Is DDR-2 the same as Dual-Channel DDR?

[Bradley]

My question is the subject of this message. Is DDR-2 the same thing as
Dual-Channel DDR? If not, what are the differences and which one should I go
with? Thank You.

 

[Papa Schultz]

No that's totally different.

Dual Channel DDR doesnt' really exist. In fact, it means that the
northbridge chipset can hold a double size bus, so before we have
64bits(single channel) then 128bits(2*64) with a dual channel.

DDR-2 is just the second generation of DDR with an enhanced design to
support higher frequencies. DDR-2 is supposed to be backward compatible with
DDR. DDR-2 start from 3200 MB/s whereas DDR range from 2100MB/s to 3200MB/s.
Therefore you could have PC3200 with DDR or DDR-2. You must check this info
before buying.

 

[Tony Hill]

My question is the subject of this message. Is DDR-2 the same thing as
Dual-Channel DDR? If not, what are the differences and which one should I go
with? Thank You.

In a word, no. They are not the same thing and they are definitely
NOT mutually exclusive. Dual-channel DDR means, quite simply, that
there are two memory channels working together. In a more traditional
single-channel setup you install a single memory module and the system
accesses only that module. In a dual-channel setup you need to
install memory modules in pairs and the system spreads memory access
across both channels. This doubles the rate at which your processor
can shuffle data in and out of memory.

Either DDR or DDR-2 (or even the older SDRAM or RDRAM) can operate in
dual-channel systems.

DDR-2 is just the next generation of DRAM interface. It can easily
work in single or dual channel setups (or any number of other setups
used in some high-end servers). It's really only a relatively minor
update to the DDR specification that will allow for higher speeds.
While current DDR memory is having trouble getting beyond 200MHz
(400MT/s, ie the stuff sold as DDR400 or PC3200 memory), DDR-2 should
easily clock up to 333MHz (666MT/s) and possibly beyond.

At this moment, the choice between DDR and DDR-2 is pretty clear. DDR
is here, DDR-2 is not, so go with DDR. In about 2 months time the
first DDR-2 systems will begin to appear, but they will probably
command a price premium in the beginning with little to no performance
advantage (the first batch of DDR-2 memory will probably only run at
the same 200MHz as existing DDR memory, or maybe up to 266MHz at the
high-end). Eventually though (6-8 months?) DDR-2 should reach
price-parity with DDR and offer higher performance.

 

[daytripper]

No that's totally different.

Dual Channel DDR doesnt' really exist. In fact, it means that the
northbridge chipset can hold a double size bus, so before we have
64bits(single channel) then 128bits(2*64) with a dual channel.

DDR-2 is just the second generation of DDR with an enhanced design to
support higher frequencies. DDR-2 is supposed to be backward compatible with
DDR. DDR-2 start from 3200 MB/s whereas DDR range from 2100MB/s to 3200MB/s.
Therefore you could have PC3200 with DDR or DDR-2. You must check this info
before buying.

DDR-2 was not "supposed to be backward compatible with DDR".

And it most assuredly is not...

/daytripper

 

[Papa Schultz]

We dont have the same information, I read that was compatible from JETEC!
Where did you read it was not?

 

[daytripper]

We dont have the same information, I read that was compatible from JETEC!
Where did you read it was not?

Ok, so this isn't a total waste of time, the short list of differences
includes:

DDR runs on 2.5v, uses SSTL-2.5 signaling, doesn't provide dynamic On-Die
Termination, has a smaller minimum burst length, is limited to single-ended
strobes, has "longer" pages, doesn't support Off Chip Driver calibration.

DDR2 runs on 1.8v, uses SSTL-1.8 signaling, provides dynamic ODT, has a larger
maximum burst length, supports differential strobes, has "shorter" pages,
provides OCD calibration, and has modified MRs and EMRs to support the added
features (those listed above plus a handful of diagnostic bits).

Back to your question. There may have been someone somewhere who honestly
thought he could plug DDR2 dimms into DDR platforms and operate them in
quasi-DDR mode. It begs questions about signal integrity - and certainly PLL
accuracy - at two operating voltages, never mind the economic wisdom in
support of the attempt.

But at the end of the day, surely the bottom line is: "Can you put a DDR2 dimm
into a DDR socket?"

Well, ask "JETEC" how you're going to fit the industry standard 240 pin DDR2
dimm into the industry standard 184 pin DDR socket...

/daytripper (Maybe they'll email you a really big hammer ;-)

 

[chrisv]

DDR2 runs on 1.8v, uses SSTL-1.8 signaling, provides dynamic ODT,

How does the On-Die Termination work? Does the BIOS tell the chips
that they are or are not the last in the chain, so they can terminate
the bus or not (and is each side of a double-sided DIMM directed
differently, if one of them is at the end of the chain)?

has a larger
maximum burst length, supports differential strobes, has "shorter" pages,
provides OCD calibration, and has modified MRs and EMRs to support the added
features (those listed above plus a handful of diagnostic bits).

What's Off-Chip Driver calibration?

 

[daytripper]

How does the On-Die Termination work? Does the BIOS tell the chips
that they are or are not the last in the chain, so they can terminate
the bus or not (and is each side of a double-sided DIMM directed
differently, if one of them is at the end of the chain)?

Much cooler than that - the key word is *dynamic*.

DDR2 provides an ODT signal for each rank of SDRAMs. During data transfer
periods (ie in real time) the On-Die Terminators for the populated ranks are
manipulated to maximize the data-phase "eye size" for significantly improved
reliability (and as these bus speeds increase, ODT becomes more and more a
"require feature").

The nature of the ODT signals are similar to CS signals and are provided by
the host memory controller (quite simple, really, as they share the exact same
state-machine that otherwise controls the ranks).

What's Off-Chip Driver calibration?

Another cool tuning "feature" that'll probably be *required* to survive decent
sized memory configurations.

OCD calibration allows the system to dynamically reduce mismatches between
output driver pull-up and pull-down strength characteristics and "tune" the
output drivers to 18 ohms to optimize signal integrity.

Using Load Mode commands to the EMR to enter an OCD drive-HIGH or drive-LOW
state, the host controller samples the voltage levels on the received "data"
and compares them to a known reference, calculates the DRAM output driver
strengths against the 18 ohm target, and then uses OCD Adjust commands to
adjust the DRAM output driver strength to achieve that goal.

Et voila, tunable transmission line drivers. Very nice.

/daytripper

 

[chrisv]

DDR2 provides an ODT signal for each rank of SDRAMs. During data transfer
periods (ie in real time) the On-Die Terminators for the populated ranks are
manipulated to maximize the data-phase "eye size" for significantly improved
reliability (and as these bus speeds increase, ODT becomes more and more a
"require feature").

Wow. Getting to be just like RF.

 

[Keith R. Williams]

Wow. Getting to be just like RF.

Well, yeah. 500MHz is getting just like RF. ;-)

 

[Dorothy Bradbury]

DDR-2 also nicely fits in with 64-bit memory growth:
o More memory sockets - requiring much bigger VRM
o Lower voltage than DDR to limit wattage - altho high in 1GB form

BTX will allow airflow along the dimms, avoiding shadowing etc which
is common on present 1U/Desktop solutions. Not seen anything to suggest
above 667Mhz yet, but that's pretty quick. Typical figures are 30W for a
server implementation, and 2m/sec airflow for junction temps.

 

[The little lost angel]

Well, yeah. 500MHz is getting just like RF. ;-)

Erm, is it just me or US radio stations don't use frequencies below
500Mhz? I thought RF begins well below that... like in the single
digit Khz regions no?

Or do you guys play only with military radars? ppPpp

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[chrisv]

Erm, is it just me or US radio stations don't use frequencies below
500Mhz? I thought RF begins well below that... like in the single
digit Khz regions no?

Well, technically speaking, I believe any frequency can be transmitted
in free space as an electromagnetic wave. It just gets less efficient
as you drop in frequency. Antenna size is inversely proportional to
frequency, as well.

 

[Keith R. Williams]

Erm, is it just me or US radio stations don't use frequencies below
500Mhz? I thought RF begins well below that... like in the single
digit Khz regions no?

In the US, AM radio is in the hundreds of kHz to low MHz (530kHz -
1.71MHz, IIRC). FM is in the 88-108MHz region, while VHF television is
in the 30-170MHz region and UHF TV in the several-hundreds of MHz.
Cell phones are in the high hundreds to low GHz. So, modern
microprocessors are right in there with all this other "RF" stuff.
....except they don't (intentionally) radiate.

Or do you guys play only with military radars? ppPpp

I could tell ya, but... ;-)

 

[The little lost angel]

Well, technically speaking, I believe any frequency can be transmitted
in free space as an electromagnetic wave. It just gets less efficient
as you drop in frequency. Antenna size is inversely proportional to
frequency, as well.

Hmm, I was of the understanding that it gets more efficient as the
frequency goes lower. That to get the same range out of a higher
frequency requires more power than a lower frequency.
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[RusH]

[email protected] (The little lost angel) wrote
:

Hmm, I was of the understanding that it gets more efficient as the
frequency goes lower.

Thats just what the man said

That to get the same range out of a higher
frequency requires more power than a lower frequency.

yup. But with higher freq you get a better chance to get an
unintentionall antenna out of some schort interconnect (a cable
stickong out of the board just loves to act as one).

Pozdrawiam.

 

[Keith R. Williams]

Hmm, I was of the understanding that it gets more efficient as the
frequency goes lower. That to get the same range out of a higher
frequency requires more power than a lower frequency.

Lower frequencies tend to bounce off the ionosphere and the
ground (ground waves), so will carry further. The higher the
frequency the more penetrating the "wave" is so it tends to go
off into space. Jacking up the (effective radiated) power simply
forces the issue. Note that it also takes a hell of a lot more
signal strength at the receiver for TV vs. radio.

 

[Anonymous Joe]

My question is the subject of this message. Is DDR-2 the same thing as
Dual-Channel DDR? If not, what are the differences and which one should I go
with? Thank You.

DDR-2, which I believe is correctly written with Roman numerals, is to DDR
as DDR was to SDR. That is to say, it is a different beast.

Dual-Channel is a function of the chipset, being able to combine two DDR (I)
DIMMS, similar to how RAID 0 works. It will double the speed of the RAM,
but you need to install it in pairs, so you need 2 DIMMs or 4 DIMMs, 1 or 3
won't work.

DDR-II would be better, but last I recalled (I've been out of this for a
while now), it isn't made yet. I sure hope that's right. In that case, you
should definetly go for the DDR (I). PC2700 or PC3200 is about as good as
it gets now. That corresponds to a so-called 333MHz & 400MHz RAM speed.
It's actually 166 and 200, but marketing made it 333 and 400 (after all it
is DOUBLE data rate....just people look at the double part and not the DATA
RATE which clearly isnt ACTUAL CLOCK SPEED

 

[The little lost angel]

Dual-Channel is a function of the chipset, being able to combine two DDR (I)
DIMMS, similar to how RAID 0 works. It will double the speed of the RAM,
but you need to install it in pairs, so you need 2 DIMMs or 4 DIMMs, 1 or 3
won't work.

This isn't technically dual channel, but with 3 DIMMs, some boards
will apparently enable dual channel for the 2 DIMM in complementary
slots and the 3rd DIMM works in single channel mode.

--
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[Walt]

I believe Keith meant that designing digital systems with
500MHz bus signals running around, is beginning to require
the same design techniques as RF designs.