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Rs-232

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    Rs-232

    For perverse reasons, I'm bit-banging serial async on a HP 200LX.

    I've done this many times before with microcontrollers - Disable
    interrupts, send your start bit, wait for the next bit frame, send
    b0, wait for next bitframe, send b1, and so on.

    Pretty simple.

    But it seems that globally disabling interrupts, "cli" in x86 asm,
    still leaves some periodic task happening in the background on
    the HP - I can see the results on my logic analyzer.

    I whittled the routine down to simply toggling the output line
    every few uS - At odd intervals I'll get a pulse that's 5 - 10 uS
    longer than it should be...

    Suggestions appreciated...

    Jack

    #2
    DRAM refresh?
    Reach me: vcfblackhole _at_ protonmail dot com.

    Comment


      #3
      Good call, Chuck.

      I was under the impression that the LX did not use DRAM, but searching the (badly indexed)
      documentation for "DRAM" turns up at several possible exceptions; video RAM for one...

      Even if that's not the specific problem, looking turned up a pile of stuff I was'nt aware of...

      I was misled by a statement I saw somewhere that the Int normally used by PCs for refresh
      is used for something else in the LX. Which may very well be true, but ...

      Thanks again.

      Jack

      Comment


        #4
        Jack, I wasn't aware that DRAM refresh in PCs used any interrupts--just a timer running a DMA channel in auto-initialize mode. Still, even without interrupts, that can cause blips in things that you've calculated down to the cycle on. Is NMI connected to anything on the 200LX?
        Reach me: vcfblackhole _at_ protonmail dot com.

        Comment


          #5
          In the LX it's a low-power scram, according to the docs.

          I'm reasonably sure low-power is'nt the problem.

          But PCs are a relatively new thing for me - I'm a

          microcontroller dweeb.

          Jack

          Comment


            #6
            If there's only DRAM for the video, then it should refresh itself as it's sequentially read.
            "Don't it always seem to go
            That you don't know what you’ve got ‘til it’s gone" (BANG )

            Comment


              #7
              Thanks, Nige. Something else I did'nt know.

              So what happens to video DRAM when I shut the display off?

              That is a low-power option...

              Never mind - Dumb question.

              Jack
              Last edited by JGardner; December 15, 2012, 08:46 AM. Reason: dumb question

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