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Memory Divider Rules and differences between DDR2 & DDR3

post #1 of 4
Thread Starter 
question for you guys. updated my system since my last thread with new phenom ii 970 be and dumped my oddball kingstons for a set of corsair 2x4gb, and i was doing more research and learning in regards to the math and concepts of overclocking. While reading the DDR2 thread with the math and rules to memory speed, divider selection, etc. i understood all of it except for one concept. I'm using DDR3 memory. Do the same rules apply, especially number 1.

Divider MUST be a WHOLE number.

When i did the math on it. i came up with a different number compared to what cpu-z showed. To get CPU-Z's memory Speed, my Divider has a decimal behind it. Does DDR3 allow decimals for the Divider with AM3 and higher systems?\\

I.E. if i follow the Rule:
16 (multiplier) x 200 (Standard Bus) = 3200mhz. / 800 (memory Speed, 1600ddr) makes a divider of 4.
16 x 240 (my current bus) = 3840 mhz / 4 (divider) = 960. so it doesn't work.

i have dram frequency set to auto so i assume it drops down 1 (i think this is the ratio?) to 1333 -> 666.5 speed.

3200mhz / 666.5 = 4.8 which by ddr2's rules should move up to 5.
3840 / 5 (divider) = 768 mhz memory speed. BUT my cpu-Z shows 800 MHZ. only way this can happen is if if the cpu can select a decimal for a memory divider.

3840 mhz / 4.8 = 800mhz. Am i doing something wrong? or just doubting my self and cpu-z and i am correct, and the rules changed slightly with the new am3 and ddr3 system?


Thanks for your help. sorry for the dragging thread and math
post #2 of 4
Usually at least with AMD mother boards your ref clock runs about 1 mhz fast which might account for some of it. Though I think you run a decimal divider, because a PLL or Phase locked loop has the ability to change the output frequency to what ever it wants within its frequency range. the system is probably trying to round to a nice number for ram frequency. I wouldn't worry about it though what is more important is a 2:1 NB to RAM ratio. I have also heard that a 3-4:1 ratio is necessary but that is only for extreme situations. SO i wouldn't sweat the ram divider much, but if it really bothers you try in the bios if you can set the ram frequency to auto, that might make it go to a whole number divider.
Sandy Thunder
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CPUMotherboardGraphicsRAM
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Phenom II x6 1090T BE Sabertooth 990FX Radeon HD 5770 1GB Corsair Veangence Blue 
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Sandy Thunder
(18 items)
 
QuickSilver X1
(15 items)
 
 
CPUMotherboardGraphicsRAM
i7-3930k C1 4.6GHz @1.344v Sabertooth X79 2x EVGA GTX 560 Ti SC SLI 900/2106 Dominator 1866 16GB 9-10-9-27 
Hard DriveHard DriveCoolingOS
Crucial M4 64GB 'Cuda 320GB XSPC Rasa 750 RX240 push/pull + Alphacool UT60 ... Win 7 Pro x64 
MonitorMonitorMonitorKeyboard
Asus VE198D 19" Samsung 920NW 19" Samsung 906BW 19" Logitech G510s 
PowerCaseMouseMouse Pad
Corsair HX850M Carbide 400R Cyborg M.M.O. 7 Corsair MM200 
AudioOther
Bose Music Monitor Nuforce Icon HDP 
CPUMotherboardGraphicsRAM
Phenom II x6 1090T BE Sabertooth 990FX Radeon HD 5770 1GB Corsair Veangence Blue 
Hard DriveOptical DriveCoolingOS
WD Caviar Black 1TB  Lite ON 16x DVD-R/RW ThermalRight Silver Arrow Windows 7 Professional 64 bit 
MonitorKeyboardPowerCase
Syncmaster 921BW 19" & Syncmaster 965 19" and S... Apple aluminum wired keyboard Corsair TX650 Silverstone TJ05 with window and thermal contro... 
MouseMouse PadAudio
logitech mx air None Monsoon MM-700 
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Reply
post #3 of 4
Thread Starter 
well it is on auto it only matters to me, so i can formultae my over clock without jacking my memory speed out of whack. i .e changing multiplier etc. i want to be able to calculate memory speed properly. and what funny is i though the about the pll. but i don't think it's what affecting it b/c if i drop the bus down to lets say 235. i end up with a 78x mhz memory speed which would be right if i calculated the divider includign the decimal (at this point 4.8.) i appreciate your response dsp1, wondering if anyone can pt this in stone for me? THANKS!.
post #4 of 4
Thread Starter 
Is it safe to assume no one currently has the answer to DDR3's (possibly new) divider choosing rule?
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