[SI-LIST] SSTL and the series resistor
- From: rajat.chauhan@xxxxxx
- To: allan.davidson@xxxxxxxxxxxxxxx
- Date: Thu, 10 Jan 2002 15:27:45 +0530
hello Allan,
Being a I/O designer, I don't know much about board design, but I feel you are
correct. Also it seems to defeat one more of it's purpose i.e. on-chip I/O power
dissipation. Series resistor minimize on-chip power dissipation, since some of
the i/o power is moved off chip and dissipated in the stub resistor, but SSTL
series resistor on-chip defeats this too.
better comments and corrections are awaited.
Thanks
Rajat
>
> Hi all:
> Let's take the example of a motherboard with DRAMs on plug-in DIMM
> cards, SSTL signaling levels, the bus on the motherboard parallel
> terminated at both ends. It is my understanding that the series
> resistors used in SSTL applications are there to effectively isolate the
> stubs (caused by the DIMM traces) from signals traveling along the main
> bus on the motherboard. Assuming that the series resistors are
> physically on the DIMM, this would imply that they should be placed as
> close as possible to the edge fingers, minimizing the stubs seen by the
> main bus. Also, when a DIMM is driving, the series resistors reduce the
> signal swing on the parallel-terminated main bus, resulting in faster
> switching. First question, is my understanding pretty much correct, or
> am I off-base here?
>
> Recently, I have seen application notes for devices where the SSTL
> series resistor appears to be integrated "on-chip". While I can see that
> this will help with reducing the output swing, it seems to defeat the
> stub-isolating purpose of the series resistor. Any stubs would now be on
> the bus-side of the resistor rather than being hidden from the bus.
> Second question, am I missing something here, or does integrating the
> SSTL series resistor on-chip defeat one of it's main purposes?
>
> I appreciate any insight you have. Thanks a lot!
> Regards, Allan
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____________________________________________
RAJAT CHAUHAN
STMicroelectronics
NOIDA
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