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Confused by HTT? LDT? FSB? A64??? READ THIS!

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a little edited, and stuck to the top...

If Athlon64 and HTT and LDT and FSB and Mhz and such are sorta making you scratch your head...please read below.

Even if you are experienced overclocker...you should give it a read ;)

Angry & RGone

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[b][color="blue"]Angry intro:[/b] [i]Some, or a lot, of us are very confused with the new AMD Athlon64 processor, especially in the area of overclocking.

Most of us here are very familiar with overclocking what we term a 'normal' cpu, like the Celeron 300A, Intel Pentium4, AMD AthlonXP etc.

AMD64 is really a superior cpu that really performs best when it is overclocked to it's max Mhz...but there is this thing called [b]HTT[/b] and it seems to be a giant bogeyman for a lot of us (me included as you will read).

Mr RGone here helped a customer with this post, and I took it from there as his post MADE ME REALLY UNDERSTAND exactly how these A64's are really no different when overclocking...we are just making them too complex!

Please, take a read below, and let us know if it hits you like a ton of bricks like it has hit a few of us (and I'm sure a few that are too embarrassed to admit that they really didn't 100% understand until reading this).[/i][/color]


The default speed of a 3700+ with 1mb L2 cache is 12 x 200 or 2400mhz; real cpu mhz. 12 is the multiplier. 200 is the buss speed AND the speed of the memory when set to 1:1 / 200 in Dram configuration. The HTT buss speed of that 3700+ on the UT250gb if LTD/FSB ratio is set to AUTO is 4 times 200 or 800HTT buss speed times 2 for a DDR rating of 1600 for single channel NF3 boards. If you set the LTD/FSB ratio to X4 the HTT buss speed itself will again be 800x2 or DDR1600 the Default for single channel NF3 chipset boards.

Again leaving CPU/FSB Ratio at the default of 12 for your 3700+ and leaving the FSB Buss Frequency at the stock 200 and changing the LTD/FSB Frequency to X3. You will then have a processor running at its default of 2400mhz same as always. The memory will still be running at 200 (DDR400) and NOW the HTT buss speed though will be 3 X 200 = 600 HTT buss speed times 2 for a DDR HTT buss speed of 1200 or 400 under the default HTT buss speed. Now that is how that works.

[b]It has been found that 600 to 900 HTT buss speed or 600 to 900 times 2 for a DDR HTT buss speed of 1200 to 1800 are all just fine when overclocking the Normal processor or memory speeds. [i]That HTT buss is in nO way nearly saturated and therefore anything from 1200 to 1800 is fine as wine. [/b][/i]

Now that is how the single channel 754 processor works and is in effect how you figure the HTT buss speed which in effect has little to do with an overclock as long as it is not below about 600 or above 900.

Now knowing that this is how it works> it makes me rather confused when you come up with other numbers for which I see no corresponding components on the board.

You do not multipy LDT/FSB Frequency Ratio times the CPU/FSB Frequency ratio to obtain any number that means anything to a Single Channel 754 A64 processor that I am aware of.


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you guys are so wound up on trying to get crazy clocks out of these crazy timings that NF2's would even scream in fear at...when you begin to realize that raw cpu speed is really what is more important than anything on these A64's.

they are a superior cpu when it comes to the things you all want them for (mainly gaming). A 2.2Ghz A64 is great, but a 2.6Ghz A64 is HOLY @#@%! great.

As Rgone says, you cant really saturate the HTT bus on this cpu (you can however have it cranked too high thereby killing efficiency and thereby ability to get data where it needs to go as quick as it would if done in an efficient manner).

some of you live and die by memory bandwidth. you should live more by getting your cpu to max fsb @ 1:1 to really give it the boost it needs (while keeping the HTT pipe in a good range). Better yet, you should live by the same rule that you did on the NF2/P4...


This is why we are happy we have the ability to drop the multiplier on the cpu. Many of you guys might be under the impression that A64 clocking just got a lot more complicated with this new-fangled A64 thingy. And it was pretty confusing for me at first too...but listening to RGone ramble on about head-scratchin stuff for so long it hit me one day...

AthlonXP we tried to maximize total cpu speed along with FSB...for max mhz + max memory bandwith.

P4 we did the same thing (and was extremely impressed by those 1200Mhz bus speeds!)

A64...AMD put some damn new number in there that is real confusing if you trying to let it be complex.

the rule was to clock your multiplier up on unlocked cpu's to find a mhz ceiling, then drop the multiplier all the way down and ratchet up your FSB to find your RAM/CPU's max FSB...then tried to match it up in a most optimal way to get the MOST FSB while getting the MOST Mhz. (this is the part a lot seem to skip the most in-detail testing and fine tuning from this point forward is where you really start finding 'sweet spots' in the setup you are running).

A64 is no different. Some of you might laugh but I was terribly confused by all the new numbers (well only really one haha but it seemed like a lot) on the A64...mainly this thing called HTT...and LDT...

and really...the only thing this little HTT/LDT thing is...is your memory controller's communication speed. Look at your motherboard, and you see...there's no northbridge! Well...not like there was anyway hehe...the NB was the memory to cpu communication point. That is now integrated into the cpu itself.

all that HTT is, as RG just explained so well, is the rate at which incoming memory meets cpu instructions...but if it is not running at optimal speed, then you start wondering why some super clock is not showing the good numbers, or isnt stable, or wont even boot haha.

the LDT or HTT multiplier should run between 1200 and 1800 (give or take a little too...there's no real concrete number...but 12-18 is a good guide to follow until again, you find a sweet spot a little lower, a little higher, or somewhere in between).

so we'll break it down a little more in case RG's explanation left you asking questions (but it was the final piece in the puzzle for me!)

you take your NF2 AthlonXP 2500+ unlocked (default of 166x11) to 260x10...2600Mhz...with a bus speed of 520Mhz DDR (260x2). 10 Multi gives us 2600Mhz total.

now lets take your NF3 A64 2800+ (default of 200x9) to 260x10...well, we can't right off because they won't go higher multiplier, only lower multiplier.

so we'll stick with x9 then, as we want to try and max out mhz along with FSB (which is misleading...FSB is normally used for cpu-to-northbridge speed...but these cpu's have a 'northbridge' built onto them and need no northbridge...so in theory if you had 1800Mhz Ram and 1800Mhz cpu...you get the idea lol).

so lets go 260x9. 2340Mhz or thereabouts. Great. But what is our HTT speed? Why, it is 2080Mhz...or 260Mhz, which is 520Mhz DDR, times 4 as that is the default HTT (LDT) multplier for these cpu's (again, just like RGone showed us).

260x2 = 520Mhz DDR (just like the NF2!)
520Mhz x4, the cpu's HTT default multiplier, = 2080Mhz.

whoa...much much too high maybe...defintely above that 1800 unwritten limit...might be stable up here, but might not be stable.

so lets say we got some killa RAM and can do 280FSB on both the NF2 and NF3

NF2 would be as follows (which we should all know!)

280FSB x2 = 560Mhz DDR

if we can only reach 2600Mhz, then we have to lower the multi a little on the AthlonXP...so lets lower it to 280x9.5, which still gives us about 2660Mhz...we'll call it stable or we'll drop to 280x9 and live with 2520Mhz (and maybe crank up to 287x9 for a total of 2583Mhz.


now what about the A64?

280x2 = 560Mhz DDR times 4x HTT multi = 2240Mhz HTT

280x9 (cpu default mutliplier) = 2520Mhz...no problem...


its not stable!

the HTT bus is too high...lets drop it down to 3x HTT...

280x2 = 560Mhz DDR time 3x HTT = 1680Mhz HTT

now we get back in the '1200-1800' range. Much more stable as we know our A64 will max Mhz @ 2660 for example just like the AthlonXP.

so now that HTT speed is within reason, we do the Mhz calculation, just like we do on teh AthlonXP:

280x9 = 2520Mhz. Which is same as above. But this time the HTT is within reason and can function at this HTT speed.


Does it make sense? Or did you already know all this from RGone's post? Or did RGone do a great job and woke a few of you up, and this sorta maybe cleared up any lingering questions?

I feel like RG explained it best...I don't think I could have done better...like I said, reading his post again finally put that last piece of the puzzle together for me...sorta like the time TCP/IP rules finally clicked into place...

a lot of you already knew all of this. A lot of you THOUGHT you knew all the HTT stuff...but now realize you didn't have a real clue.

and some of you were awoken to the mystery of the A64, and how you were making it way more complex than you really should have.

but we havent even gotten into A64 DDR Ratios (dividers)!!!. Haha thats for another day.

thanks RGone! (and yes, even AG has to learn things the hard way sometimes!)

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@RG and AG...I finally got it! I have had this board for how long...? I finally understand. Mind you I am at 2000 htt and fully stable, but until now I didn't even know I was at 2000, lol!

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Well, I'm still not sure if I understand everything totally. I have saved and printed this thread so I can read it a couple hundred times so maybe I will finally understand completely. I'm not sure if I understand the "unwritten" 1800 HTT bus speed rule. My 3000 is overclocked from stock 2.0 Ghz (200X10) to 2.4 Ghz (240X10) and my HTT is running very stable at 1920 Mhz, without a single lock-up or crash for 3 weeks now. I have not touched the memory timings at all, they are set on auto. I did up the voltage a tad on cpu. I have actually overclocked to 247x10 with no problems. I tried to bump up to 250x10, but that was a no-go...with a friendly BSOD.

Do you think memory timing adjustments can get me over the 250 hump, even though I have 0 stepping instead of G? Currently at 240x10 I'm getting a temperature of 34c-36c. Under a load my temp goes to around 40c-44c.


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It isn't an unwritten rule it is an unwritten rough guide, like I said I am at 2000htt well more actually 255x4=(x2)2200 and it makes no difference if I drop the ldt ratio to 3 (255x3(x2)=1530).
However if I was unstable at 2200 htt it would be worth me trying 1530 htt to see if it made a difference. ( I hope this make sense).

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Yep, stowenrat and if I had your board, my pieces and my knowledge I could break the HTT record. Hehehehe. That was so talked about even before the UT 250 was released. But that whole record HTT thing was just so much hype really anyway since that buss is 'nowhere' near 'full-up'.

Speeding HTT out of spec has about the same significance as 8 cars traveling the same direction on a 16 lane highway and you suddenly make the highway 20 lanes wide. It does not speed up anything. All 8 cars already had their own lane and adding lanes did nothing to speed up anything.


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im gonna copy this to the admin section and chop it up a bit and then bring it back as a sticky.

the HTT rule is not really a rule as stow says...its more like a guide.

the important thing is that if you are at some crazy thing like 2532HTT and its not stable...you should drop the HTT (LDT) multi...some of us all think the HTT/LDT multi is something complex that was added in when in reality it is not.

[quote]Do you think memory timing adjustments can get me over the 250 hump, even though I have 0 stepping instead of G? Currently at 240x10 I'm getting a temperature of 34c-36c. Under a load my temp goes to around 40c-44c.


honestly...not likely...C0 cpu's can be real dogs...not just because they cannot do 2T (you always want to do 1T anyway), but because they are previous generation A64 with weaker memory controllers, came out of lower yield batches of chips...

ive seen some good C0 cpu clocks...but the general rule is the CG is gonna show you much more love ;)

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Ok. I think I'm getting it now. I have been running at 2.4 Ghz. (240x10) with an HTT of 1920 (fsb240x4=960x2). For experimental purposes I changed the LDT/FSB Ratio in Bios from Auto (4) down to 3. This gave me an HTT of 1440 (fsb240x3=720x2=1440). I was then able to raise my FSB up to 255 stable which gave me a cpu speed of 2.55 Ghz.

So, If I understand correctly 2.55 Ghz with an HTT of 1440 is faster and more efficient than 2.40 Ghz with an HTT of 1920. Makes sense. I wasn't sure until this was explained in detail that the trade off between HTT speed is really not as important as raw CPU speed as long as you fall in the HTT guideline range. Correct?


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David I wish more had your inclination man. You get right in there and do homework and dig and then just flat amaze me and yourself. Hehehehehehe.

[quote]Makes sense. I wasn't sure until this was explained in detail that the trade off between HTT speed is really not as important as raw CPU speed as long as you fall in the HTT guideline range. Correct?[/quote]

You got it about as close as need be man. HTT which is a data buss has no problems toting the amount of data it is called on to tote. So it does not matter really how slow or fast you run it except that it will not bootup if too high or too low.

The memory does NOT transfer thru or over that buss so it has no bearing on Sandra memory bandwidth testing either. The memory talks directly to the cpu as the memory controller is there in the cpu. So for all practical purposes any discussion of HTT at the level of the 754 is mute and only needs be in a 'range' and that range must include enough to boot and not so high it does not boot.

If I can find it I will try to do a little write-up on "what" is traveling in the HTT "tunnel". Hahahahaaha. New word or description and makes way more sense once you think of throwing your data thru a tunnel. Not memory thru put but data really.


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