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The Celeron Overclocking F.A.Q.

   Celeron Overclocking FAQ   

By Frank Monroe

Version 1.0 - 10/03/98


Introduction and Disclaimer

I created this FAQ because, after reading literally thousands of posts, I still see the same requests for basic instructions over and over in each Newsgroup and forum.  There are many web sites with similar information,  but many people either can't find these sites or they don't have web access.  Since I have never seen a FAQ like this posted in any of the Newsgroups I read, I took it upon myself to offer this small contribution to novice overclockers everywhere.  I don't claim to have all the answers and I can't guarantee that everyone will be able to overclock their Celeron, but after reading this FAQ you should be well on your way to a successful experience.

Overclocking is not recommended by any manufacture (especially Intel) and will void your warranty.  I do not advise anyone to follow these instructions unless they are willing to assume all associated risks.  I have consolidated in this document information that I've learned while overclocking my own system or that I have read about the experiences of others.  Overclocking can damage your system.  Working inside your power supply or wiring 110 volt fans can cause serious personal injury if done by the inexperienced or without the proper precautions.  If you're unsure or in doubt about any of these procedures, seek professional advice.  I am providing this document for informational purposes only.

If any one out there in Net-land has suggestions, comments or contributions for this FAQ, feel free to contact me. Frank Monroe - Email:

So you want to overclock a Celeron?

You've read a few post, maybe visited a few web sites.  Everyone is reporting their success and claiming fantastic speeds from a lowly 266 or 300 mHz CPU.  You're excited at the prospect of a high performance CPU for, essentially, small change and you want to get in on the action.  The speed of a P2-400 or -450 for $90 or $150 sounds too good to be true.  But wait, they're talking about S-codes, multiplier locking, Pin B21, CAS-2, and other esoteric terms.  Names like Deschutes, Klamath and Mendocino are bandied about while you wonder what these words have to do with computers.  Now you're confused.  How hard is this going to be?  Is it worth it?   Do you need to be an Electrical Engineer to overclock a Celeron?  In a word, no.  With the right hardware and a little luck, it should be a snap.

Why is the Celeron so overclockable?

As you may know, a given chip design is used for CPU's of many different speeds.  The P2 and Celeron designs are named after Western US counties: Deschutes, Klamath and Mendocino.  More on this later.

In theory, a CPU is tested first at it's maximum speed.  The ones that pass the testing process at this speed are marked as such and sold as top-of-the-line CPU's.  Those that fail at the fastest speed are tested at successively lower and lower speeds until they run reliably.  These slower cores are then marked with the speed at which they passed the testing process and sold as slower processors.  At least, that's the theory.  No one really knows how Intel decides which cores get marked for a given speed.  Several other factors, such as customer demand and production quality, affect how many processors of each speed are produced.

A CPU of any given speed can usually be made to run somewhat faster if one is willing to play around with the motherboard settings. This is the overclocker's bread and butter.  Now, through a convenient turn of events, Intel has produced a CPU with an unusually high capacity for overclocking.

Intel has long controlled the high-end CPU market while it's competitors, Cyrix and AMD were gaining market share in the low- and mid-price range because of the popularity of lower priced PC's.  Intel finally realized what was happening and wanted to recover the low ground while also keeping the high end market (can you say "total market domination"?).  When Intel designed the CPU core for their newest line of processors, the P2, they changed the way the CPU was mounted.  All P2's are mounted on a circuit board, called an SECC (Single Edge Contact Cartridge), that plugs into a special, patented CPU slot (Slot 1) similar to a PCI slot.   [Intel calls the Celeron packaging a SEPP (Single Edge Processor Package) but it's still compatible with the Slot 1 connector, go figure.]   AMD and Cyrix do not have a Slot 1 CPU, so if you want high-end speed, you need to buy an Intel processor.   Thus the high-end market is preserved for Intel.  Now, Intel needed a cheap Slot 1 CPU to corner the low-cost PC market.

Enter the Celeron line. To reduce production costs, Intel left out the expensive Level 2 cache.  Also, to eliminate design costs, the original Celerons (C266 and C300) used the same CPU core as the new 350-450 mHz P2's (code name Deschutes).  [Remember, design costs account for a huge percentage of the total cost of a CPU.  Once in production, it costs exactly the same to manufacture a core destined for use as a 266 mHz processor as it does to use that same core in a 450 mHz processor.]  Many media pundits immediately dubbed the Celeron a backward-stepping piece of crap because of the lack of the L2 cache.  Later, perhaps due to the poor reviews from hardware critics, Intel released the Celeron 300A and 333 with 128 Kb of built-in cache. Again, they used basically the same core design with some modifications to incorporate the on-die cache.  The C300A and the C333 modified Deschutes core carries the code name Mendocino.    Since Celerons use a Slot 1 motherboard, you can't upgrade to one of Cyrix's or AMD's fast new CPU's later, when prices come down.  They don't have Slot 1 CPU's and Intel has the patent.  Now Intel has again regained a foothold in the below-$1000 PC market and insured that the upgrade dollars also come home to Papa Intel too.

Here's where it gets interesting.  The fastest P2 CPU's (350 to 450) require a relatively new type of Slot 1 motherboard with the BX chipset.  The BX motherboard runs at a bus speed of 100 mHz.  They can also run at 66 mHz bus which allows them to accept slower P2 CPU's (233, 266, 300 and 333) and Celerons. The Celerons are supposed to be used on the earlier EX and LX generation of Slot 1 motherboards which run at 66 mHz only.  Since the Celerons have the exact same core as the new architecture P2 CPU's,  there's nothing to stop you from setting the bus to 100 mHz and running a Celeron at 400 or 450 mHz.

People started buying BX motherboards and Celerons and overclocking the hell out of them by setting the bus speed to 100 mHz. A chip meant to run at 266 running at 400 mHz and more was unheard of previously.  It's all because Intel is trying to capture the low-cost CPU market without the R & D costs of a new chip.  It's really a marketing stroke of genius when you think about it.  Produce one type of CPU.  Take the best ones, add 512 kb of fast, expensive cache and sell it as the top-of-the-line CPU for $700+.   Take the rejects, leave off the expensive L2 cache and sell them as cheap Celerons.  Except they're too smart for their own britches.  The production yield of 450 mHz cores is too good and the "rejects" are too few and far between.  Because they want to flood the market with $100 CPU's, they have to mark them as 266 to 333 mHz Celerons and sell them cheap anyway.  It doesn't cost them any more since both chips came off the same production line.  Because the P2-450 market is relatively small compared to the low- and mid priced market, the demand is greater for Celerons.

What does stepping mean?

Celerons come in four flavors.  The C266 and C300 without L2 cache and the C300A and C333 with 128 Kb L2 cache.  Each type of Celeron has several slightly different variations, called a "stepping".  Stepping 0 (zero) cores are the original production run.  When minor imperfections (bugs) are found in the instruction programming (micro-code) of the core or in other parameters of the chip, they are fixed and the next batch of cores will incorporate the changes.  This batch will be identified as stepping 1.  If another change is required later, the stepping number will be incremented again.   As each successive refinement to the chip is made, the next higher stepping number will be assigned.  For many reasons, one stepping may be easier to overclock than another, but usually the higher stepping cores make the best, most stable CPU's.

What is an S-code?

An S-code (Intel actually calls it an S-Spec.) is a 5 character designation beginning with 'S' used to identify the various different types, stepping, voltage and packaging of Celerons and other Intel processors.  There are currently 14 (as of  27 Sep 98) different S-codes for the Celeron family of CPU's.  OEM packaging is just the SEPP in  a plastic container.  There is no heatsink/fan attached, so you need to buy your own.  The warranty, if any, is usually only for 30 days and from the vendor, not Intel. The retail Celeron (sometimes also called a "boxed" Celeron)  comes in a cardboard box with a pretty good heatsink and fan already attached.  You also get a Certificate of Authenticity and an Installation Notes booklet in 11 languages, a cute sticker for the front of your computer and, most important, a three year warranty from Intel.  The S-code can be found on one end of the retail box or on the back, left side of any Celeron SEPP printed circuit board.

C266  (Deschutes core without cache)

SL2SY   Stepping 0   OEM
SL2QG   Stepping 1   Retail
SL2TR   Stepping 1   OEM
SL2Y3   Stepping 2   Retail
SL2YN   Stepping 0   Retail

C300  (Deschutes core without cache)

SL2YP   Stepping 0   OEM
SL2Y2   Stepping 1   Retail
SL2X8   Stepping 1   OEM
SL2Y4   Stepping 2   Retail
SL2Z7   Stepping 0   Retail

C300A   (Mendocino core with 128 Kb L2 cache)

SL2WM Stepping 0 OEM
SL32A Stepping 0 Retail

C333    (Mendocino core with 128 Kb L2 cache)

SL2WN   Stepping 0   OEM
SL32B   Stepping 0   Retail

What is 'multiplier locking' and 'bus locking'?

No processors since the early 80486 CPU's have taken the motherboard bus clock and used it internally at the same speed.  Remember the 486DX2?  It took the 33 mHz bus clock from the motherboard, multiplied it by 2 and ran at an internal speed of 66 mHz.  Modern, BX-chipset motherboards now provide a 66 or 100 mHz bus clock to the Slot 1 connector.  Today's Celeron and Pentium II processors multiply this to achieve their designated speed.  Without multiplier locking, circuitry inside the processor reads the multiplier jumpers on the motherboard via the Slot 1 connector.  Depending on the setting of these jumpers (or BIOS setting for the Abit boards) the CPU then multiplies the clock by 3.5, 4, 4.5 or 5.  Multiplier locking forces the CPU to use a multiplier that is pre-determined by Intel, ignoring the settings on the motherboard.  All Celerons are multiplier locked.  The C266 is multiplier locked at 4; the C300 and C300A is locked at 4.5; and the C333 is locked at 5.

Multiplier limiting (only affects P2-350/400 processors made before mid-August '98) uses a signal from the motherboard to detect the bus speed and then places an upper limit on the multiplier based on the bus clock speed.  For example, with the bus set to 66 mHz, the processor can be set to a higher multiplier than it can when the bus clock is set to 100 mHz.  In effect, this limits the CPU to a maximum internal speed while allowing lower speeds.  With a 66 mHz bus, a "multiplier limited" P2 would accept higher multipliers than at 100 mHz.  [The BH6 BIOS has a setting under the SoftCPU menu called 100/66#SEL.  With the LOW setting you can defeat the clock limitation on certain P2 processors.  It will not work on the newer 400's and 450's and it will not unlock the Celerons.]  Intel says it uses multiplier locking and multiplier limiting to prevent unscrupulous retailers from re-marking processors to higher speeds.

Bus locking is a myth, at least at the present time.  If it was implemented, it would prevent a processor from being used at a higher bus speed than it was designed for.  For example, since all Celerons are meant to use a 66 mHz bus clock, bus locking would prevent the CPU from running at any other bus speed.  Since bus speed is set on the motherboard,  Intel would need to design and incorporate special circuitry in the CPU to detect the bus speed and compare it to the "proper" clock rate.  Below is a newsgroup post about bus locking from a member of the Intel support team.  As far as I was able to determine, this is a genuine post.  [I have edited format but not content.]

From: "Randy S." <>
Subject: Re: Bus locking and clock locking
Date: Mon, 14 Sep 1998 11:52:16 -0700
Newsgroups: intel.microprocessors.celeron
Organization: Intel Corp. - 100003


First of all, "bus locking?" how feasible is this in a processor when the bus timing and clock generation is sourced on the motherboard?  Think about that.

Yes, of course, you could specify something in the design guidelines for the processor. Given the current state of art in board design, I do not think this would happen on the processor side.  In fact, what are you currently doing now?  Would not a better description be "Overbussing" (i.e. using a bus speed higher than specified).

You can do what you want with your processor like anything else, but as long as there are those who wish to misuse and misrepresent Intel products to others for their own personal gain, we will take whatever means needed to prevent or foil these attempts.

To be honest, the needs of 'overclockers' pale to insignificance when compared to the specter of remarking and fraud. Intel is acutely aware of activity in this regard, and thus, the accommodation of variable multipliers and the like must be sacrificed to preserve the basic integrity of our product.

That said, perhaps you would like to convince the average consumer who was duped into buying an overclocked system of the need for this ability.

It seems to me your discontent is misplaced.  It is the unscrupulous people that should demand your attention in this regard, not Intel.

To paraphrase a quote I read somewhere:
"Chaos offers multiple solutions" - but who will support it?  Perhaps you?
Randy S.
Intel Internet Technical Support
*All other brands and names are property of their respective owners.

Which type of Celeron should I buy?

The key, of course, is getting the right stepping of the right type of Celeron.  First of all, the "wrong" Celerons are the C300 (without the 'A' designation) and the C333.  If you plan to overclock your Celeron, stay away from these two altogether.  Because of multiplier locking [see "What is 'multiplier locking' and 'bus locking'?"], the C333 severely limits your options.  Reports of successful overclocking are very rare with these CPU's.  While it's true that some Celerons have successfully run at speeds up to 500 mHz, these instances are rare [estimated at 5-10%].  If your C333 won't operate at 500 mHz (100 X 5), your only option (since the multiplier is locked), is lowering the bus speed to 83 mHz (415 mHz) or 75 mHz (375 mHz).  Seventy-five and 83 mHz require the PCI and AGP devices to operate at higher than normal speeds since motherboards only provide the correct clock step-down dividers for the 66 mHz and 100 mHz clock speeds. [Some motherboards may correctly divide the 133 mHz bus speed, but then SDRAM timing becomes a major problem.]  Many devices do not react well to this condition. Obviously, you are more likely to have problems at 83 mHz than at 75 mHz.  The C300 and the C300A should theoretically have the same likelihood of reaching 450 mHz [estimated at 60-75%].  However, there is very little information on the success rate of the C300.  [The reason may be that improvements were incorporated into the C300A that were not made to the C300.   Remember the C300A uses the Mendocino core while the C300 uses the cache-less Deschutes core.]  If you're are going to for 450 mHz with a 300 mHz CPU, why get a C300 when the C300A has the L2 cache for a few dollars more.

Between the C266 and the C300A, it becomes a matter of your needs and your wallet.  Both Celerons are quite likely to run well on a 100 mHz bus.  The success rate for the stepping 1 C266 at 400 mHz (estimated at 85-90% for the SL2QG and the SL2TR) is somewhat higher than for the C300A at 450 mHz (estimated at 60-75%).    The C266 is cheaper, roughly 1/2 the price of the C300A, but it doesn't have any L2 cache.  Though the lack of the L2 cache may not make as much difference as you might expect, it does affect overall performance some.  In most 3D games (except Unreal) the lack of secondary cache makes almost no difference.  Most other applications do use the L2, but remember, SDRAM at 100 mHz is pretty damn fast and you do still have the L1 cache.  After you factor in the wasted CPU cycles looking in cache for data that's sometimes not there (cache miss), the speed disadvantage is less than you might imagine.

That said, the C300A seems to be the current favorite.  Many people have decided that the extra cost and added risk is more than outweighed by the [IMHO, slight] performance gain offered by the 128 Kb of cache.  A word of caution, the C300A does seem to be a little trickier to overclock.  It occasionally needs more than the default 2.0 volts to be stable and often needs more than a stock CPU heatsink and fan.  I've  seen many more cries for help from people with an unstable C300A at 450 mHz than first-time success stories.  Most people usually achieve their goal but some just never make it, no matter what they do.  There are no guarantees with either CPU.

OK.  We've narrowed the field somewhat, but there are still the different stepping variations to consider.  For the C266 the best stepping is stepping 1 carrying S-codes SL2QG and SL2TR.  Since the stepping 2 C266 is so new, there is very little known about the success rate of the SL2Y3.  It may very well be as good or better than the stepping 1 CPU's.  Before the release of the C300A, the S-code with the most successful reports was by far the SL2QG.  The fan and heatsink provided by Intel with the retail boxed SL2QG has proved to provide more than enough cooling and there is very little price difference between it and the OEM SL2TR.

Both the of the C300A CPU's, the OEM SL2WM and the retail boxed SL32A, are stepping 0.  Many people have reported that additional cooling helps improve the success rate of reaching 450 mHz.  With this in mind, you may want to consider the cheaper OEM version and buy your own quality heatsink and fan setup rather than find out later that you need to remove the Intel-provided heatsink on the retail version.  There is one additional point in choosing your C300A that bares mentioning.  There seems to be a consensus that certain manufacturing plants have a higher success rate.  The Malaysia plant has gotten the most attention as producing the best CPU's, but few vendors even let you request specific S-codes, let alone trying to specify country of origin.  [This last thing about the country of fabrication is unscientific at best and most likely just wild speculation.]

Which motherboard should I use?

There are basically two motherboards of choice for overclocking a Celeron, the Asus P2B and the Abit BH6.  The BH6 is by far the most popular with owners of the C300A for a couple of reasons.  First, it's the only motherboard that doesn't need a BIOS upgrade to recognize the C300A.  Secondly, it allows you to increase the CPU voltage from the BIOS SoftMenu.  This feature is particularly attractive since many C300A CPU's need a voltage higher than the default setting of 2.0 volts to be completely stable.  Additional features that make the BH6 popular are it's lower cost, BIOS SoftMenu setup for all settings and an additional PCI slot.

In all fairness, the Asus P2B is also a very good board.  Though it costs about $40 (US) more, some staunch Asus supporters maintain that the P2B is more stable and has a higher success rate when overclocking.  The P2B does not have any built-in provision for changing the CPU voltage if it's necessary to do so, but it does have 3 ISA slots for those legacy ISA cards while the BH6 has only 2 ISA slots.  The biggest drawback to the P2B is that you need to flash the BIOS to the newest version (1005) in order for it recognize the C300A.  This can be problematic, to say the least,  since you need a CPU to flash the BIOS and it won't recognize your CPU until you flash the BIOS.  Catch 22.

There are other motherboards that can be used, however, I recommend at least considering one of these two if at all possible.

What kind and how much memory?

If you are going to use a 100 mHz bus speed, you should plan on getting PC-100 SDRAM memory.  Many people have reported successfully using their old PC-66 memory, however, if you do try it and have problems overclocking, memory would be a likely suspect.  You should also plan on starting with a minimum of 64 Mb.  It's best to get 64 or 128 Mb DIMM modules since the number of memory sockets is usually only 3, or 4 at the most.   Two 32 Mb DIMM modules will limit your ability to upgrade memory in the future.

Not all PC-100 is created equal and you usually get what you pay for.  Fortunately, if you only plan to try the 100 or 112 mHz bus speeds, most good quality PC-100 memory will work.  You only need the high-priced, premium "CAS-222" memory if you want to try 124 or 133 mHz bus speeds.  CAS stands for Column Address Strobe -OR- Column-Address Select.  CAS latency refers to the number of processor cycles from when a Read command is registered to when the data from that Read command becomes available.  CAS-222 certainly won't hurt if price is no object, but the added cost will only buy you a few tenths of a percentage point of overall performance.

There is an excellent web site, called PC100, that will tell you more than you'll ever want to know about memory.  The URL is listed in the learning resources section at the end of this FAQ.  Here are some excerpts from that page (used with permission).

From the PC100 web page--
The fastest cas 2 parts that operate at cas 2 at either 66 mHz or 100 mHz will have a PC100 label on them that says PC100-222-xxx. Cas 2 parts that operate at cas 2 at 66 mHz and cas 3 at 100 mHz will have a PC100 label on them that says PC100-322-xxx. The 222 or 322 refers to the actual data programed into the spd eeprom chip located on the memory module that informs the motherboard exactly what the SDRAM module is capable of.

brand   nano-sec  rated max real world cas @ 66mHz cas @100mHz PC100-label
many -12  83 MHz 66 MHz n/a  n/a
many  -10 100 MHz  83 MHz  n/a  n/a
most -8  125 MHz  100 MHz 2 3 322-620
Samsung -G8 125 MHz  100 MHz  2 222-620
Gold Star -7J 100 MHz  100? MHz  3 322-620
Gold Star -7K 100 MHz  100? MHz  222-620

What about cooling?

There are two aspects of system cooling that need to be considered, case cooling and CPU cooling.  The power supply fan alone normally does not provide sufficient air flow to eliminate heat build-up inside your case.  Hot air trapped in the case forces all components to operate at higher temperatures and reduces the effectiveness of convection cooling throughout your system.  Many overclockers find that heat is their main enemy, especially if you find that you need to raise the CPU voltage.  There are several things you can do to ensure that your case stays cool.

First, check the direction of air flow from the power supply fan. The best cooling is obtained by having the power supply fan draw air out of the case.  If it draws air into the case, you may want to try reversing it. It's a simple procedure than can make a significant difference in case temperature. [Caution:  Capacitors in the power supply can store a charge even after the power has been off for several hours.  Make sure that the unit has been unplugged for 24 hours or more.]  Remove the supply from the case and remove the cover.  Most power supply fans are held in place by four screws.  Remove these four screws and flip the fan over.  Generally, both sides of the fan will have a set of holes so you should be able to re-attach the fan with the same screws.  Reassemble and install the power supply.  You should see a drop of several degrees inside the case just from this simple, free procedure.  Opening the power supply will probably void the warranty on it so, if you're worried about that sort of thing, you'll be relieved to know that there are other things you can do to lower your case temperature.

Adding a second fan is a good idea even if you aren't overclocking.  Many cases provide a location at the lower front that is designed for a second fan.  Even if your case doesn't have a ready-made mounting point, you should be able to find a spot to install a second fan.  Depending on the type of connector your fan has, you can plug it into the motherboard fan connector or use one of the extra drive power cables for it's 12 volt supply.

Leaving the case cover off is also a possible solution to overheating.  Though not ascetically pleasing, it is a free solution that many overclockers employ.

Now that your case is maintaining a near ambient temperature, you need to think about the CPU.  If your system crashes or seems to become unstable after a few minutes of operation, you may find that heat build-up is the problem.  The fan and heatsink that is attached to retail Celerons is usually adequate to achieve the 400 or 450 mHz speed with the C266 or C300A.  If you bought an OEM Celeron or if you're having suspected heat problems with your CPU, you'll need to buy a good heatsink and fan combination and install it on the CPU.  Many vendors offer cooling packages with heatsinks and one, two or even three fans.  One vendor (STEP-ThermoDynamics) even offers an electronic peltier system ($85 US) and another (Kryo-Tech, mentioned at Tom's hardware site) offers a $500 refrigeration system.  While these expensive cooling systems work very well, most people find that a simple heatsink setup with one or two ball-bearing fans will provide all the heat dissipation that your CPU needs. A number of vendor and cooling information web sites are listed in the learning resources section at the end of this FAQ.

Be sure to remove all of the original thermal tape from the CPU if you're replacing an old heatsink.  It's important to apply a very thin layer (.003 to .005 inch) of thermal interface material between the heatsink and the CPU.  A little goes a long way since you're only trying to eliminate air gaps, not frost a cake.  Too much is worse than not enough.  Some interface materials are conductive so you'll want to be careful not to get any on the CPU pins or circuit board. There are several different types of thermal compound and some work better than others.  Thermal greases are made by dispersing thermally conductive ceramic fillers in silicone or hydrocarbon oils to form a paste. Thermally conductive compounds are an improvement on thermal grease as these compounds are converted to a cured rubber film after application at the thermal interface. Thermally conductive adhesive tapes are double-sided pressure sensitive adhesive films filled with sufficient ceramic powder to balance their thermal and adhesive properties.  As you can see from the table below, thermal tape is only slightly better than nothing at all.  Radio Shack and most electronic supply stores sell small tubes of the thermal grease or thermal compound for $2-$5 (US).

Thermal resistance of CPU to heatsink (Source: Electronics Cooling Magazine,  Sept. '96)
Thermal compound = 0.8
Thermal grease = 0.9
Thermal Tape = 2.7
Dry joint = 2.9
(Lower numbers are better)

How do I overclock?

First, set up your system and get it running at it's normal speed.  Set the SoftMenu or the jumpers as directed in your motherboard manual.  Install all your peripheral cards and software and test out the system.  Run a few benchmarks at the standard speed so you can compare the before and after results.  Only when you're satisfied that the system is behaving as it should and that it's stable at the rated speed, should you begin to push the performance envelope of your system.

Now you're ready.  Neither the Asus or the Abit motherboards require you to cover pin B21 on the Slot 1 edge connector.  It is ignored by the motherboard.  Most other motherboards do require you to cover this pin to fool the bus speed setting circuitry into selecting the 100 mHz speed.

On the Abit boards you should reset your system and enter BIOS setup.  Change the following options in the CPU SoftMenu:

CPU Operating Speed:   User Define
- External Clock:   100 Mhz
- Multiplier Factor: x4 (or 4.5 for the C300A)
- AGP/CLK:   2/3
Speed Hold Error: Disabled

On the Asus P2B you have to change a jumper on the motherboard.  If you are set for the 66 mHz bus speed (as you should be if you followed the advice at the beginning of this section), you should only need to change one jumper.  Power down and unplug the power cord.  The jumper block that you need to set is located just above the primary IDE connector and it should be labeled "BUS FREQ".  Your current setting for 66 mHz should be: FS0 1-2 (pins closest to CPU), FS1 1-2 (pins closest to CPU), FS2 2-3 (pins away from CPU).  To set the bus to 100 mHz you need to change FS2 to 1-2 also.  Now all three jumpers should be on pins 1-2, the pins closest to the CPU.

That's it.  If it works when you power up, you'll be at your new, overclocked frequency.  A C266 will be 400 mHz; If you're lucky, a C300 or C300A will be at 450 mHz; and, if you're really, really lucky, the C333 will be at 500 mHz.   If your computer completes POST (Power On Self-Test),  boots into Windows and seems stable, try running some applications.  Run a benchmark or two.  Let it stay on for several hours, cycling a game demo or benchmark.  If it acts normally, except FASTER, of course, congratulations!

If it doesn't work at first, don't worry (yet), there are several things you can try before you give up and admit that you've got an "unlucky" CPU.  Read on.

What if it doesn't work?

There are many things that can be done to coax a stubborn CPU into working.  I'll try to mention as many as I can here.  Above all, don't give up until you have exhausted all of your options.  Some of the things you can try are free or low cost, while others may require replacing some expensive components.  Whenever possible, try to eliminate the cheaper options first.  Then, if you suspect you may need to buy a new DIMM or video card, try to borrow one from a friend first or try your CPU in another, successfully overclocked system.  Remember, it might not be the CPU at all, but something else in your system that's giving you problems.

As mentioned in the section on cooling, heat build-up is one of the most common problems.  It manifests itself usually after several minutes to an hour after start up, especially when running CPU intensive applications.  If you system won't POST (Power On Self-Test), heat is probably not one of your problems.

  • 1. Try leaving the case open with a table fan blowing into the case.  If the system stays up longer or seems more stable with the table fan and open case, try some of the cooling methods mentioned above.
  • 2.  Check the temperature of the CPU (front and back sides) and your video card by gently placing your finger on the heatsink.  Be gentle and only touch the heatsink while your system is running.  If it's too hot to leave your finger in place, you definitely have a heat problem.
  • 3.  If the heatsink is applied with thermal tape, try removing it (be sure to get all of the tape off) and using some thermal grease and/or a better HS and fan.  [If you have a retail Celeron that came with an Intel fan, I'd save this for last since many, many people have used this fan without problems.  Lately though, I have seen cases reported where changing the HS/fan did help a retail C300A.  It may be that the thermal tape does not provide enough heat transfer to the heatsink.  You could try just replacing the thermal tape with good old thermal grease and reinstalling the same Intel HS/fan setup.]

There have been many reports of what is being called "burn-in effect".  After running the CPU at an elevated voltage or even at the normal voltage while "exercising" the CPU (cycling CPU intensive applications), the processor somehow becomes more likely to run at the desired speed.  The time required varies and it doesn't always work, but it's worth a try.  If it doesn't run at 100 mHz bus, try 75 mHz or 83 mHz for a few days.  Leave the computer on for several days straight.  Give it a workout, then try it again at 100 mHz.  [I have found some documentation on this effect in respect to audio components.  Some suggest that it may have something to do with dopant stabilization and the dielectric properties fully forming in the tiny, in-circuit semi-conductor junctions, capacitors and other components.]

Sometimes a little extra voltage is all that's required to encourage your recalcitrant CPU to straighten up and fly right.  You can adjust the voltage quite easily with the BH6 SoftMenu.  It's a little harder with the P2B but you can still do it.  With the BH6 you can increase the voltage in small increments.  Put your system through it's paces after each step.  If it still crashes, bump the voltage a little more. You can fry your CPU by increasing the voltage too much.  Use some caution and common sense here.   If voltage is your stumbling block, 2.2 volts usually does the trick, though some have required as much as 2.3 or more. The BH6 BIOS will not let you set the voltage higher than 2.3 volts without a special procedure (found on Andy Drake's site).   All Slot 1 motherboards read the required CPU voltage through contacts on the SEPP (CPU board).  By selectively blocking certain contacts, you can "tell" the P2B (or any motherboard) to raise the CPU voltage.  Teflon tape is one of the best materials to use, however, some people have used nail polish (dry 24 hours before inserting the CPU) or other non-conducting varnish.  You can even cut the trace, but this technique, while effective, is difficult to reverse (you need to re-solder over the cut).   This procedure is fraught with pitfalls and, if done incorrectly, can jolt your CPU with 2.6 volts or more.  Since I can't include pictures with this text document, I recommend examining a web site that illustrates and details this procedure, if possible (see the list of web resources at the end of this FAQ).  Your choices are quite limited with this method, but if you have a P2B or other motherboard, it may be your only option.  Notwithstanding those words of caution, here is the list of pins to cover to get the specified voltages.
Cover up these pins to attain:

2.2v-- A121,A119,B119 (if A119 breaks through, you get 2.6!)
2.4v-- A121,A120,B119 (if A120 breaks through, you get 2.6!)
2.6v-- A121,B119 (not recommended)
2.8v-- A121,A119,A120 (don't even try it)

Heat production increases when you increase voltage, so don't forget about needing more than ordinary  cooling if you need to raise the voltage.  One note of encouragement,  there have been many reports of users being able to revert back to the normal 2.0 volts after a few days and still maintaining stability (see the section on "burn-in" above).

Drivers and Peripherals
Try different versions of drivers.  Try new video drivers, try old video drivers, try Direct X 5.0 and 6.0.   Remove all your cards except the video card.  Disconnect the harddrive and boot from a floppy.  If you have two DIMMS, try with each one individually.  Borrow better, or at least different RAM.  Borrow or use an old video card.  Overclocking pushes your whole system to the edge.  There is no predicting what device may be extra sensitive to slight timing errors, data errors or excess heat.  Many of the new video cards, especially the AGP cards run very, very hot.  Be sure that video chip overheating is not what is keeping you from your desired speed.  [My G200 AGP card has hit 145 F when I forgot to turn on my extra fans.  You can mount an old 486 fan on it's heatsink for added insurance.]  If you find a card, harddrive or device that's keeping you from running at 100 mHz, you'll need to replace it.

Try getting it to run after disabling the L2 cache in the C300A (L2 should be disabled anyway for the C266 and C300 which have no L2 cache).  Set memory delay settings to higher values.  Be sure the AGP setting (Abit motherboards only) is at 2/3.  Set harddrive mode to PIO  4 or 3.  [This is an area I hope to receive comments on from other experienced Celeron users.  How about it?]

Will it damage my CPU or other components?

There are a number of things that could happen to your system through overclocking but, with a little common sense, you can remove most of the risk.  Heat is the primary concern.  The CPU is the system component that is most likely to suffer from excess heat.  Higher clock speeds and/or higher voltage create more heat.  Extreme heat can literally fry the CPU.  Keeping the voltage as close as possible to the default of 2.0 volts and using a quality heatsink and fan will keep the CPU temperature within reasonable limits.

Long term effects of higher current produced by faster clock speeds can have more subtle effects.   A process called electro-migration can slowly erode the microscopic circuits inside the CPU--causing the traces to spread and the semi-conductor junctions to break down--until the CPU eventually fails.  This is a very slow process and it takes years.  A modern CPU has a design life of 10 to 15 years.  While the life of your overclocked CPU may be somewhat shortened, do you really expect to be using your current 300 mHz processor, even overclocked to 450 mHz, in even as little as 5 years?  By then we'll all be overclocking 1000 mHz (giga-Hz) CPU's.

Since BX motherboards like the BH6 and P2B are designed for a 100 mHz bus speed, you are not likely to hurt the mainboard with speeds up to 100 mHz.  The other components, however, can be negatively affected by bus speeds other than the standard speeds of 66 or 100 mHz.  At all other speeds the PCI and AGP clocks are higher than normal (see the table below).  Besides the obvious effects of increased heat generation, some peripheral devices are especially sensitive to timing problems when the PCI bus is over clocked.  Some hard drives will trash your data if the PCI bus is clocked too high. 

66 mHz  33 mHz 2 to 1  66 mHz  1 to 1
75 mHz  37.5 mHz  2 to 1  75 mHz 1 to 1
83 mHz  41.5 mHz 2 to 1 83 mHz 1 to 1
100 mHz  33.3 mHz 3 to 1  66 mHz  3 to 2
112 mHz  37.3 mHz 3 to 1  74.6 mHz 3 to 2
124 mHz  41.3 mHz  3 to 1  82.6 mHz 3 to 2
P2B 133 mHz 44.3 mHz 3 to 1 88.6 mHz 3 to 2
BH6 133 mHz 33 mHz 4 to 1 88.6 mHz 3 to 2
   The End    

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