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"Overclocking" my IBM AT 5170-319

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    "Overclocking" my IBM AT 5170-319

    Okay AT experts, here's one for you guys:

    So you may or may not remember that several months ago I was talking about the possibility of boosting my AT from 8Mhz to 20MHz. We'll, I have gone ahead and tried just that. But, I have encountered an interesting problem. I will first explain what exactly it is I have done to my AT before I go into specifics.

    -5170-319/339 planar (sometimes described as Type II or Type III). This is the one that looks like an XT-286 motherboard without the SIMM slots.

    -512kb 100ns 256kbit DRAMs. The motherboard comes with 150ns DRAMs onboard. Running faster than 10MHz requires faster chips. My 100ns should be good up to about 16MHz. Remember that the AT board has 1 waitstate.

    -Award 286 Modular BIOS v3.03Z. As everyone and their dog already knows, IBM did something sneaky with their own BIOS to prevent overclocking. I replaced it in order to install faster crystals.

    -Make it 486 CPU upgrade. This is a 486SLC 33MHz chip with extra circuitry that tricks it into being "clock doubled". So for example, if the motherboard runs at 10MHz the CPU will operate at 20MHz internally.

    The Problem:

    I purchased three addtional crystals from mouser electronics. They are 20MHz (10MHz AT), 24MHz (12MHz AT) and 32MHz (16MHz AT) parts. I started with the 20MHz crystal and worked my way up. Both 10MHz and 12MHz tests were successful. I threw everything I could think of at the system, and was unable to crash it. Naturally at this point I wanted to try 16MHz. However, after installing the 32MHz crystal I discovered that my system was running very slowly. Benchmarking determined that the system was operating at just under half the speed of a 6MHz AT. I thought this was very strange indeed. One would think that running the system too fast would simply prevent it from booting or generate some strange errors. But it still boots and functions perfectly....just very slowly. So, it leaves me wondering if maybe I got a bad crystal or if there is some extra circuitry on the motherboard that prevents me from running faster than I should. I guess the only thing I can do at this point is order more crystals. I suppose going from 12MHz to 16MHz wasn't a good idea. I'll order some more crystals in between and see what happens.

    Does anyone know what's going on?
    Last edited by Anonymous Coward; January 12, 2007, 02:31 PM.
    "Will the Highways on the internets become more few?"

    V'Ger XT

    After doing a little bit of searching on the internets for people with Make it 486s, I discovered a person who was having trouble getting his 486SLC to run at 33MHz. Apparently adding a heatsink solved the problem. I am thinking that perhaps there is some circuitry in the 486SLC that prevents the chip from overheating by stepping down the frequency.

    Does anyone know about these ti486SLC chips? Mine seems to be designed for laptops and has a bunch of power saving features.

    I think the best way to determine if the CPU is causing the problem is to remove the make it 486, and try a standard 286-16.
    "Will the Highways on the internets become more few?"

    V'Ger XT


      I did some pretty crazy things today. First I tried swapping out the 486SLC CPU and installed a 20MHz 286. This did not help my problem at all, so obviously it is not CPU related. However, it is interesting to know that a 286 is indeed slightly faster than a 486SLC at the same clockspeed...I benched at 12MHz in both configurations.

      The next thing I did was replace the BIOS chips. I thought maybe the manufacturer of the 12MHz 286 I swiped my AWARD chips from might have implemented their own overclocking prevention. I only had two extra sets available. One turned out to be an earlier verision of the AWARD chips I was already running, and the other set was from DTK and they didn't work at all. I ran out of 286 boards to salvage so I thought "what the hell, I'll just try an AMI 386SX's close enough anyway". Installing the 386SX chips did not work with the 286-20 chip installed, but after replacing it with the 486SLC it worked beautifully at 12MHz. Though, operating at 16MHz gave me the same results as the previous BIOS chips.

      While I didn't solve my problem, I did learn something interesting about transplanting 386SX BIOS chips. I guess it doesn't do anything terribly useful, but it includes a few interesting diagnostic programs in ROM. I think there's a low level format program in there too. It gives me the standard AMI splashscreen found on 386/486 boards, and even reports my CPU accurately as an 80486. Finally it counts up my memory quite quickly.

      If anyone is interested in doing this, I am using two (HI and LO) 1989/1990 AMI chips from an early model SX motherboard with a VIA chipset. I did the usual tests and wasn't able to get the computer to generate strange errors.

      The behaviour of the system at 16MHz is still a mystery, and I intend to order some extra crystals to figure out exactly what's up.
      "Will the Highways on the internets become more few?"

      V'Ger XT