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RAM advice needed for project.

Discussion in 'Electronic Basics' started by GTR, Oct 1, 2005.

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  1. GTR

    GTR Guest


    I'm a newbie whose been playing around with miscellaneous circuits using TTL
    chips. Now, as a learning experience, I want to attempt to build a small
    computer around the 6809 processor. I am curious if anyone can recommend a
    RAM chip for this project with the following characteristics:

    a) Just needs 5 V supply.
    b) Can interface with TTL chips and has similar leg spacing(so I can
    breadboard it)
    c) Common enough I have a good chance at finding it at a reasonably stocked
    electronics store.

    I had considered using the 4164, are there other better alternatives?
    Perhaps a chip that meets the above criteria and has 16 bit addressing in a
    single pass, I believe the 4164 needs the low 8 bits of the address,
    latched, then the high 8 bits of the 16 bit address.

  2. If it's just the 6809, then there's little sense in using dynamic
    ram at this point. The 6809 can only address 64K, and that can
    be done in one or a few 8bit wide static rams. Even 20 years ago,
    such high density static rams were still uncommon and expensive, so
    dynamic ram was used despite the extra circuitry needed for the interfacing.
    But you should be able to get 32K by 8bit or even 64K by 8bit static
    ram, filling the 64K address space with one or two ICs.

    If you were adding a memory management unit or even some sort of latch
    for bank switching, allowing for more than 64K of ram, then one has
    to re-examine static versus dynamic at some point.

    When this sort of thing has come up, people have pointed out that
    cache ram off not quite recent computer motherboards are 32K by 8bit
    static, and unlike the old standard that had the pinouts of 24pin ROM,
    the cache ram is a narrower package.

  3. GTR

    GTR Guest

    Thanks for your response Michael. I'll download some data sheets from Jameco
    and see what the've got in low memory SRAM.

  4. Jamie

    Jamie Guest

    That's kind of old technology isn't it?
  5. Chris,

    Use SRAM. Conrad offers 6264 (8kx8 100ns) for about €4, the 62256 (32kx8
    100ns) for about €5 and the 681000 (128kx8 100ns) for about €10. The latter
    fills the addressspace of the old 6809 twice. The same amount of DRAM will
    hardly be cheaper (maybe even more expensive) and much harder to implement.

    petrus bitbyter
  6. Not sure how much RAM space you'd need, but I think you'll like the 6116.
    It's cheap and plentiful and easy to work with.
  7. GTR

    GTR Guest

    Actually need very little, I'll probably split up the 6809's address space
    in to 32KB (15 bits) for a bootable image in EPROM, and 32 KB for the RAM
    and maybe use the 16th bit as a chip select, something like that. Eventually
    I'd like to add serial support and write a small assembly based shell for
    it. Thats probably as far as I'll go with it. 32KB should be plenty for
    this, and likely much less.
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