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Command Rate Question


JLB768

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Hope I'm in the right forum for this question. I am confused as to T1 T2, and what is preferred. While I have read that timings are 2.5-3-3-6, my timings are at 2-3-3-6, with the command rate on auto by default. My system is extremely stable, and has been since I built the system last february. Can I set my command rate from auto, to T1 or T2? if so, which would be best?

 

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Test your system with SuperPi and Prime95 for stability. Run SuperPi for 6+ hours. If no errors occur, then set your Command Rate to 1T and retest. If you have issues with the 1T Command Rate, then set it back to 2T.

 

Contrary to popular belief, the Command rate or Command Per Clock rate is NOT a memory timing.

 

The CPU and most types of memory have an 8 byte data transfer bus. They transfer 8 bytes of data once, twice, or four times per clock tick. However, for efficiency the basic unit of data transfer is a 32 byte "burst" of data around the address of the data the CPU needs. Any CPU reference to memory is rounded up to a complete 32 byte block. This block is transferred in a burst of 4 consecutive 8 byte transfers. Your timing settings effect this transfer, not the Command per clock.

 

The Command Per Clock sets the Command Rate for the memory controller. At 1T, the controller can issue commands on every clock cycle, if set to 2T the controller can only issue commands only on every other bus cycle. With DDR, the 2T command rate had some poor transferrals since there was little use for the bus interleaving (Dual Channel Mode). Let us say that if you ran at a 2.5 - 3 - 3 latency then the read command would be issued 2 cycles after an bank activate command. This would negate 1T vs 2T correct? Since the issuance was held to 2 cycles there would be no need for a single cycle command rate.

 

But we have DDR running in Dual Channel Mode and this mode allows for an opening of more than one page in different banks of the same memory device (slots related to dual channel ie. Bank 1 and Bank 3, etc.). So the second bank activate command that follows after the first bank activate command can not be issued since a read command for the first bank has already been scheduled. This is considered a "Bus Contention" and because of it, the next available time slot for the next bank activate command would collide with the second read command for the possibility of a page hit. This would continue until there is a no-op (no operation) return. When there is a no-op return or interval then another bank activate command can be initiated. Because of this issue, dual channel is largely useless, not only is it useless but it can even slow bank activation. This issue follows with DDR2 but one of the reasons for the creation of the DDR2 standard was to eliminate this previously mentioned bus contention. The DDR2 standard created the Posted CAS characteristic, so that there was no need to wait for the Trcd function to be completed. The Posted CAS feature allowed for the Read command to be issued immediately after the bank activate command. But this "Feature" can only be useful with a 1T command rate since a 2T command rate would eliminate the use of this characteristic.

 

Thus in a DDR2 system, with dual channel bank interleaving enabled there will be a more responsive system. Now there are added latancy issues with DDR2 vs DDR1 so for this benefit to be more effectively seen, one needs very low latency DDR2 DRAM.

 

When one compares DDR at 3 - 4 - 4 - 8 and DDR2 at 3 - 4 - 4 - 8 (Both Dual channel, bank interleaved) there becomes no doubt that the DDR2 is a far better memory transaction at 1T vs 2T and the difference at the DDR level is far less pronounced.

 

So, don't expect miracles with DDR and 1T but with (high quality and low timing) DDR2 you can expect a far greater response from 1T vs 2T.

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Will I damage my system running @ 2.75v 2-3-3-6 1T on my AMD setup? Its been set at 2.75v, and 2-3-3-6 since the day I installed the ram last Feb, I just have no idea what the Command Rate is on auto, which is what its been set to all along. I havent experienced a system crash since I put it together.
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Thanks for the reply. When I bought the sticks, I read that on my MB, I would have to manually set the correct timing. Now I can't even recall what brought the Command Rate to my attention this week, but figured it was worth a closer look into. As it sits, its 2.75v 2-3-3-6 1T, and appears to be running as usual.
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