Overclocking the Hard Way

Unfortunately, not everyone can overclock Phenom processors by simply changing their clock frequency multiplier. Besides, Phenom 9600 Black Edition, AMD also ships other quad-core processor models that do not belong to the Black Edition series and hence feature locked clock frequency multiplier. If you own a processor like that, then you have to resort to overclocking method that stood the test of time: increasing the clock generator frequency. That is why we decided to continue our overclocking experiments and try push Phenom 9600 Black Edition to higher speeds by just changing the clock generator frequency. Especially, since it is all pretty clear when it comes to clock multiplier adjustment, however, overclocking by adjusting the clock generator speed may be a little trickier.

Most of questions arise from the fact that Phenom overclocking is generally very much different from Athlon 64 overclocking. The thing is that Phenom uses different algorithm for obtaining the memory frequency and features an integrated North Bridge (with the memory controller and L3 cache) that runs at its own independent frequency. So, in case of Athlon 64 overclocking we had to increase the clock generator frequency watching out for three parameters: processor frequency, memory frequency and HyperTransport frequency. However, during Phenom overclocking we also have to monitor North Bridge frequency.

All four Phenom frequencies depend directly or indirectly on the clock generator speed - HT Reference clock - that is also called CPU Host clock or something like that. Processor cores, HyperTransport bus and built-in North Bridge frequencies are formed as [HT Reference clock] x [clock multiplier], which is unique for each of the three. Memory frequency of Phenom processors is also set using HT Reference clock and appropriate dividers. Note that unlike Athlon 64, the memory frequency in new quad-core processors is not connected with the CPU frequency in any way. As a result, all Phenom processor models support the same DDR2 SDRAM operational modes: 400, 533, 667, 800 or 1066MHz.

Formally it looks as follows:

• [CPU Frequency] = [CPU multiplier] x [HT Reference clock];
• [HT Frequency] = [HT multiplier] x [HT Reference clock];
• [NB Frequency] = [NB multiplier] x [HT Reference clock];
• [Memory frequency] = [Divider] x [HT Reference clock].

Contemporary Phenom processors working with DDR2 SDRAM support a set of memory frequency dividers including the following ones: 6:6, 7:6, 10:6, 12:6 and 16:6.

So, our Phenom 9600 processor with 2.3GHz nominal clock frequency and 1.8GHz HyperTransport and North Bridge frequencies was configured as follows when working with DDR2-1066 SDRAM:

• [CPU Frequency]: 2300 MHz = 11.5 x 200 MHz;
• [HT Frequency]: 1800 MHz = 9 x 200 MHz;
• [NB Frequency] : 1800 MHz = 9 x 200 MHz;
• [Memory frequency]: 533 MHz = 16:6 x 200 MHz.

Now that we have discussed the theoretical part, let’s try and find out what will happen if the clock generator frequency is increased.

We decided to start our experiments with lowered 10x clock multiplier: in this case the results should look more convincing. Besides, we will have more opportunities to investigate Phenom overclocking techniques by raising HT Reference clock parameter.

So, knowing the peculiarities of our CPU, we have immediately increased its voltage to 1.45V, and the voltage of the integrated North Bridge – to 1.3V. We picked a lower divider for the memory – 12:6. Since we were using high-speed DDR2 SDRAM, we didn’t expect it to cause any problems.

Once this prep work was done, we managed to increase HT Reference clock to 230MHz, i.e. overclock our processor to 2.3GHz. As we know, it is far not the maximum for our CPU but just its nominal clock speed, however, the system wouldn’t boot when we increased the clock generator frequency a little more.

We have already come across problems like that during Athlon 64 overclocking experiments. It is true, once we increase HT Reference clock, the HyperTransport frequency rises unacceptably high, which may cause the system boot-up to fail. In this case the HyperTransport frequency rose to 2070MHz considering the default HyperTransport bus frequency multiplier was 9x. Could it be too high? It is actually really easy to check: DFI LANParty UT 790FX-M2R mainboard BIOS allows lowering the corresponding multiplier. However, as we found out from our experiments, it is not the higher HyperTransport bus frequency, but the lowering of the corresponding multiplier that causes the system to stall.

As a result, we suspected the North Bridge frequency to be the one to blame here, since it also increased to 2070MHz, while its nominal value for our CPU is 1.8GHz. Luckily, “proper” Socket AM2+ mainboards, such as DFI LANParty UT 790FX-M2R, of course, allow adjusting the multiplier for the built-in North Bridge, too. You can change it together with the processor clock frequency multiplier, on the BIOS CPU Feature page.

And it is set with parameters similar to those used for CPU multiplier. These one are called NbFid and NbDid. As a result, the North Bridge multiplier can be derived from the following formula:

Or the simpler formula modified by DFI engineers by removing the power function from the denominator part:

I believe that if you own an old Socket AM2 mainboard you have already realized by this time that you will not be able to fully overclock AMD Phenom processors. As we see, overclocking in this case implies that the North Bridge built into the processor allows to independently adjust its frequency and voltage, and it is one of the peculiar features of Socket AM2+ only. Therefore, the maximum old mainboard owners should hope for is raising the clock generator frequency to 230-240MHz, not more than that.

Returning to our own overclocking experiments, I would like to say that dropping the North Bridge multiplier from 9x to 8x (by lowering the NbFid parameter from 5 to 4) did solve the problem: the system could work stably at HT Reference clock frequency over 230MHz.

Here I would like to point out one more thing we discovered: the HyperTransport frequency multiplier shouldn’t be bigger than North Bridge frequency multiplier. Otherwise, Phenom processor will not work.

Now that we have fished out all the underwater rocks on the way to successful Phenom overclocking, it all becomes a very easy and fast procedure. The main trick here is to increase the clock generator frequency carefully and watch for the HyperTransport and integrated North Bridge frequencies not to get too far beyond 2.0GHz. In fact, you can obtain all the multipliers and coefficients for that in advance using the formulas above. And of course, you shouldn’t forget to increase the main voltages, too.

As a result, we managed to get our system to run stably with Phenom 9600 Black Edition working at 2.69GHz. So, HT Reference clock hit 269MHz with the processor clock frequency multiplier of 10x. It required us to additionally lower the North Bridge and HyperTransport multipliers down to 7x.

In the end, all the frequencies turned out as follows:

• [CPU frequency]: 2690 MHz = 10 x 269 MHz;
• [HT frequency]: 1883 MHz = 7 x 269 MHz;
• [NB frequency]: 1883 MHz = 7 x 269 MHz;
• [Memory frequency]: 538 MHz = 12:6 x 269 MHz.

It turns out that if you know some of the fine tuning tricks, Phenom overclocking becomes not any harder than Athlon 64 overclocking. However, you definitely need a high-quality Socket AM2+ mainboard. Older mainboards designed for Socket AM2 interface do not have all the features you may need during Phenom overclocking. That is why if your mainboard is a Socket AM2 one, you should probably got for the AMD Phenom Black Edition series. Overclocking with the clock frequency multiplier can be performed easily on any mainboard.