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Troubleshooting Overclocking Problems ( Part 2 )

In the last article we covered heat-related issues and overclocking. As promised, in this article we’ll continue with basic overclocking fundamentals and tactics for specific components. We’ll also delve into specific problems with certain components and memory. I asked Al Drake to cover the fundamentals of overclocking since I concentrated more on troubleshooting tips. Here is Al’s contribution:

Overclocking Fundamentals

What is overclocking?

To put it very simply, overclocking is increasing the speed of your CPU beyond its rated speed. This is achievable by increasing the CPU clock multiplier, the front side bus speed (FSB), or a combination of the two. The way to determine a possible overclock speed is to multiply the clock multiplier by the FSB speed.

AMD K6/2 266 --> Defaut:4.0 x(clock) 66.8 FSB = 266MHz

Overclocked to 300 --> 4.5 x (clock) 66.8 FSB = 300MHz

Overbussed to 300 --> 3.0 x (clock) 100 FSB = 300MHz

It is noteworthy to mention that some CPU’s are "clock locked", which means the clock multiplier can not be adjusted and therefor the only option is to increase the FSB speed and now you are overbussing.

Intel Celeron 300A --> Default: 4.5x (clock multiplier) 66.8MHz FSB = 300MHz

Overbussed to 450: 4.5x (clock multiplier) 100MHz FSB = 450MHz

If at all possible it is preferable to overclock the clock multiplier as opposed to changing FSB speeds as other peripherals such as memory, video cards, SCSI cards, and hard drives, will be affected. IDE hard drives like to operate at 33Mhz IDE speed…. If you overbus to 75 MHz your IDE speed becomes 37.5, if you increase the FSB to 83 then the IDE speed becomes 41.5 MHz and here’s where the problems start. Some hard drives will operate quite happily at these bus speeds (Quantum for example) while others will reward you with corrupted data, or total inoperation (Maxtor, Fujitsu, and Seagate come to mind). With these reluctant drives it is necessary to disable the UDMA in the BIOS and possibly lower the drive’s PIO mode. I have run overclocked machines with Maxtor drives before so it is a step that needs to be taken if your drive is fussy about the IDE speed.

Video cards are picky too, the most notorious being the AGP variety. Since the AGP speed is 66Mhz increasing the FSB to 83 MHz @ 1/1 AGP divider, or 124 MHz 2/3 AGP divider (gives the same AGP speed) will be more than some cards can handle. If this is the case you could have a machine that won’t boot or lockup when loading to the windows desktop. If you absolutely have to run 133+ MHz on your FSB then you currently are limited to PCI video cards until mainboard manufacturers start offering ˝ AGP divider settings.

Memory is limited by pushing it far beyond its rated speed (example PC66 SDRAM used with 100 MHz FSB speed). Although it is possible to run memory beyond its designed speed you can be left with an unstable machine. Try to match your RAM with the FSB speed you are using (greater than 83Mhz use PC100…. Greater than 112 MHz use PC133).

So now that you have your peripherals sorted out, it’s time to actually overclock your CPU. Take small steps, and don’t try to clock your CPU too high too fast. Normally OEM supplied heatsink are not up to the job and will need to be replaced with a good quality CPU cooler and thermal compound. Hard lock ups are a sign that the CPU is getting too hot and therefor require better cooling.

When you overclock your processor there is an increase of the amount of "noise" within the CPU core that can cause errors, crashes and BSOD (Blue screens of death). The way to overcome this extra noise is by increasing the voltage to the CPU core… usually 15% is the maximum you want to deviate from the default voltage. Compare it to driving down the road with the windows down and the stereo on, at 30Mph the music sounds fine, but when you drive at 70 Mph you need to turn up the volume so you can still hear it.

L2 Cache can make a CPU with a beautiful core a rotten overclocker. The Celeron processors are still the overclocker’s choice. The integrated L2 cache runs at the processor speed and benefits from being cooled by the heatsink sitting on the CPU. The Pentium II and III’s have L2 cache chips that are external from the CPU and reside on the PCB next to the processor, these are not cooled by the heatsink and get very hot. In addition the speed of the L2 Cache chips often dictate how fast you can push the processor. An example of this is the PIII 450 with 4.4 Ns L2 cache chips Vs the 3.3 Ns L2cache chips. PIII with the faster L2 cache chips are being regularly overclocked to speeds of 600 MHz and in most cases higher than that…. Where as the slower 4.4 Ns chips are allowing owners to reach only into the 500Mhz range.

What Else Can Go Wrong?

First thanks to Al. I think he explained it about as easily as anyone can without getting overly technical. That’s quite an accomplishment considering the task I gave him. If you assessed the heat-related issues brought up in the first article you are ready to continue. Onto new and uncharted country: problems that appear after you have begun your overclocking endeavors. As with the first article I’ve tried to group areas of problems together but many of these tips may, in fact, help you to improve system problems whether you are overclocking your computer or not.

First we will cover issues with CPUs and motherboards:

  • Most Intel chips overclock well. Less likely to overclock are the AMD K series chips (Athlon o/cing is good but requires modifying the CPU). Cyrix and Winchip units are the least likely to overclock. Spend a little more money on the chip if you plan to overclock it.
  • Intel Pentium II, Pentium III, and Celeron are multiplier locked from the factory and can only employ front side bus modifications to reach higher clock rates.
  • Many vendors will sell a CPU that has been tested to a certain speed. This is a more sound investment than buying a CPU and hoping for good results. Many vendors tell me that they find one Celeron 366 in ten that will reach 550 MHz.
  • Very few chips will overclock at their stock voltage rating. Most will require an increase of at least 0.1 volts and often 0.2 volts. Don’t push your chip more than 0.3 volts over stock. This is a very common cause of a "burnt chip".
  • As you increase voltage to your CPU proper cooling of it becomes even more crucial as the extra voltage will create more heat in the chip.
  • If you are shopping for a motherboard look for one that has several front side bus settings and allows for CPU voltage manipulation. A big plus is a board that allows voltage manipulation in 0.05-volt increments.
  • Also consider a motherboard that offers front side bus, multiplier, and voltage manipulation from within the BIOS. Depending on the case your motherboard is mounted in it may be difficult to reach the jumpers on a motherboard.
  • When overclocking a chip with a locked multiplier it is easier to use a chip with a lower multiplier. A Celeron 366 with a factory locked multiplier of 5.5 is more likely to overclock than a Celeron 400 with a multiplier of 6.0.
  • If your CPU features an adjustable multiplier it is better to resort to a low multiplier with a higher bus speed. Ex: an AMD K6 233 MHz (3.5 x 66) will achieve higher performance set to 225 (4.0 x 75). The faster bus speed will allow for increased performance of the chip’s cache and the system memory.
  • Sometimes using a higher multiplier with a lower bus speed will allow a chip to overclock. I recommend you benchmark the CPU before and after to see if there are any gains. Ex: K6-3 400 MHz (4.0 x 100) may be stable at 428 MHz (4.5 x 95).
  • All bus speed manipulations are very dependent on the memory used. PC100 (CAS 3) will probably not reach 125 MHz while CAS 2 PC100 routinely does. EDO seldom handles more than a 75 MHz bus speed while most 66 MHz SDRAM will run up to 83 MHz.
  • Many boards will allow you to adjust the refresh rate of your memory from within the BIOS (CAS setting). Higher numbers will yield less performance but will allow higher stability.

Issues with PCI and AGP components:

When overclocking the system with a higher than normal front side bus it is important to remember that this will affect the PCI and AGP components since their bus speeds will be changed. PCI and AGP buses are fractionally based on the front side bus of the system. PCI bus is normally/3 for systems with 100 MHz bus) and AGP bus is normally 66 MHz (2/3 of system bus). Modern harddrives use a PCI bus. At high system bus speeds most of your PCI hardware may not work at all or will have problems such as harddrives that corrupt their data or registry.

  • Because Celeron processors normally run at 66 MHz adjusting their frontside bus to 100 MHz allows them to continue to have PCI and AGP components run normally. These are the settings for Pentium II and III which are in the same family of processors.
  • An added benefit of using a Celeron this way is that the L2 cache on a Celeron runs at CPU speed and not ˝ CPU speed such as with the Pentium II or III. A Celeron running at 550 MHz can actually have marginal performance over a similarly clocked Pentium II.
  • Most PCI components will operate fine at slightly higher than normal system bus speed. For truly adventurous overclocking experiments you may find yourself pulling your PCI modem and soundcard and inserting ISA substitutes since they run at a constant bus speed that is not determined as a fraction of the system bus.
  • For more stability within Windows you should consider disabling DMA support for your drives as the system bus is increased. Some motherboards BIOS also allow you to adjust DMA support (reducing PIO settings).
  • Certain harddrives cannot handle large increases in the PCI bus speed. Maxtor drives are particularly notorious for this. You should definitely disable DMA support in Windows and adjust your PIO setting higher in the BIOS before you overclock your processor.
  • Consider purchasing drives that handle an overclocked PCI bus such as Western Digital, Quantum, and Seagate.

Overclocking is one of the hottest topics on the web these days. Unfortunately for someone who is new to overclocking or has not obtained their overclocking goals there seems to be little left to try before giving up. These guides were written to introduce beginners to basic concepts in overclocking and to give troubleshooting advice. Obviously not every concept can be covered in two articles and to that end we are going to be featuring articles with our ongoing overclocking projects in the future. Many will cover heat-related issues but we have a few interesting projects up our sleeves. Remember to reach your goal in small steps and keep the safety of you and your processor in mind and you should have better results.

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Victor Oshiro & Al Drake
Victor Oshiro @ Al Drake
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