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Thread: 486-66 timum settingCLKIN

  1. #1
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    Default 486-66 timum settingCLKIN

    I've recently resurrected a 486 VLB motherboard and have a question or two about the AT Bus Clock. The CPU is an Am486 DX2-66. Question: What is the optimum setting for the AT Bus Clock (and why)?

    Options:

    1) 7.189 MHz
    2) 1/10 CLKIN
    3) 1/8 CLKIN
    4) 1/6 CLKIN
    5) 1/5 CLKIN
    6) 1/4 CLKIN
    7) 1/2 CLKIN

    I know this has been kicked around here on the forum but I'm just not straight on it?

    Thanks in advance.

    Tom
    Surely not everyone was Kung-fu fighting

  2. #2

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    The "AT Bus Clock" is probably the ISA clock frequency.

    A 486DX2/66 uses a system clock of 33 MHz, so "1/4 CLKIN" leads to 8.25 MHz ISA clock, which is the fastest option within reason: Your ISA cards should be able to take this without issue. Otherwise, you can use the first option, which is independent of the system clock and therefore probably asynchronous to it (i.e. there will be some clock synchronization overhead).

    I'd go with "1/4 CLKIN" and if it works, fine. Some stone-age peripherals might want something slower; the original 4.77 MHz PC clock is between "1/8 CLKIN" and "1/6 CLKIN".

  3. #3
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    Quote Originally Posted by Svenska View Post
    The "AT Bus Clock" is probably the ISA clock frequency.

    A 486DX2/66 uses a system clock of 33 MHz, so "1/4 CLKIN" leads to 8.25 MHz ISA clock, which is the fastest option within reason: Your ISA cards should be able to take this without issue. Otherwise, you can use the first option, which is independent of the system clock and therefore probably asynchronous to it (i.e. there will be some clock synchronization overhead).

    I'd go with "1/4 CLKIN" and if it works, fine. Some stone-age peripherals might want something slower; the original 4.77 MHz PC clock is between "1/8 CLKIN" and "1/6 CLKIN".
    Thanks for the reply - I'll try to get some benchmarks done using "1/4". The default for this MB is 7.189.
    Surely not everyone was Kung-fu fighting

  4. #4

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    The EISA specification standardized the ISA bus clock to 8.33Mhz, so there shouldn't be any issues with modern cards.

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