Question about HT Bus & Memory BW

[imported_inspire]

So, I wake up this morning with a crazy idea. Apparently, my subconcious has been trying to manufacture a reason why my overclock is giving me hell. The idea is:

HT Bus = HTT x HT Mult.
and
Memory BW = HTT x 4 x (Z/800)

Where Z is one of (800 / 667 / 533 / 400). My question comes up when I follow Zebo's guide - which says:

VERY IMPORTANT to keep in mind at all times is hypertransport needs to be around 1000, meaning LDT x HTT (aka FSB) ~1000 or less.

I've lowered the HT Mult. from 5x to 4x with any overclock, and then from 4x to 3x at 250 MHz HTT, and then from 3x to 2x at 334 MHz HTT (yeah right... :roll: ). As an example, say I'm trying to hit 2.8 GHz at a 667 or 533 value for Z...

......HT Bus...................Memory BW
280 x 3 = 840 < 933 = 280 x 4 x (667/800)
280 x 3 = 840 > 746 = 280 x 4 x (533/800)

This is just an example of where this question may be relevant. But, I got to wondering after I read that AT ran a AM2 4800+ X2 in a Foxconn C51-XEM2AA with a HT Bus of 1280 MHz. I'm fairly certain, still, that my problems are coming from the my board's inability to give my memory more than 1.95v, but I want to rule out any other possibilities. Anyway - what happens when memory BW exceeds HT Bus ?

 

[BitByBit]

The first thing to keep in mind when overclocking the Athlon 64 is that the Hypertransport (HT) bus has nothing to do with the memory bus.
The HT bus is the transport medium between the CPU and peripherals, not memory.
The Athlon 64 has a separate, dedicated bus for memory access, and therein lies the elegance of AMD's implementation, but also the source of much confusion.

The Hypertransport bus, even at speeds as low as 200MHz can provide sufficient bandwidth for the CPU to communicate with peripherals. Changing the HT bus does not affect CPU-to-memory bandwidth. The memory bus speed is however determined by the HTT speed, which is 200MHz by default.
A 200MHz HTT bus (400MHz effective due to DDR) can accomodate the bandwidth of DDR400 memory. 400MHz HTT would be used with DDR2-800 and so on.

So for overclocking, you could set the HT multiplier down to 1 or perhaps 2, as this is entirely independent of the memory bus, and as mentioned above, can easily satisfy the bandwidth requirements of peripherals.

Hope this helps.

 

[imported_inspire]

Yes, thanks a lot!

 

[cliesucks]

Mine case is interesting too. I can set the HT Mult. to 5 and memory BW to 400MHz(all default) and just increase the FSB by 10% to 220MHz and the system is always stable. I am using A8N-E. So the HTT is actually operating at 1100MHz and the memory is at 440MHz. My 3200 venice couldn't go any faster(even with 1.45V VDD and all those tweakings of HTT and memory BW). Now I have 3800 X2 and I can again use the same setting. Since the 3800X2 can overclock much better, I now push the FSB to 260MHz. Now supposedly I can set the HTT Mult. to 4(<1100MHz/260MHz) but I can't since the system is not stable. The memory BW can't work with 333MHz(333*260/200=433<440) setting and I have to lower it to 266MHz(266MHz*260/200=346MHz effectively). It seems that the maximum memory BW and HTT are affected by the FSB.

When memory BW is set to 333MHz, the machine won't boot. I can tweak the timing and increase the DDR supply to 2.9V. Then the machine will boot but will fail the stress test. Now I will probably live with the lower memory BW.