[SI-LIST] Re: Question on DIMM's

  • From: Jim_Pankratz@xxxxxxxx
  • To: sidney16481@xxxxxxxxx, si-list@xxxxxxxxxxxxx
  • Date: Fri, 31 Oct 2003 09:29:52 -0600

Hi Sidney,

Your question is a little broad, not sure if you have a specific memory
technology or application in mind (e.g. SDRAM, DDR1, etc.) but the basics
are the same.

However, the differences in module types aren't too tricky.

First, the term buffered and registered, with respect to modules, are the
same thing.  There are unbuffered DIMMs and Registered DIMMs.  The term
"buffered" is not generally used (to my knowledge) when referring to DIMMs.
Yes, it's strange to have an unbuffered DIMM and not a buffered DIMM, but
that is the terminology chosen.

The most significant difference between unbuffered and registered DIMMs are
that the command/address and control signal groups are "registered" on a
Registered DIMM.  That is to say, they are driven from the memory controller
to a register on the DIMM that re-drives the signals to the DRAM devices on
the module.  Also, the clock signals are driven to a PLL on the registered
DIMM and then fanned out to the DRAM devices.

The primary benefit of this scheme is a reduction in the number of DRAM
inputs (i.e. think capacitive loading) that the memory controller must drive
directly.  This is pretty much required for the memory densities necessary
in server/workstation systems these days.

One of the down sides of this is an increase in latency as compared to the
unbuffered DIMM.  Generally, you must suffer a roughly 1 clock cycle latency
for command/address and control to move through the register (don't quote me
on the exact latency, check data sheets/specs).  

Conversely, unbuffered DIMMs do not have any registers on the modules.  All
signal groups are simply driven directly from the memory controller to the
DRAM devices on the module.  This means the I/O cells on the memory
controller must be able to properly drive the maximum amount of memory loads
you would need to support in your system design (i.e. timing, signal
integrity requirements, etc.)

Also, note that the data signals (as well as strobes and data mask) are
driven directly to the DRAM devices on the module for both unbuffered or
registered, no significant differences.  However, in general the data I/O
drivers usually drive a significantly lower number of DRAM inputs than
address/command and control.

Example of loading difference: using unbuffered DIMMs with x8 DRAM devices
and having two slots would require your address/cmd signals to drive a
possible maximum load of 32 DRAM inputs (assuming one copy of address
signals, no ECC and double sided modules)  However, with
registered DIMMs you could reduce the load on your address bus (from the
controller's view) to basically four register inputs (i.e. two per module or
slot).  This is a very general example and is simply meant to illustrate
possible loading scenarios.

Hope this is helpful.  I'm sure others will answer as well.

Also, you can talk with your friendly neighborhood memory vendor apps
engineer for more details.

Regards,

Jim P.

Jim Pankratz
Dell Client Signal Integrity Group
512.723.5357
jim_pankratz@xxxxxxxx
 

-----Original Message-----
From: Sidney S [mailto:sidney16481@xxxxxxxxx] 
Sent: Friday, October 31, 2003 7:23 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Question on DIMM's





Hello,

Could anybody provide more info on unbuffered , buffered, registered
DIMM's?? What are the differences and advantages and disadvantages of each?

Thanks,

Sidney

 




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