Memory timings - basic explanation... please!

[jmp_78]

Please can someone shed some basic light on this subject please? I have read and read and read articles and posts... but have come to conclusion I must be thick!

My HTPC is an Athlon II X2 245 overclocked to 3.62GHz. The PC2-6400 memory has been downclocked in the BIOS to 266, so is now running effectively at 665MHz.

Let's assume that if I set the memory to 333, the resulting effective speed of 833MHz is too great (it's not, but let's assume!) and I therefore have to leave it at 665MHz.

Can I 'tighten up' timings or something?

V confused... apologies for waffle!!

 

[PhoenixUK]

Why not just change memory multiplier or unlink the memory totally from CPU so you can set the speed of ram yourself ?


Right now it looks like your RAM is linked to CPU hence the funny ratio.

And well, 833mhz shouldn't be a big deal. Any good DDR2 6400 will do up to around 880-930mhz on stock timings.

And regarding timings, you can tighten them up at any time up to the point where it becomes unstable, it's just another way of 'overclocking'.
However, it is best to just find best speed / timing ratio rather than just simply leaving it on low mhz and try to get tighter timings.

 

[Jamiman]

CAS (tCL) Timing: CAS stands for Column Address Strobe or Column Address Select. It controls the amount of time in cycles between sending a reading command and the time to act on it. From the beginning of the CAS to the end of the CAS is the latency. The lower the time of these in cycles, the higher the memory performance.

e.g.: 2.5-3-3-8 The bold “2.5” is the CAS timing.


tRCD Timing: RAS to CAS Delay (Row Address Strobe/Select to Column Address Strobe/Select). Is the amount of time in cycles for issuing an active command and the read/write commands.

e.g.: 2.5-3-3-8 The bold “3” is the tRCD timing.


tRP Timing: Row Precharge Time. This is the minimum time between active commands and the read/writes of the next bank on the memory module.

e.g.: 2.5-3-3-8 The bold “3” is the tRP timing.


tRAS Timing: Min RAS Active Time. The amount of time between a row being activated by precharge and deactivated. A row cannot be deactivated until tRAS has completed. The lower this is, the faster the performance, but if it is set too low, it can cause data corruption by deactivating the row too soon.

tRAS = tCL + tRCD + tRP (+/- 1) so that it gives everything enough time before closing the bank.

e.g.: 2.5-3-3-8 The bold “8” is the tRAS timing.
(The 2.5-3-3-8 figure is just an example for memory timings.)

These are the four timings that you would see when memory is being rated. It is in the order of CAS-tRCD-tRP-tRAS. The lower these timings, the higher the performance of the memory. Some motherboard manufacturers (DFI for example) list the timings in their bios CAS-tRCD-tRAS-tRP.

Certain memories can take tighter (lower) timings at higher speeds. These are the more expensive memory modules out of the bunch. There are also other timings to consider in your BIOS. Not all boards will have options like these.

So basically, you can decrease the amount of time the memory waits between operations to improve performance, but too fast and it will error.

So, like over clocking, you just need to be patient with it

Hope i helped!

(couldn't be bothered to type up a full explaination, so sourced it from http://www.techpowerup.com/articles/overclocking/AMD/memory/131)