Why 166Mhz DDR?

250MHz DDR:

200MHz DDR

450MHz DDR

We use 312.5MHz DDR in one of our projects.

So what the f**k are you talking about?

Kolja Sulimma

Hi Rohit, It's because the woman who invented DDR memory had polydactylism. See

. This meant she could count up to

6ns on one hand, so she made the operation frequency 1/6ns = c.166MHz HTH, Syms.

Perhaps this number is how slow one can go, not how fast? If you're not meeting timing with your FPGA, the low frequency number could be important.

DDR does have a minimum frequency...

G.

Pleasant! Errr. but I dont completely agree.

DDR200 = PC1600 = 100MHz ...

Has a bit more detail which may give the OP some information about why

166MHz was chosen... just a stepping stone really.

Ben

Why is PCI 33MHz?

If you look at it from the PC angle, almost all clocks are multiples of ~33.3MHz and the reason for this is to have integer clock multipliers and dividers to simplify PLL and related circuitry design.

PC2700 DDR-DIMMs operate at 166MHz, CPU FSBs of that time operated at

333/667MHz, PCI operated at 33MHz as always, AGP at 66MHz.

So, the 166MHz figure is simply due to PC computing legacy. Within the next two years, after PCIe will have completely replaced PCI and AGP, we will see other system clocks shift from multiples of 33/66MHz to multiples of

125/250MHz... or multiples of 75/150MHz if Intel/AMD decide to reuse the SATA reference clock instead.

The minimum operating frequency for DDR DRAMs comes from the DLL circuitry used to capture data. My 166MHz Infineon DRAMs say the minimum operating frequency for its DLL is 100MHz.

The maximum operating frequency is determined by how fast the DRAM is able to get data into or out of the active row (data) register and other control structures with the maximum amount of pipelining enabled. Since the high-K process used for DRAMs yields slow logic, latency cycles (intermediate registers) pile up really fast on high-speed DRAMs.

Well, both clock frequency and data rate are measured in Hertz. The OP did not say which 166MHz number he was talking about so it could have been both. AFAICT it could also be the number of alpha decays in the solder balls.

Kolja

Thanks Ben & Daniel ! I think you answered my curiosity question :)

-Uday

If your curiosity wants to see something extreme, download datasheet for some DDR and DDR2 devices and compare these timings with the latest 1.6GHz Samsung GDDR4 devices... there is a pretty drastic difference in the number of latency cycles, particularly CAS.