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Thread: Acer Labs M5818 A1 24-pin chip datasheet. RTC on a 486

  1. #1

    Default Acer Labs M5818 A1 24-pin chip datasheet. RTC on a 486

    I've got this 486 board with this chip on it, that has an oscillator heading to it via another chip. AFAIK it is a RTC, but I can't find a datasheet for it anywhere. Pin 24 isn't connected to 5V, and pins 1 and 4 are tied together on the PCB. on the Motorola part that would be NC and AD0, which doesn't make a lot of sense.

    I'm not getting a CLK signal on the ISA bus. So I'm trying to follow traces around, but this thing is confusing me. Additionally half the traces disappear under the RAM slots making it awkward. All I really need is the pinout, is it MC146818 compatible?

    Chip is labeled:
    M5818 A1
    9240 TS2
    A28595

    Underside:
    TAIWAN
    T1067
    ACER LABS
    Looking for: OMTI SMS Scientific Micro Systems 8610 or 8627 ESDI ISA drive controller, May also be branded Core HC, Please PM me if you want to part with one.

  2. #2

    Default

    Ok, well I've been getting the RTC oscillator confused with the CPU/BUS oscillator, so this is all irrelevant anyway. Turns out that pin 24 (VDD) is connected to a bunch of resistors and whatnot and connected to the 'Clear CMOS' jumper. On this board, if the jumper is in the normal position, it give the chip ~4.5V, in the clear, ~-1V. I had a skim through the MC146818 datasheet and couldn't see anything related to this, but it isn't text searchable so it may explain all this.

    Anyway, my problem is elsewhere, the chipset has a clock going in, but the bus clock isn't coming out
    Looking for: OMTI SMS Scientific Micro Systems 8610 or 8627 ESDI ISA drive controller, May also be branded Core HC, Please PM me if you want to part with one.

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