps from VTT to VDD (1.5V) (and not
>>> to GND)
>>>
>>>
>>>
>>> Does item d) make sense, if the signals are GND referenced?  Wouldn't it
>>> make more sense if those signals we referenced against VDD?
>>>
>>>
>>>
>>> The vendor's rationale for bypassing with caps from VTT to VDD is quoted
>>> as follows:
>>>
>>>
>>>
>>> "The reason for bypassing to 1.5V instead of GND is that VTT is both
>>> sinking and sourcing current. When high side mosfet of the buffer is
>>> turned ON, for logic HI, the current flow is from 1.5V to MOSFET to
>>> termination resistor to VTT power supply. Signal is switched referenced
>>> to VTT, so we need to decouple the buffer's 1.5V pin to VTT, not to GND.
>>>
>>> No need for the decap between buffer's GND to VTT, because GND is much
>>> more robust than 1.5V rail (usually a massive plain) and can supply
>>> necessary instantaneous current for logic LOW."
>>>
>>>
>>>
>>> What do you think?
>>>
>>>
>>>
>>> Thanks, Stefan M.
>>>
>>>
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>>
>> -- 
>> Steve Weir
>> IPBLOX, LLC
>> 150 N. Center St. #211
>> Reno, NV  89501
>> www.ipblox.com
>>
>> (775) 299-4236 Business
>> (866) 675-4630 Toll-free
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>>
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>>


-- 
Steve Weir
IPBLOX, LLC
150 N. Center St. #211
Reno, NV  89501
www.ipblox.com

(775) 299-4236 Business
(866) 675-4630 Toll-free
(707) 780-1951 Fax

All contents Copyright (c)2012 IPBLOX, LLC.  All Rights Reserved.
This e-mail may contain confidential material.
If you are not the intended recipient, please destroy all records
and notify the sender.

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