CPU question

Discussion in 'General Electronics Chat' started by cjdelphi, Feb 2, 2011.

  1. cjdelphi

    Thread Starter New Member

    Mar 26, 2009
    I was scrounging around for anything I could find on Crystal Oscillator i still don't know what I need to pass to get a stable frequency out, I was trying to send the out from a 555 doing as many cycles as possible feeding the out to a crystal of 3khz it just did not seem to be working at all, so once i work out how to use one properly in a circuit....

    Anyway while looking, It turns out a lot of the older computers, gameboy, some calculators, sega, N64 etc, just by changing the frequency of the CPU it runs at, so if the CPU had a 30mhz crystal, you could replace it for 60mhz, doubling the clock speed... now here's what I don't get, what is the limit? I remember my old 486 pentium 33mhz with 4 meg of memory, it did not require a heatsink or fan at this point in time although a heatsink was probably a good idea!

    Now, if I decided to change say the 33mhz crystal in small device like a gameboy or faster, what's my limit to how much of an overclocking i can give the CPU? (sounds rude lol)

    Could i got 200mhz could I stick my existing HeatSink and Fan (2.8gighz) and get a crystal of say 900mhz or even 1000mhz would that cpu which is designed for only 33mhz be able to cope providing 1000mhz if it was cool enough?.. and seriously to get a device which is only 7-8mhz like a gameboy what was the reason for not using a 20mhz crystal give it some kick?... the only thing i can think of is battery life... it seems to me games could have been a lot more fun on even quite outdated hardware simply by increasing the frequency and not having to worry about heat???
  2. Wendy


    Mar 24, 2008
    Most CPUs are clocked at their maximum speed they are designed for. You may be able to increase the speed a little, but not like your thinking.

    Part of the problem is the outside devices. Things like the LCD screen are designed for a specific speed. If you change the CPU you are also changing a lot of interfacing designs outside what they were designed for.

    Computers are designed to allow for various speeds, things like game boys are not.
  3. magnet18

    Senior Member

    Dec 22, 2010
    Not all are clocked for maximum speed, Ive seen gameboys clocked at 1.5X speed, ti graphing calculators can increase a lot with the right couple components, but i think this is changing a capacitor not a crystal

    I think it depends on the application, home computers are probably a lot harder to speed up than something like a calculator, and i doubt it can be done simply by changing the crystal and nothing else, i know for the gameboy it was a module that needed replaced and some wires run throughout

    it aint simple and its really easy to fry something
  4. Wendy


    Mar 24, 2008
    Actually overclocking home computers is a time honored tradition, or at least it used to be. On another forum, MajorGeeks, they have a whole forum devoted to the subject. A lot of it is devoted to cooling though, TANSTAFL.
  5. magnet18

    Senior Member

    Dec 22, 2010
    Me and a friend of mine seriously considered re-purposing a minifridge for a computer housing one summer, his would overheat all the time...
  6. bitrex

    Active Member

    Dec 13, 2009
    As you probably know, on modern PC hardware overclocking the system can usually be done from the BIOS - the CPU's clock frequency is a multiple of the front side bus frequency. It's been a while since I've had a 486, but I believe even back then the arrangement was similar except that overclocking, if at all possible, had to be done with jumpers on a motherboard that had them. I'm not terribly familiar with the details of PC clock generation schemes, but I'm pretty sure that just substituting one crystal for another won't work properly.

    The limits to overclocking are not just power dissipation, as you mention, but also the ability of the logic in the chip to respond properly to a faster clock rate without locking up or otherwise behaving improperly. This usually has to do with the parasitic capacitances and inductances of the internal features of the chip and is a function of the process size of the IC and also varies from unit to unit.

    It is probably possible to overclock simpler processors like the Z80 derivative in the Gameboy with a simple crystal swap. The problem is though that the rest of the system, such as the subsystem for generating the LCD output, and the audio subsystem, was designed to operate at the original frequency. Even if you could get the processor to operate with a different crystal it's unlikely that the Gameboy would put anything intelligible onto the screen.

    I don't like being a Debbie Downer, but you might get better results in experimenting with overclocking a chip such as an AVR microcontroller or a PIC via swapping in higher frequency crystals. If you do decide to give it a shot with the Gameboy or other consumer hardware I think everyone would be interested to hear the results. Remember that if you change the crystal you also have to change the values of the loading capacitors.
  7. CWMan

    New Member

    Feb 2, 2011
    Every microprocessor has a maximum recommended frequency that comes from the manufacturer. In real world applications, some microprocessors are implemented at less than their maximum frequency (increases battery life, increases reliability, decreases heat) and a few at above their maximum frequency (better performance). There are a few important factors that limit the maximum frequency a micro will run stably at:

    The most obvious factor is heat. Once a microprocessor is heated to above its maximum specified operating temperature, anything goes. There are a number of failures high heat can cause, and some are fatal to microprocessors.

    The speed at which the microprocessor's transistors can switch at also plays a role. It takes time for a transistor to change logic states. If a transistor is not given enough time to transition, it will cause huge stability problems.

    Next is communications. To understand why communication busses are likely to fail at overclocked frequencies, you must first know that transistors take more time to transition between logic states as the voltage they are switching increases. Many older/legacy communication busses communicate with higher voltages than those that are used internally in microprocessors, so the switching speed becomes more of a problem. Unstable communications can cause a number of problems - for example, the inability to read external memory chips or control a display.

    Any of these three issues (and a couple more) can cause a device to fail when it is overclocked too aggressively.

    If you want to experiment with overclocking hands-on, I would suggest this Intro to Circuit Bending guide by Joseph Thibodeau.
  8. cjdelphi

    Thread Starter New Member

    Mar 26, 2009
    CWMan, thanks, finally someone who actually understands what I was saying :p

    Now you hit the nail on the head, if the CPU when overclocked can have stability issues, so it's down to the manufacture who specifies the recommended clock speed, so take a RISC cpu of 7mhz, if was to change it to 14mhz, you're not saying it will crash it's just not recommended and it COULD crash...

    I was getting it in my head, you could clock a 10mhz calculator to a 100mhz calculator just by changing the frequency of the crystal.

    If the device is digitally controlled you should be able to use firmware to overclock it like you'd do in your computer bios, clock speed * bus speed , eg 100mhz computer, to get the ratio, Clock Speed * bus = 100mhz in this instance most people would choose 100mhz for the bus (if acceptable) and then clock multiplier of 1 clock multiplier of 2 (2*100) would give you a 200mhz computer.

    Older computers prior to when AMD and Intel's CPU's could be installed in 1 motherboard, then you'd have a jumper for Voltage (AMD or Intel) then a few jumpers to indicate Intel or AMD or other, then we have the Jumpers for BUS speed and then the clock Multiplier, set all the jumpers up and boot up, if you don't you get nothing or worse, fry the CPU.

    But to CWMan

    Even if you kept an old 11mhz CPU the same temperature even if you ran it at 55mhz even with good heat removal, you think it would become unstable purely because the transistors can't switch fast enough or unable to perform operation properly.

    That answers my question..

    Now to answer the other questions about how to OC an old RISC chip, have a look at a couple of these videos from youtube

    http://www.youtube.com/watch?v=fDU2ljszxeU [modding a gameboy]
    http://www.youtube.com/watch?v=lbFxseisiSs&feature=related [overclocking an N64]
    http://www.youtube.com/watch?v=8SmGHAbZsnc [OC a Sega Genesis, to a 10mhz crystal, indepth details on how to!]

    It seems a lot of Nintendos hardware, Sega, probably Game Gear it seems a lot of the older RISC cpu's use a Crystal Oscillator.

    But my question was, with the right cooling can i do extreme speeds? The answer seems no, but you can OC just fine as long as it's cool enough :)
  9. cjdelphi

    Thread Starter New Member

    Mar 26, 2009

    Overclocking the SNES with a 23.292Mhz Crystal giving it a core clock speed of 3.882Mhz, comparing it with the original. Thumbs up and nice screen = original. Thumbs down and graphic glitches = overclocked.

    That answered my question with visuals :) lol

    one more :)

    Comparison between standard Nintendo 64 (62Mhz) and overclocked Nintendo 64 (125Mhz). The game testes is Goldeneye... the difference is awesome...

    http://www.youtube.com/watch?v=KXesXdk6Atg&feature=related //watch the diff
  10. tom66

    Senior Member

    May 9, 2009
  11. marshallf3

    Well-Known Member

    Jul 26, 2010
    As with most anything if you're going up in frequency there are precautions that have to take place. I see some good files listed above but getting rid of the extra heat and having the correct power supply voltage are imperative in addition to a few other things.

    Given proper cooling it's amazing what you can do with some of these chips, they pushed an older AMD CPU up to nearly 8 GHz with liquid nitrogen cooling. I tend to leave mine alone although you can unlock the extra cores on many of the AMDs given the correct motherboard. How much speed do you really need?

    Just some basic info regarding PC CPUs: (and most of this is old)

    That cheap little TI 430 kit can open up a game console like you wouldn't believe.
    Search around for that as well, I'm no gamer so I don't know how they're doing it.