DDR=Double Data Rate
Basically, the current/old way CPU, Mobo, RAM and other hardware handle things by sending 1 signal per clock cycle. DDR RAM has been in the works for some time, and DDR mobos were introduced with the Athlon. It works by send not 1, but 2 signals per clock cycle...I'll try to make an ascii picture to clearfy things...
...\_/-\_/-\_/-\_/-...
\_/- is 1 clock cycle. Notice there's an rising edge "/" and a droping edge "\". I forgot which edge it was done before, I think it was the dropping edge where a command was sent. Now, there's a command signal for rising AND falling edge, thus 2 signals, hence double the data rate.
DDR is must faster than before, but not truely 2x speed, because certain chip have certain amount of delay times, there are other delay times that must be there for CPU/RAM etc timing to work right, which means sometimes, 1 full cycle is necessary for 1 signal, but because that RAM is one of the biggest bottleneck currently, most instructions/signals can be processed at the DDR. I'm not sure if that's clear or not...I'm not too good at explaining this, since the whole thing is new to me too (since I became an EE major this past year).
Also, know the RAM speed codes like the 2-2-2 or 3-2-2? That's actual timing in ns. Since the RAM is rated as fast as 4ns (as the DDR RAM used on the GeForce2Ultra), I guess that means 2 ns/signal is the speed of which DDR need to operate on. Theoretically, with 3-2-2, (3 being the CAS delay if I recally correctly) will require 1 full cycle while the 2-2-2 will require half a clock cycle for CAS to be pulled low (active) after certain conditions are met (write enable, etc). My guess is that the speed code for the DDR RAM is higher than 2-2-2 (ie 2-1-1 or something like that...). It helps a lot to read some intro CPU manuals. I've read the 188 manual and numerous data sheets, and most EEs will tell ya that the DRAM (SDRAM being one flavor) is rather painful when it comes to timing/interfacing with computers...at least at the introductory EE classes...(I have yet to finish my 256k DRAM interface with the 188...)
In much simpler words, DDR is a cheap/fast way to get close to 2x the speed of current hardware. That's why Athlons claim they can support 200Mhz Mobos when they were released back when 133Mhz mobos just hit the market. It was actually 100Mhz DDR...DDR RAM yields are rather high, much higher than Rambus, which is why DDR RAM price has dropped significantly. Now, what I'm wondering is that is 4ns DDR really 8ns made 4 because of the double data rate? or is 4ns DDR really 4ns and is actually 2ns/signal?
[This message has been edited by Evangellydonut (edited 08-18-2000).]
Top
