"Ron" said:
Greetings. My trusted colleagues: I am wondering about the cryptic
notations which characterize P/N's for memory.
Aside from the basic part number -- which looks something like
"KVR266X64C25", or "CT3264Z265" -- there is sometimes an additional
notation. Something like "1T" or "16T".
Additionally, I am confused about the term "latency" versus the term
"voltage". I have seen specifications that say "2.5". (latency?) And I
have also seen "1.6V", which is presumably voltage. But I'm apprehensive
because it seems that [without o'c'ing] some sticks are intended to run at
1.6V, while others are intended to run at 1.5V, and then there is this
latency thing which seems to have values of 2, or 2.5, or 3. And probably
more! Here is a small cut 'n' paste from the Crucial site:
most motherboards!<<
And, in response to my query for a P/N from an online retailer, I received
this:
Hmm. Everything before the dot is perfect...but what the heck is "16T"?
Thank you very kindly for your anticipated clarification.
Ron
Our friends at Crucial make this tougher than it has to be.
I don't see any datasheets on the Crucial site, whereas I can get
them from Kingston.
Crucial is the sales arm of Micron, and Micron memory modules are here:
http://www.micron.com/products/modu...count=184-Pin&version=Unbuffered&package=DIMM
You may notice a correlation between portions of the Crucial and
Micron module part numbers. The "265" part, for example, is a speed
grade, and in this case equates to PC2100 memory.
I've never gotten too much intuitive value from 3264 type numbers.
All these DIMMs are 64 bits wide (non-ECC) or 72 bits wide (ECC) and
the 64 number doesn't allow working out the all important chip
dimensions (32Mx8) or number of chips per module.
If I had to guess, I'd say 16T stands for 16 chips on the module.
And, from that, I would further guess that 8 are on one side and
8 on the other, making the module double sided. (My guess is based
on the normal size of chips, and the available space on a module.)
As for voltage, I think the original SDRAM ran at 3.3V nominal,
whereas DDR SDRAM (the stuff you are looking at) is 2.5V nominal
operating voltage. Some enthusiast memory now mentions voltages
higher than 2.5V (like 2.6V etc), but it is all based on a
nominal design point of 2.5V.
The next memory, DDR2, may involve 1.8V, so will require
different power circuits and motherboard designs.
If you see a voltage like 1.5V, that is useful for AGP
video card I/O, or perhaps processor interfaces of some
sort.
About the only other parameter with a T in it, would be
the "command rate", which is 1T or 2T. 1T command rate
allows one cycle for some address ??? info to be clocked
into a module. Generally, with two or more modules
on a memory bus, the loading is too great for that aggressive
a timing, and then 2T command rate is used. I don't
have any tutorial info on "command rate" to give you,
so again, I'm pretty foggy on command rate.
CAS (Column Address Strobe) Latency, is the delay between
CAS being clocked into the chip and some operation inside
the chip. The smaller the number, the less time you wait
to get your data. The inverse of time is transfer rate,
so the aggregate bandwidth can be higher, with a lower
CAS number. Lower CAS is a good thing, something you pay
good money to get.
To see CAS Latency at work, download TN-46-05 from this page
and look at page 6. It shows a CL2 RAM delaying two cycles
before giving some data.
http://www.micron.com/products/modules/ddrsdram/technote.html
Actually, TN-46-07 document has this to say about command rate:
"Further, the address bus can be clocked using 1T or 2T
clocking. With 1T, a new command can be issued on every
clock cycle. 2T timing will hold the address and command
bus valid for two clock cycles. This reduces the efficiency
of the bus to one command per two clocks, but it doubles the
amount of setup and hold time. The data bus remains
the same for all of the variations in the address bus."
but that file is an awful big download just to get that
paragraph. Too bad there are no pictures of a -2T ram access
in there.
An Altavista search dug up this:
http://www.lostcircuits.com/advice/bios2/7b.shtml
BTW - the best way to shop for memory, is to use the Crucial
memory selector and just enter your motherboard model number,
to get a list of compatible memory.
HTH,
Paul
