Modified from http://www6.tomshardware.com/guides/overclocking/index.html by Wattana Rattanaprom

Three things are necessary for overclocking:

The CPU
So far, Intel manufactures the CPUs with the highest quality, hence the probability of a successful overclocking is highest with Intel CPUs. In case you want to overclock a Pentium 133, try to avoid the 'SY022'. 33% of the Pentium 133 with this S-spec have been disabled for multiplier settings of more than x2. In my database there are 22 of 66 entries with the 'SY022', where the CPU wouldn't support multipliers of x2.5 and x3. A small number of SU073 also seem to suffer from the same problem. Check to make sure your Pentium isn't faked. If you can peel off a black sticker underneath the CPU, it's definitely a re-marked one. In this case your CPU is most likely already overclocked.

The Motherboard

The quality of the motherboard is crucial for successful overclocking! Due to the fact that the CPU produces fewer 'clean' signals in overclocked mode, reflections and other flaws on the bus can cause the system to crash or hang. The reverse situation is also true - in overclocked mode the CPU is more sensitive to unstable signals from the bus and will crash if the motherboard can't deliver clean signals. Always go for a branded motherboard! You will have to decide if you want to go for a higher bus speed or if you will stick to a maximum of 66 MHz.

Motherboards with 75 MHz bus speed support are hard to find and motherboards with 83 MHz bus speed support are even more rare. Refer to my The 75-83 MHz Bus Speed Project to find out which boards support these higher bus speeds. Read in The Bus Speed Guide why bus speed is so important! The board should obviously support a wide range of CPU supply voltages. Minimum are 3.3 and 3.45 V, for STD and VRE voltage. If you want to use P55C, M2 (the new M1/6x86), or the new K5/K6 CPUs, you will need support for 'split voltage'. This means that the core of the CPU requires a lower supply voltage than the I/O ports of the CPU. The latest boards all support 2.5 up to 2.9 V in 0.1 Volt steps. If the board offers you an even higher voltage than 3.45 as well, you should be happy, because this might be the last trick to get your CPU successfully overclocked.

The RAM

This topic is new on this page, but it is very important indeed. You will have to consider decent RAM if you want to run your system at bus speeds of more than 66 MHz. If you want to run an HX board, such as the Asus P/I-P55T2P4 at 83 MHz bus speed, you will require high-end EDO. I've experienced myself, that the marking of the RAM is less important than it's brand. My official 45ns EDO is not as stable as Fredi's 60ns EDO of a good brand. I will try to find out the best brands of RAM, but so far I know Siemens (well, after all one German company is in the computer industry, hehe) and naturally Micron to be of outstanding quality. Be careful, however, that you don't get second-rate chips from the manufacturers being sold in some stores. These chips still say Siemens, Micron, or whatever on them, but their quality won't live up to your expectations. In the case of high bus speeds always go for SDRAM if you can. SDRAM relieves a lot of the worries of running at 75 or especially 83 MHz, and runs flawlessly in any case.

The Cooling

I can't proclaim it often enough, the cooling of the CPU is extremely important ! If you have been able to boot your system with an overclocked CPU but it crashes within the first minutes, it's most likely due to insufficient cooling of your processor. Don't think the average small heat sinks with their small fans designed for a Pentium are able to do this job properly! Their job is only to keep a normally clocked CPU cooler in case you have very hot surroundings (e.g. SCSI or Video cards, which can get very hot as well). They are not designed to save your overclocked system from crashes due to overheating. This doesn't mean you always have to have better cooling. If you've got a new SSS CPU, using the 0.35µm die, it just won't get that hot. If your CPU is of the old 0.6µm die size type, however, you will require decent cooling.

To accomplish this, you can use heat sinks, fans, or both, peltiers, or peltiers with fans. I personally don't believe in peltiers. Peltiers are elements which transport heat using an electrochemical method from one side of the element to the other, consuming energy. You will still need a heat sink to dissipate the heat from the non-CPU side of the peltier and most likely will also require a fan.

My opinion is that you should go for a heat sink, and most importantly THINK BIG !! If a big heat sink still can't do the job, add a fan on top of it. My overclocked P133 -> 180 has a temperature of about 30°C, which is hand warm or less. If you achieve this cooling effect, you can be sure that any crashes which do occur are not a result of overheating. So how to get a decent heat sink ? Don't even think of finding anything in a normal computer shop. You'll find professional heat sinks only in professional shops which sell electronic equipment such as transistors, resistors, chips, etc. (e.g. Hobby Electronic Stores).

You can tell how good a heat sink is by looking at the K/W value. K/W means degree Kelvin per Watt of power dissipation . K/W tells how hot the heat sink gets per each Watt of heating power of the device it's meant to cool. If you were able to follow that, you will understand that the smaller the value, the better the heat sink. If you can get a heat sink which has a value below 1K/W, you've found a good one. You'll need to make the surface of the heat sink that will attach to the top of the CPU match the size of your CPU (maybe the electronic shop will cut it for you, otherwise you'll have to do some sawing and grinding).

Be careful that this surface stays completely flat, so that there are no gaps between the heat sink and the CPU surface. Finally, you only need to affix the heat sink to the CPU which is best done with some thermal compound (also available in every electronic shop). You can also use super glue, but it should be applied very sparingly with just enough to attach the heat sink. Do realize that you might not be able to remove the CPU from the heat sink if the super glue is good stuff. If required, attach a good (powerful + quiet) fan to the top of the heat sink (how, I will leave up to your imagination). I will try to find a heat sink manufacturer that produces large, cool heat sinks for our overclocking community. I'll keep you posted about that
 

Correct Overclocking - The Goals

First and foremost, we want to improve overall system performance. Also, we want the system to be just as stable as it was before it was overclocked. Finally, we want to keep our CPU alive.

As you may have learned from my bus speed guide, the best way to improve system performance is to increase the bus speed. If you can't do that, either because your motherboard doesn't support higher bus speeds or your RAM or your PCI devices aren't up to it, you can change the multiplier instead. Don't expect much gain in performance, however, if you increase the multiplier but you decrease the bus speed!!! For example, changing from 166 @ 2.5x66 MHz to 180 @ 3x60 MHz will actually decrease your overall performance. The same rule applies to changing from 133 @ 2x66 to 150 @ 3x50. These types of changes will not make your system any faster!!

This is some touchy news for 6x86 users, who should really only overclock their CPUs to a slightly higher speed than the original. The 6x86 only has multiplier options for x2 and x3. Don't let yourself be told otherwise!! Of course you can try jumpering the board to all of the different Intel Pentium settings, but it won't make a difference for the 6x86 CPU

• Changing the bus Speed

To understand how you can overclock a Pentium, Pentium Pro, 6x86, or K5 CPU, it helps to realize that the internal clock in these CPUs runs at a different speed than the external clock or bus speed. The external clock is the speed at which the cache and the main memory run and when divided by two yields the speed of the PCI bus. There are only three different official bus speeds used by the Intel Pentium, Pentium Pro, and the AMD K5 CPUs - 50, 60 and 66 Mhz. The 6x86 uses five bus speeds: 50 MHz, 55 MHz, 60 MHz, 66 MHz, and 75 MHz. There are also new boards available which support the unofficial bus speed of 83 MHz.
To change the bus speed, look in your motherboard manual for something like 'CPU External (BUS) Frequency Selection' - these are the jumpers you will have to change. If you are lucky and happen to have a motherboard with the new SoftMenu^TM technology, you can change these settings in the BIOS setup menu right from the comfort of your chair. You don't even have to open the case.
Always go slowly and increase the bus speed one step at a time (e.g. go from 60 Mhz to 66, not 60 Mhz to 75). This is the most successful way to overclock. Using this method, almost every P150 CPU runs at 166 MHz and most all 6x86 P150+ CPUs run at a P166+ level or 133 Mhz.
• Changing the Multiplier

The internal clock is controlled by an internal clock multiplier in each CPU which is programmed via CPU pins. Intel Pentium CPUs support the following multipliers: x1.5, x2, x2.5 and x3. Intel Pentium Pro CPUs support x2.5, x3, x3.5, x4. 6x86 CPUs so far only support x2 and x3, but the upcoming M2 will support x2, x2.5, x3, x3.5. The K5 is kind of a difficult fellow here, because it doesn't seem to be affected by the external settings of its multiplier. So far it only uses the x1.5 multiplier for each of the PR75, PR90, PR100, PR120, PR133 CPUs. The new PR150 and PR166 K5 CPUs will use the x2 multiplier, but it seems fairly likely that you won't be able to change it - although I have not been able to verify this.
To change this setting, find something like 'CPU to BUS Frequency Ratio Selection' in your motherboard manual. There are usually two jumpers used to change these settings. Again, you can do all of this in the BIOS setup menu if you have a SoftMenu^TM motherboard such as the new Abit motherboards.
• Changing the CPU Supply Voltage

You might not like it, but this is something which is often required to make the CPU run more reliably.
First of all, I'd like to stress that the Intel Pentium and Pentium Pro CPUs can run at a supply voltage of up to 4.6 V. This, of course, requires serious cooling because the chip is producing a lot more heat than usual. I have tried this with my own P166 and the chip is still doing fine. It didn't really help my problem either, however, since it was the memory that would not run reliably at 208 MHz, not the CPU.
Often the change from STD to VRE voltage is the whole trick to successful overclocking. This is due simply to a bigger voltage difference between the digital HIGH and LOW conditions, which results in 'cleaner' signals for the CPU and other motherboard devices. If you can't run your CPU reliably at one particular clock speed, it's always worth considering changing to a higher supply voltage. The silicon of STD and VRE CPUs is identical, so you won't damage your STD voltage CPU with VRE voltage, the CPU will only run a little hotter. The Abit IT5 motherboards offer a voltage which is even higher than VRE, of 3.6 V in their SoftMenu^TM BIOS CPU setup. I'm running my CPU at this voltage and it runs completely stable at 205 MHz @ 2.5 x 68, even as I type this document.
[Top]

Have a look at the The 75-83 MHz Bus Speed Project

To be honest, the easiest and most sensible answer to that is: TRY IT OUT !!! Switch the multiplier to the lowest setting and then put the bus speed jumpers in all the different configurations. For non mathematical people, there are 2 to the power of the number of jumpers configurations. 4 configurations for two jumpers, 8 configurations for 3 jumpers, and so on. Just boot up to the DOS prompt and run ctcm, or a similar program which will tell you the CPU speed. You then only have to divide it by the multiplier setting to find your bus speed. Again, if your motherboard uses SoftMenu^TM BIOS, you only have to look in your BIOS setup menu to find all of the different bus speed settings from which you can choose.

There is also, however, a different approach:

• Motherboards with the PLL chip PLL52C59-14 can run at up to 75 MHz and they also support the 'turbo frequency' feature, which increases the bus speed by 2.5% (officially approved by Intel's CPU specifications).
• For 75 MHz

Pin 8 via 2.2 k Ohm to 0 V ('0 V' means 'ground' and NOT 'disconnected'!)
Pin 12 via 10 k Ohm to 5 V
Pin 13 via 10 k Ohm to 5 V
• For 68 MHz, the 'turbo frequency' for 66 MHz

Pin 8 via 2.2 k Ohm to 0 V
Pin 12 via 10 k Ohm to 0 V
Pin 13 via 10 k Ohm to 5 V
• For 61.5 MHz, the 'turbo frequency' for 60 MHz

Pin 8 via 2.2 k Ohm to 0 V
Pin 12 via 10 k Ohm to 5 V
Pin 13 via 10 k Ohm to 0 V
Measured on the Abit boards IT5H, IT5V, PR5, which all use the PLL52C59-14. The PR5 also comes in a version with the PLL52C61-01, however, the setting below seems to set the board at 61.5 MHz instead of 83 MHz.
• Motherboards with the PLL chip PLL52C61-01 can run at 83 MHz bus speed as well and also theoretically support the 'turbo frequency'. There obviously is, however, a way of configuring this chip (circuitry), so that it would not run at 83 MHz. I'm working on that.

These are the conditions:
• For 83 MHz or 61.5 MHz 'turbo frequency' for 60 MHz. Unfortunately this depends on the circuitry on the motherboards.

Pin 5 via 10 k Ohm to 0 V ('0 V' means 'ground' and NOT 'disconnected'!)
Pin 12 via 10 k Ohm to 5 V
Pin 13 via 10 k Ohm to 5 V
• For 75 MHz

Pin 5 via 10 k Ohm to 5 V
Pin 12 via 10 k Ohm to 0 V
Pin 13 via 10 k Ohm to 5 V
• For 68 MHz, the 'turbo frequency' for 66 MHz

Pin 5 via 10 k Ohm to 5 V
Pin 12 via 10 k Ohm to 5 V
Pin 13 via 10 k Ohm to 5 V
I have measured this on the Asus P/I-P55T2P4 rev. 3 board and was able to verify the exact same settings on the FIC PA-2006 board. The FKI SL586VT II or Magic Pro MP-586VIP board also use this chip, but you can't get to 83 MHz. Instead you get the pathetic speed of 61.5 MHz.
In terms of jumpers, this means that you will have to find out which of the three jumpers is connected to the particular pins. It's circuited via the pull up/down resistor of 10 k Ohm. In case you only have jumpers with ON/OFF positions instead of 1-2/2-3 positions, the ON condition is the condition for 0 V, and the OFF or open condition is for 5 V.

There are several motherboards that use one of the two PLL chips, depending on what was available when the motherboard was assembled. The Abit PR5 is such a fellow (of course I was unlucky enough to receive a board with the PLL52C59-14) and it seems the shuttle HOT 557 is another. There are a few reports of 83 MHz bus speeds with these boards, but most simply can't run at this faster bus speed. This is a real pity, because both of these two boards can use SDRAM - the best RAM for the 83 MHz bus speed.

Using this higher bus speeds includes some important restrictions which you should be aware of.

• The PCI bus runs at 37.5 or even 41.6 MHz. This can lead to several problems with PCI devices. Typical trouble makers are SCSI controllers, some video cards, and network cards. SCSI controllers and network cards often refuse to work at the faster speed, but some video boards just get much hotter than usual. If you find a way to cool these video cards, you shouldn't have any trouble. My Diamond Stealth 64 Video VRAM isn't affected at all by those higher bus speeds. I hope the 75/83 MHz bus speed survey will help us find out which PCI devices run at higher PCI bus speeds.
• The speed of the EIDE interface included with the chipset is not only determined by the PIO or DMA modes, but is also highly dependent on the PCI clock. This is one reason why the EIDE interface is always slower in systems with 60 MHz bus speed or less. This is also valid in the other direction, meaning your interface will be faster when you are running at 75 or 83 MHz bus speeds than at 66 MHz. At first this sounds fine, but often either the interface or, in more cases, the hard disk isn't up to the faster bus speeds. My HDDs work fine at 75 MHz bus speed, but at 83 MHz I have to reduce the PIO down to 2. The same is valid for EIDE CD-ROM drives. This could be the cause if you are running into strange lock-ups in windows.
• The Asus P/I-P55T2P4 is one example of a board that does not allow you to adjust the ISA bus speed. It seems to be a fixed divider from the PCI clock. This can cause sound cards to run into trouble if they don't like the higher ISA bus speed. I haven't come across this problem myself yet, but I've heard of one fellow whose AWE 32 produced strange sounds when running at a faster speed. If you run into this, increase the ISA wait states in the BIOS setup to try to remedy the problem.

Let me say again that the RAM type and quality is of great importance. Most 60ns EDO will run fine at 75 MHz bus speed, but for 83 MHz you'll need high-end EDO or SDRAM (as long as the motherboard supports it).
  

[Top]