DDR2 Roundup: Reaching for 667 and Beyond

tRAS and DDR2

It made little sense to test 8 new DDR2 modules at memory timings that were less than optimal for best performance. We were able to verify quickly that the traditional CAS, Ras-to-Cas delay, and RAS Precharge timings all improved performance as timings were lowered. However, tRAS at 11 or 12 is a significant departure from the best timings that we have seen with DDR on the Intel platform - it is more like the 9 to 11 optimal tRAS seen on current AMD chipsets/processors.

To determine if 11 or 12 is the best tRAS timing for DDR2, we first ran Memtest86, a commonly used memory tool that measures memory bandwidth from a custom bootable OS. Memtest86 showed absolutely no change in memory bandwidth in the range of 4 to 15 when memory timings were fixed at 4-4-4 and only tRAS was varied.

To further check our Memtest86 results, we ran a complete series of tests with Aida 32, which is also available for free download on many shareware sites. The Aida 32 memory test module is very useful because it reports both memory Read and memory Write operations as separate results. We ran both tests and also added a Total Memory Bandwidth to offer a better comparison of the test results.

Micron PC2-4300U - 4-4-4-tRAS
tRAS	Aida 32 Read	Aida 32 Write	Aida 32 Total
4	5316	2328	7644
5	5264	2338	7602
6	5289	2341	7630
7	5289	2326	7615
8	5302	2340	7642
9	5303	2349	7652
10	5314	2344	7658
11	5303	2344	7647
12	5289	2336	7625
13	5303	2339	7642
14	5264	2334	7598
15	5289	2275	7564

As you can see clearly, over the full range of tRAS 4 to tRAS 13, there is virtually no difference in memory performance when tRAS is adjusted. While there is a slight falloff at tRAS of 14 and 15, it is still minor. We can conclude that a tRAS setting of 4 to 13 makes no difference in memory performance. Since higher tRAS settings usually mean better memory stability, we can well understand why a 12 setting is standard. For our memory tests, we chose a value of 10, which represents the upper mid-range of best performance and stability. However, we did find that some high density (1GB) memory modules that were not included in our tests performed with greatest stability at a tRAS setting of 11 or 12. Overall, we had no difficulties at tRAS 10, but there is no performance penalty when using 11 or 12, if it improves the stability of your DDR2 memory.