How do you figure out DDR memory speed from ns rating?

[Krk3561]

How do you figure out DDR memory speed from ns rating? Thanks

 

[pm]

It's 1/((ns number)x10^-9) to get the base frequency that the memory operates at and then twice this to get the double-data rate frequency. If it's 5ns DDR then this would end up with 200,000,000 - or 400MHz double-date rate.

If you want to get to the product naming from this (PC2700, etc), then take the base frequency number, divide by a million and multiple by 16 and round to the nearest hundred.... or at least that's how I've been doing it. So, 100MHz DDR, divided by 1000000 and multiplied by 16 would be 1600 => PC1600 DDR.

 

[Lord Evermore]

Or more simply 1/(ns rating) * 1000 = MHz.

1/5 = .20
*1000 = 200MHz

1/4 = .25
*1000 = 250MHz

1/7.5 = .13333
*1000 = 133.33MHz (which means if you have 7ns PC133, it should be able to run at the best timings or slightly overclocked)

Physical signalling of course.

Can't wait till memory starts being rated in GHz.

It's a little more marketing friendly (and "equivalent frequency" friendly, and more technically focused) to say to take the "rated" speed and multiply it by 8, for SDRAM and DDR, since they are 8 bytes wide. That way PC133 is properly multiplied to get 1066MBps and DDR memory is properly multiplied by the DDR speed (333MHz = 2666MBps; considerable rounding is done in some cases).

RDRAM is rated differently. RDRAM is only 2 bytes wide (yes, a full quarter the width of SDRAM). After rounding, a single RDRAM module at 800MHz (400MHz physical signalling) is 1600MBps; two channels gives the 850E chipset a total of 3200MBps. PC1066 RDRAM (533MHz signalling) is 2100MBps per channel, or 4200MBps for a dual channel chipset.

There is also the new "32 bit" RDRAM module available. This is essentially two channels of RDRAM mounted to a single module. Motherboards have to be specifically designed to use these, because they use a different number of pins, and the traces have to be laid out so both channels connect to the same slot. In this case, to figure out the bandwidth for that single module, you'd multiply the rated speed times 4, even though each chip is still only 2 bytes. So a 1066MHz 32 bit module is 4200MBps (notice that they don't always round to the nearest, since 1066x4 is 4264), or "PC4200". An 800MHz 32 bit module is "PC3200".

As for why I explained RDRAM, two reasons: one, people should know both instead of only knowing the bare minimum; two, RDRAM is also DDR memory, and DDR SDRAM wasn't specified.

 

[Bovinicus]

The easiest way is to divide 1000 by the number of ns. This will tell you the clockspeed, by you have to multiply that by 2 to get the effective clockspeed.