Which high end memory should you choose? Between capacity, frequency and timings, this alone can be difficult to determine. After this, you also have to find the best on the market in your category. Need help ? Here it is!
First off, all chips used in this test come from store shelves in order to avoid specially prepared modules for the press from dishonest manufacturers. We would like to thank LDLC, Materiel.net and PC-Look for lending us the material.
DDR2, frequency, timings
The DDR which AMD introduced in our PCs, is different from SDR because it uses both edges of each signal to send data. This means DDR can send two words of 64 bits each clock cycle. Therefore, at a frequency of 200 MHz and on one channel we can attain 3.2 GB /s.
DDR2 functions like DDR externally, but like QDR internally. So DDR2-533 communicates with the rest of the PC via a DDR bus (Dual Data Rate, two transfers of data per cycle) at 266 MHz, but internally it functions at 133 MHz QDR (Quad Data Rate, four transfers of data per cycle). This reduced internal functioning frequency allows an easy increase in a chip’s pure transfer rate to the detriment of latency time. Power decreases from 2.5 to 1.8V, while the number of pins goes from 184 to 240.
There are two principle parameters in memory; functioning frequency and timings. The arrival of DDR2 especially had an impact on frequency at the expense of timings, because the frequency on chips remained unchanged. This resulted in the following names for the most widely sold chips:
- 133 MHz : DDR266/PC2100, 2.1 GB /s per chip
- 166 MHz : DDR333/PC2700, 2.7 GB /s per chip
- 200 MHz : DDR400/PC3200, 3.2 GB /s per chip
- 200 MHz : DDR2-400/PC2-3200, 3.2 GB /s per chip
- 266 MHz : DDR2-533/PC2-4200, 4.2 GB /s per chip
- 333 MHz : DDR2-667/PC2-5400, 5.4 GB /s per chip
- 400 MHz : DDR2-800/PC2-6400, 6.4 GB /s per chip
Remember that here we are speaking of the frequency of the external bus of the memory chip. Internally, a DDR2-800 chip, which fits on a PC2-6400 module, has memory cells functioning at 200 MHz … like DDR400 !
Be careful though, as these are frequencies officially recognized by the JEDEC, the organization that is in charge of memory standards. Some manufacturers go even further and offer DDR2-1066, 1200 and we even saw DDR2-1400 at the CeBIT!
Memory frequency isn’t everything, otherwise it would be too simple. Depending on the quality of memory chips, they are more or less fast in responding to the desired tasks. The four principle characteristics on this level are (and there are others, of course) tCAC, tRCD, tRP and tRAS :
tCAC : Is the minimum time necessary to access a column in a bank
tRCD : Is the wait time between a accessing a row and a column
tRP : Is the minimum time separating two RAS signals (activation of a bank)
tRAS : Is the minimum time necessary to access a line of a bank
These values are expressed in nanoseconds (ns), but it is the value in clock cycles (which takes the external memory frequency as a reference) that is used in the bios. We divide these figures by the tCLK, which corresponds to the value for one clock cycle.
The following formulas are used to find latency time, which is expressed in cycles:
CAS Latency : tCAC / tCLK
RAS to CAS Delay : tRCD / tCLK
RAS Pre-load Time : tRP / tCLK
RAS Active Time : tRAS / tCLK
Generally, these timings are given in order, which gives us, for example, 4-4-4-12 or 5-5-5-15.
