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Cyrix Cx5x86 Underclocking, Downclocking

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    Cyrix Cx5x86 Underclocking, Downclocking

    Cyrix Cx5x86 Underclocking, Downclocking

    Correction " The Micro is not a Cyrix cX5X86, it is an Am5x86 "

    will like your opinion on this matter:

    I have a PC Compatible Cyrix Cx5x86 running at 133MHz

    I will like to Underclock the 5x86 and run the system at 16 Mhz
    (in order to make it compatible with old software)

    the mainboard has a 50Mhz integrated oscillator

    the jumpers are set to: 50 Mhz x (2/3) x 4 = 133.33 Mhz
    changing the jumper setting I've been able to run at 75Mhz, everything ok


    I will like: 6 Mhz x (2/3) x 4 = 16Mhz

    one solution is to replace the integrated oscillator, but I will like the
    Pc to run at 133 Mhz and only when needed run at 16Mhz, (using the turbo switch)

    I found this chip on Digikey LTC6903 ($3.90)
    (1kHz – 68MHz Serial Port Programmable Oscillator)

    this is from the datasheet
    http://cds.linear.com/docs/Datasheet/69034fc.pdf

    Underclocking.jpg


    I plan to:
    - Remove the 50 Mhz oscillator
    - solder a socket (machined pins type) and try a 6Mhz oscillator (just to check)
    - build a small PCB with a LTC6903 + pic, program the pic
    - connect the turbo switch to the board
    - plug the board, I intent to run MSDOS on that machine

    Do you see any problems ?
    timing issues with bridges, memory access ?
    Last edited by lxx5101; August 14, 2011, 08:45 AM. Reason: my mistake

    #2
    Wait, so you want to take the fastest pre-Pentium CPU and underclock it to that of a 286?
    [Need something to waste time on? Click here to visit my YouTube channel CelGenStudios]
    --------------------------------------------------------------------------------------------
    [No time for videos? Click here to visit my Twitter feed @CelGenStudios]

    = Excellent space heater

    Comment


      #3
      Its going to be a LOT easier to just disable the onchip cache, I've heard of people using a pentium-133 and with just disabling the cache, it would run as fast as a 486.

      But honestly if you have to have EXACTLY 16mhz, i think you are better of getting a 286 up and running.
      It is a mistake to think you can solve any major problems just with potatoes.

      Comment


        #4
        The datasheet doesn't say you can't run the clock down to something stupid slow, (OTOH, the Am5x86, no relation, does). I'm not optimistic, but you can give it a try without making too much magic smoke.
        Reach me: vcfblackhole _at_ protonmail dot com.

        Comment


          #5
          How is the (2/3) multiplier is working its way into the equation? Is it through the BIOS or a jumper? Either way, the 2/3 multiplier option usually drops the PCI bus rate down, not the front-side bus. In which case, your 4X Cyrix is seeing 50 MHz x 4 = 200 MHz. Did 486 motherboards have an option to drop the whole FSB by some multiplier as well? If so, I've never seen it. With the Cyrix register bits, by way of DIR0, you can control the Core/bus ratio to 1x, 2x, 3x, or 4x. So it would seem to be, you can run this CPU at 25 MHz. If you wanted to run it closer to 16 MHz, you can probably see if your PLL has an unused ratio division output and rewire. You can also put in a different PLL circuit, or make your own. You can also try putting in a slower crystal oscillator. If your crystal is at 14.3 KHz, you'd want to find one that is 9.15 KHz. This way you can just use your existing PLL chip.

          Comment


            #6
            What exactly are you trying to run? Your best bet is to disable the cache. PM me and I can give you a "Cache ON/OFF" utility. Also, you can pick-up a 286 mobo for next to nothing on eBay.
            Surely not everyone was Kung-Fu fighting

            Comment


              #7
              I'll trade you a 286/386sx 12 or 16mhz motherboard, ram and cpu for the 586 and motherboard if your interested...
              '. \ / .'
              '. .'``'. .'
              ......:::::::`.....`::
              Currently seeking a Compaq Deskpro 386

              Comment


                #8
                To run a Cyrix 5x86 CPU at 133 MHz and at 16 MHz with the push of a button will take some custom design work, as noted above.

                If you let me know more about your application, maybe we can find another way to satisfy your requirements. I am open to trading your Cyrix CPU with other, pehaps more suitable, CPUs. I have all sorts of 386/486 CPUs, some at 16 MHz. Please include an image of your Cyrix CPU.

                Comment


                  #9
                  Thanks to all of you for your quick response

                  feipoa

                  How is the (2/3) multiplier is working its way into the equation? Is it through the BIOS or a jumper? Either way, the 2/3 multiplier option usually drops the PCI bus rate down, not the front-side bus. In which case, your 4X Cyrix is seeing 50 MHz x 4 = 200 MHz. Did 486 motherboards have an option to drop the whole FSB by some multiplier as well? If so, I've never seen it. With the Cyrix register bits, by way of DIR0, you can control the Core/bus ratio to 1x, 2x, 3x, or 4x. So it would seem to be, you can run this CPU at 25 MHz. If you wanted to run it closer to 16 MHz, you can probably see if your PLL has an unused ratio division output and rewire. You can also put in a different PLL circuit, or make your own. You can also try putting in a slower crystal oscillator. If your crystal is at 14.3 KHz, you'd want to find one that is 9.15 KHz. This way you can just use your existing PLL chip.
                  after your comment I checked the board and turns out that I was totally wrong

                  The clock is not a 50 Mhz integrated oscillator (some another board has that kind of oscillator)

                  IT has a UM9515 + 14.318 Mhz crystal (sorry my mistake)
                  and the chip is not a cyrix It's a Am5x86 (again my mistake)

                  see the attached image
                  Um9515.jpg

                  Am5x86.jpg

                  The board has 3 jumpers for:
                  25 MHz
                  33 Mhz (that's where I got the 2/3, thinking in the 50 Mhz oscillator)
                  40 Mhz
                  50 Mhz

                  there is another jumper to select 4x or 3x (Amd and Cyrix share the same config,
                  the manual was checked as a cyrix chip )


                  I like your Idea to check for PLL unused ratio division, but haven't found the data sheet yet


                  I will like to have the normal 133 Mhz operation and a stupid slow mode if possible


                  ------------------------------------------------------
                  Chuck(G)
                  (OTOH, the Am5x86, no relation, does)
                  turns out this is the case (8 Mhz minimum) thanks for the tip

                  ------------------------------------------------------

                  Agent Orange
                  What exactly are you trying to run? Your best bet is to disable the cache. PM me and I can give you a "Cache ON/OFF" utility. Also, you can pick-up a 286 mobo for next to nothing on eBay.
                  if there is a software solution, perfect I will PM you, thanks

                  I intent to run

                  copywrite (max freq 33 Mhz, as far as I know)

                  copyIIpc (max freq 20 Mhz, as far as I know)

                  Disk explorer (max freq 33 Mhz, as far as I know)

                  all of them work fine at 16 Mhz in a 286 machine

                  --------------------------------------------------
                  luckybob
                  you have to have EXACTLY 16mhz
                  no, may be anything below 33Mhz will work

                  --------------------------------------------------
                  Why ?? ( for almost everybody), NeXT

                  I have a 286 @ 16 Mhz but I don't have HDD for that machine
                  my LPT Zip died last year, I also live overseas, hard to get almost anything here
                  (my company has a PoBox in the US, but this is personal use, the shipping is expensive for me, I only import small parts)

                  my FDDs and/or Floppy Disks are giving me hard time, I have to copy everything twice

                  I prefer the 5x86 It has LBA HDD support and a PCI USB card
                  that works fine with MSdos

                  ----------------------------------------------------------
                  twolazy

                  I'll trade you a 286/386sx 12 or 16mhz motherboard, ram and cpu for the 586 and motherboard if your interested..
                  thanks I already have a 286 board, thanks for your offer
                  Attached Files

                  Comment


                    #10
                    I think the moment you mentioned it was an AMD X5-133 instead of a Cyrix 5x86-133, you lost half your audience! To the best of my knowledge, I have the only currently in-use Cyrix 5x86-133. It was working fine with this forum up until about 1 month ago. There may have been some host-side software change that made it run too slow with NT4-IE6. Oh well, vogons still works fine.

                    I may have mixed up my KHz and MHz units for the oscillator. I'd have to hook it up to the scope to see. The concept is the same though. They often leave off the units on the package. I think you'll have a very difficult time finding the spec. sheet for the PLL circuit. Please let me know if you find it.

                    Comment


                      #11
                      feipoa
                      I think the moment you mentioned it was an AMD X5-133 instead of a Cyrix 5x86-133, you lost half your audience!
                      not half, 95%

                      I am really sorry guys for the Cyrix fiasco

                      I think you'll have a very difficult time finding the spec
                      if I get it I'll publish it

                      Comment


                        #12
                        Thanks Agent Orange for the files !!!

                        It worked !!!!!!!


                        check out the results:

                        Jumper setting: 75 Mhz (25Mhz x 3)
                        Amibios detects: Am486Dx4-SE
                        Speed tool: quaplus 4.2

                        Cache Enabled

                        Turbo on: 72.50 Mhz
                        Turbo off: 72.32 Mhz

                        Cache Disabled:

                        Turbo on: 71.80 Mhz
                        Turbo off: 28.02 Mhz <-- Disk explorer, copywrite, copy II pC Work OK


                        Jumper setting: 133 Mhz (33Mhz x 4)
                        Amibios detects: Am5x86
                        Speed tool: quaplus 4.2

                        Cache Enabled

                        Turbo on: 128.82 Mhz
                        Turbo off: 128.27 Mhz

                        Cache Disabled:

                        Turbo on: 126.64 Mhz
                        Turbo off: 37.38 Mhz <-- Disk explorer, copywrite, copy II pC Work OK


                        also thanks to luckybob (same idea)

                        Comment


                          #13
                          There's a DOS program called 'slowdown that can be used to slow the speed down purely with software. You can use it to get any speed you want.

                          It can be downloaded here: http://bretjohnson.us/

                          Comment


                            #14
                            The problem with DOS "slowdown" programs is that they do so with rather large granularity; somewhat like taking your Maserati and driving 120 MPH between dead stops to get a longer average travel time.

                            Some systems that still use DMA channel 0 for memory refresh can be slowed by bumping up the refresh rate.

                            Another method would be to design a little ISA board that added wait states to the CPU.

                            One could also enable single-step interrupts (x86 interrupt 1) and kill some time between instructions. A few programs are smart enough to disable this, but most aren't.
                            Reach me: vcfblackhole _at_ protonmail dot com.

                            Comment


                              #15
                              FishFinger
                              There's a DOS program called 'slowdown
                              I tried that before start this post, Didn't work, thanks

                              Chuck
                              Another method would be to design a little ISA board that added wait states to the CPU.
                              Chuck do you mean like holding the /READY signal high for some cycles, until the end of a T2 cycle, release and wait for some more cycles, and then start all over again ?

                              Comment

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