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Llano Overclocking: Theory

Before we start our overclocking experiments with the new Llano processor, we should define clearly what parts of the processor we are going to overclock. It is not a common processor, but a hybrid one, which consists of the computational cores as well as a graphics core working at its own clock frequency. Moreover, Llano has one more part that could in the end affect the overall system performance. This is a memory controller. And one shouldn’t underestimate the importance of its high performance. The memory in Socket FM1 system is shared between the computational cores and integrated graphics that is why the practical bandwidth of the DDR3 SDRAM in this system is of utmost significance.

So, Llano processors have three major frequencies, which have direct effect on performance and which could be overclocked:

  • CPU clock frequency;
  • GPU frequency;
  • DDR3 SDRAM frequency.

All these three frequencies are derived from the same exact formula, when the resulting value equals base clock generator frequency (BCLK) times corresponding multiplier. The nominal BCLK frequency for the Llano based systems is set at 100 MHz.

Each processor model has its own clock frequency multiplier, because they determine the clock frequency. This multiplier may be changed, but only to a lower value. In other words, you can’t overclock Socket FM1 systems by increasing the processor clock frequency multiplier.

Unfortunately, the multiplier for the graphics core frequency cannot be increased either. Each processor series has the same constant multiplier that is why it is impossible to overclock graphics core independently from all the other processor units.

However, the multiplier for the memory frequency gives you some flexibility and freedom. In nominal mode Socket FM1 processors support DDR3-1067, DDR3-1333, DDR3-1600 and DDR3-1867 SDRAM. It means that Llano memory controller offers you a choice of four multipliers for the DDR3 frequency: 10.66x, 13.33x, 16.0x and 18.67x.

Since there is no way to increase the clock frequency multipliers for the computational and graphics core of the Llano CPUs, the only possible overclocking strategy is to increase the BCLK clock. However, you will encounter one very serious problem here. The thing is that BCLK frequency is used not only for the processor and memory, but also for the I/O frequency of the AMD A75 chipset. Therefore, when you increase the BCLK clock, we automatically speed up PCI Express, USB, SATA and either bus controllers. Unfortunately, it may sometimes quickly lead to system instability, which in this case isn’t caused by the CPU or memory.

I believe I don’t have to remind you that the same exact issues killed the clock generator frequency overclocking approach for Sandy bridge processors in LGA1155 systems. Luckily, things are not so hopeless with Socket FM1. Firstly, the maximum threshold for the BCLK frequency is not as close to 100 MHz as it is in LGA1155 systems. Secondly, In Socket FM1 platforms this threshold depends a lot on the peripheral devices, hard disk drives in the first place. So it is quite possible to put together a more overclocking-friendly configuration for Llano processors. And thirdly, once you exceed 133 MHz BCLK frequency, AMD A75 chipset automatically adjusts the multipliers for frequencies of peripheral controllers, which guarantees normal operation of the system.

As a result, when we overclock a Llano based system, we have two BCLK frequency ranges at our disposal: starting at 100 MHz and at 133 MHz. Note that the maximums in both ranges are determined by the specific configurations and in certain situations these intervals may merge into one.

Our practical tests show that in normal conditions maximum BCLK frequency that can be used in Llano systems is 140-150 MHz. In most cases it is enough for overclocking of the A8 series processors. Although AMD has now started using 32 nm process, the highest Llano frequency that can be achieved with reasonable voltage increase and air-cooling is around 3.6 GHz. So, taking into account relatively high nominal multipliers, we can expect BCLK overclocking approach to help us fully uncover the frequency potential of the computational Llano cores even with a default processor clock frequency multiplier. We may only experience a problem if the BCLK frequency we need falls into the “instability interval” before it hits 133 MHz, but it can be resolved by lowering the processor clock multiplier.

However, overclocking approach when we raise the BCLK frequency is not the most optimal one. Llano is not a common processor, but an APU with an integrated graphics core inside. When you increase the BCLK frequency, the graphics core also gets overclocked together with the computational processor cores, but the tricky part is the higher relative frequency potential of the Radeon HD 6550D graphics core in Llano processors compared with their computational cores. While 25% frequency increase is considered normal processor overclocking, the graphics core can work just fine at up to 800-900 MHz, which is 40-50% higher than its nominal 600 MHz frequency. And the only way to utilize this potential is to additionally increase the BCLK frequency, since the GPU multiplier is locked. So, if you simply increase the BCLK speed, it will eventually hit the overclocking maximum for the computational cores. Therefore, Socket FM1 systems will benefit much more from increasing the base clock frequency if you previously lower the processor clock multiplier. As a result, BCLK frequency will be higher and the integrated graphics core will speed up more.

There is one more thing to keep in mind. The memory frequency in Socket FM1 platforms greatly affects the graphics core performance. The integrated Radeon HD 6550D graphics core shares the system memory with the processor cores that is why the memory bus width becomes a bottleneck. This happens in all integrated systems, but in Llano’s case the situation is aggravated because Radeon HD 6550D is a pretty fast graphics accelerator that requires high-speed memory. Llano processors offer a choice of multipliers for memory frequency, but the intervals between them are way too big. Therefore, in most cases it may be better to set the BCLK frequency a little below its maximum, so that you could set higher multiplier for DDR3.

Summing up everything we said in this chapter of our review, we can put together the following Llano overclocking plan that will produce the best result in terms of overall system performance.

  1. Preparation stage. Seriously lower the processor clock multiplier and the memory frequency multiplier. This should allow you to determine the maximum BCLK frequency without any limiting factors.
  2. Increase the CPU Voltage (computational cores) and CPU NB Voltage (graphics core). The extent of this increase may depend on the efficiency of the CPU cooling system, but we wouldn’t recommend hunting for millivolts, as it doesn’t affect llano overclocking success that much. The most optimal increase would be from +0.1 V to +0.15 V for both parameters.
  3. Find the maximum BCLK frequency, at which the system still works. Maximum frequency may depend on the potential of the integrated graphics core, as well as on the peripheral devices connected to the mainboard (primarily those connected via SATA interface, as they are the trickiest ones). Therefore, we recommend to start from the second frequency interval beginning at 133 MHz. This will give us maximum overclocking of the graphics core integrated into the processor.
  4. Once we found the maximum BCLK frequency, we have to determine the maximum processor clock frequency multiplier. As a result, we now have not only an overclocked GPU, but also and overclocked CPU. However, priority should be with graphics, because its frequency has more significance for gaming performance.
  5. We add memory overclocking. Try setting the highest possible multiplier for the DDR3 SDRAM and don’t be afraid to use less aggressive timings for that matter. Keep in mind that higher memory frequency is worth losing a few megahertz of the BCLK speed, as the resulting performance won’t be any worse in this case. But if we have to lower the BCLK, then repeat step 4 one more time.
 
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