Playing with a simple 8-bit output port

Home Forums Z80 Playground Early-Adopters Playing with a simple 8-bit output port

Viewing 4 posts - 1 through 4 (of 4 total)
  • Author
    Posts
  • #1046
    teg
    Participant

    Just for fun (re-living teenage days messing with ZX80s/81s and Spectrums), I’ve been playing with a simple 8-bit output-only port for the Z80Playground. I wanted to find a simple glue-logic free configuration, and came up with a two TTL chip approach using a 74HC138 3-to-8 decoder and 74HC374 8-bit latch. The 74HC138 usefully provides three-input enable logic. This needs one high signal and two low signals – just perfect for an address line, nIORQ and nWR. Also, I found that the Z80Playground fits nicely into a large breadboard (if the two legs by the expansion connector are removed).

    Details are on this schematic, which I’ve drawn for neatness only, I don’t have a PCB design.
    David

    image1
    image2

    • This topic was modified 5 months, 3 weeks ago by teg. Reason: Added note about loading images
    • This topic was modified 5 months, 3 weeks ago by teg. Reason: Still trying to fix images
    #1049
    Phil_G
    Moderator

    All good stuff David, you can run Richard Russell’s Music Compiler with just one output port 🙂
    A port opens all sorts of opportunities:-)
    Did you change the cpu crystal? to what speed? I much prefer 4mhz as the old loop-timed stuff runs perfectly, I think music has a max of 6mhz iirc.
    Cheers
    Phil

    #1051
    teg
    Participant

    Hi Phil,

    I wanted a nostalgia feeling with my Z80Playground so I have fitted a 4MHz XTAL and a 80’s vintage Toshiba TMPZ84C00, and it boots from an old EPROM (see my earlier posting on that topic).

    On a related note, I think the memory timing margins at 10MHz are out-of-spec.
    An instruction fetch allows 1.5 T-cycles’ access time, so at 10MHz that is 150ns. Since the control line decoding logic cuts another ~20ns off that, it’s actually ~130ns at the memory devices. The modern SRAM is OK at this speed, but the EEPROM is only rated at 150ns. Whilst technically out of spec, looking on a scope reveals that it actually presents the data with plenty of margin, so in practice it’s fine at 10MHz, but I wouldn’t push it further. Maybe this should be a topic in its own right?

    CHeers
    David

    #1052
    Phil_G
    Moderator

    Mines switchable 4 or 10mhz, but its almost always on 4mhz as a lot of my old CP/M stuff assumes a 4mhz Z80 when loop-timings are used (eg the DAC stuff)
    A couple or 3 years ago I started using EEPROMs exclusively, I find them so much more convenient than EPROMs for development. Even my old Kitbug SC/MP board has a 2k byte 28C16 🙂 And for my 6800 project I bought a 48Z35 battery-backed 32k x8 sram, half of which is write-protected with the monitor & microbasic and half read-write for ram, one chip does both jobs.
    The problem is with the current silicon shortage, electronics is becoming an expensive hobby!
    Cheers
    Phil

Viewing 4 posts - 1 through 4 (of 4 total)
  • You must be logged in to reply to this topic.