How to calculate AM2 memory frequency

[Beomagi]

I figure people are going to be confused as hell trying to figure this out : "why the hell is my memory running like that..."

I used info derived from :
vrzone asus AM2 board review
Anandtech AM2 platform review

Reference Clock * Clock Multiplier = CPU Frequency

CPU Frequency / Memory Divisor = Memory Frequency

Memory divisor is based of the calculated memory frequency and depends on your cpu speed.

The memory divisor is the highest integer that can produce a memory frequency equal to or less than the rated memory speed, or ddr800 (limit)

So what's the most important thing now for people that want to push memory? The new divider. vrzone did a good job, but missed going through the bios options that controlled it - i.e. If i DONT want mt DDR675 to operate at 630 but rater 730, can the user change the divider to the next number?

here's a table some may wish to use when selecting memory

using divider = roundup(clock*2/spd memory frequency)

           auto set div:Hz
AM2 GHz   667           800          667:div-1   800:div-1
1.6       5:640         4:800        800         1067
1.8       6:600         5:720        720         900
2.0       6:667         5:800        800         1000           
2.2       7:628         6:733        733         880
2.4       8:600         6:800        686         960
2.6       8:650         7:743        743         867
2.8       9:622         7:800        700         933
3.0       9:667         8:750        750         857
3.2      10:640         8:800        711         914

div minus one is just what that memory and chip selection would have gotten on the next highest divider. for the 2.4GHz, 686 is awefully close to ddr2-667, and i'm sure most modules can take that with ease.

Note : exact frequencies are achieved on the common multiples of the chip and base memory frequency (half effective). So for ddr2-667, you'd get exact values on k*(333MHz) where k is an integer - 1000, 2000, 3000MHz.(1666, 2333 etc dont exist) For 800, it's k*(400MHz) - 1.6, 2.0, 2.4, 2.8.

There's still a base clock governing HTT and chip speed, running at 200MHz.

original formula for chip and HTT remain constant.
i.e.
chip speed = base clock * cpu multiplier
HTT speed = base clock * HTT multiplier

[TriniXaeno]

damn right

I'm confused.

[Beomagi]

hehe, use the table then

The base bus (200MHz) was used with DDR. For DDR2, they for some reasons could not use the base bus, but had to use the final cpu frequency. Thus, a different cpu multiplier, which changes the cpu frequency, changes the speed of the memory, and different cpu speeds required a different divider - the problem is they only use integer dividers.

It's is a direct integer division on the final cpu frequency to get the highest base memory frequency at which the memory is still within the operating limits specified in the SPD chip.

e.g. from table
2.4GHz AM2 means with ddr2 667, the divider is 8, giving an effective memory speed of 600MHz (2400/8=300 - the base memory frequency). If you were to drop in ddr2 800 in there the memory divider is 6 (2400/6=400) giving a base speed f 400, ddr2 effective rate is 800MHz.

I included the next fastest speed if the divider were any higher for people that want to tweak.
e.g. For 2.4GHz, if you have DDR2 675, why run at the autoset 600MHz, when you can set the divider one less(7 instead of 8 ), and run at  686MHz which is barely any overclock?