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6674Re: [cosmacelf] Re: Free Running .pdf

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  • Lee Hart
    Apr 5, 2010
      RickC wrote:
      > At this stage I just want to see if the CPU works!<grin>

      I suggest getting one of those white plastic solderless breadboarding
      sockets. Plug the 1802 into it. Wire the lines shown on that "1802 free
      running" high through pullup resistors to +V (resistor value doesn't
      matter).

      Power supply: Anything from 3v to 6v is fine. Two AA batteries is
      simple. Put a flashlight bulb in series as a current limiter. If you
      short something or connect power backwards by mistake, it will light and
      prevent any damage.

      Oscillator: The simplest scheme is to connect a 1 megohm or greater
      value resistor between 1802 pins 1 and 39. Then stick your finger or a
      long piece of wire on pin 1 to pick up random 60 Hz noise. This is
      enough to clock the 1802. :-)

      Move the resistor on the DMA-OUT pin so it goes to ground instead of +v.
      This tells the 1802 to start doing DMA-OUT (memory read) cycles. An LED
      with a resistor in series will serve as a "logic probe". You can probe
      the address pins, and will see them toggling high/low like an 8-bit counter.

      Wire up your 6810 RAM. Now when you do these DMA-OUT cycles, they will
      be reading random data from the RAM, which you can see on the data bus.

      You can build up your Elf like this, one chip at a time, wiring and
      testing each one as you go.
      --
      Lee A. Hart | Ring the bells that still can ring
      814 8th Ave N | Forget the perfect offering
      Sartell MN 56377 | There is a crack in everything
      leeahart earthlink.net | That's how the light gets in -- Leonard Cohen
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