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Could you use DMA_Out to read memory?

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  • mikemclaren6502
    Greetings guys. Just wondering if you could use the DMAOUT pin to read memory, as opposed to using DMAIN along with the write protect function Mr.
    Message 1 of 4 , Mar 22, 2014
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      Greetings guys.  Just wondering if you could use the DMAOUT pin to read memory, as opposed to using DMAIN along with the "write protect" function Mr. Weisbecker implemented in the original Elf article in PE?  Is it because it might have interfered with the upcoming '1861 upgrade in some way?

      Cheerful regards, Mike
    • bill rowe
      so Lee, if you didn t care about reading back what you d written could you somehow use dma-in to simulate a LOAD mode on one of the chips that doesn t support
      Message 2 of 4 , Mar 22, 2014
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        so Lee, if you didn't care about reading back what you'd written could you somehow use dma-in to simulate a LOAD mode on one of the chips that doesn't support it?


        To: cosmacelf@yahoogroups.com
        From: leeahart@...
        Date: Sat, 22 Mar 2014 14:24:50 -0600
        Subject: Re: [cosmacelf] Could you use DMA_Out to read memory?

         
        mikemclaren6502@... wrote:
        > Greetings guys. Just wondering if you could use the DMAOUT pin to read
        > memory, as opposed to using DMAIN

        Certainly! DMA-OUT works exactly like DMA-IN, except that it read a byte
        of memory at address R0, so it can be latched by something on the data
        bus. It then increments R0, ready for the next DMA-IN.

        LOAD mode has a special trick so that all you need is DMA-IN to both
        read *and* write* memory. When DMA-OUT goes low, it writes the contents
        of the data bus into memory at R0, just as you'd expect. But it then
        *reads* memory, so you can latch the byte on the data bus to see if it
        really went into memory or not.
        --
        In life’s great adventures, keep in mind thy cause.
        Be neither moved by critics, nor multitude’s applause.
        -- Nasreddin
        --
        Lee Hart's EV projects are at http://www.sunrise-ev.com/LeesEVs.htm

      • Lee Hart
        ... Certainly! DMA-OUT works exactly like DMA-IN, except that it read a byte of memory at address R0, so it can be latched by something on the data bus. It
        Message 3 of 4 , Mar 22, 2014
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          mikemclaren6502@... wrote:
          > Greetings guys. Just wondering if you could use the DMAOUT pin to read
          > memory, as opposed to using DMAIN

          Certainly! DMA-OUT works exactly like DMA-IN, except that it read a byte
          of memory at address R0, so it can be latched by something on the data
          bus. It then increments R0, ready for the next DMA-IN.

          LOAD mode has a special trick so that all you need is DMA-IN to both
          read *and* write* memory. When DMA-OUT goes low, it writes the contents
          of the data bus into memory at R0, just as you'd expect. But it then
          *reads* memory, so you can latch the byte on the data bus to see if it
          really went into memory or not.
          --
          In life’s great adventures, keep in mind thy cause.
          Be neither moved by critics, nor multitude’s applause.
          -- Nasreddin
          --
          Lee Hart's EV projects are at http://www.sunrise-ev.com/LeesEVs.htm
        • mikemclaren6502
          Ok, now I see why Mr. Weisbecker did it that way. You need the write signal from the DMA_IN operation to latch data onto the output LEDs. I missed that
          Message 4 of 4 , Mar 22, 2014
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            Ok, now I see why Mr. Weisbecker did it that way.  You need the "write" signal from the DMA_IN operation to latch data onto the output LEDs.  I missed that (doh!).

            Thanks, guys...

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