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Multiplexing displays without a microcontroller — Parallax Forums

Multiplexing displays without a microcontroller

John A. ZoidbergJohn A. Zoidberg Posts: 514
edited 2011-06-16 11:17 in General Discussion
Hello there,

Basically, most of the time, it's easier to program a microcontroller to multiplex a 7-segment or a LED matrix display. One at the time.

But one thing - there could be taking up too much microcontroller time and stall the system if the microcontroller isn't fast enough. There are chips to multiplex the 7-segment and LED displays easily, but expensive.

So, I think of external hardware multiplexing - by using the counter, and then the 7-segment decoder (for 7-segment) and static RAM to store data temporarily. The microcontroller/system only sends data into the RAM and let the entire row of chips doing the multiplexing. The space taken by these chips are a lot, but I'm a bit curious about doing that.

Is it possible? :D

Comments

  • kwinnkwinn Posts: 8,697
    edited 2011-06-14 18:46
    This was quite common on the early instruments I worked on. Typically the data to be displayed was from a counter or register chip rather than a memory IC. There were some 3 digit counter chips with built in hardware for driving a multiplexed display. This can still be done fairly easily with the logic chips available now, and there are single chips to do it as well.
  • RDL2004RDL2004 Posts: 2,554
    edited 2011-06-14 19:02
    That's quite a coincidence, I just bread-boarded one of those up a couple of days ago. I think I'm going to make a permanent version once I get it working good. It uses the 4553 counter. Started from a circuit by Forrest Mims.
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  • kwinnkwinn Posts: 8,697
    edited 2011-06-14 19:32
    RDL2004 wrote: »
    That's quite a coincidence, I just bread-boarded one of those up a couple of days ago. I think I'm going to make a permanent version once I get it working good. It uses the 4553 counter. Started from a circuit by Forrest Mims.

    Thanks for posting that. I clearly remembered how the chip functioned but for the life of me could not recall the part number. I even laid out a small pc board to hold the CD4553 with the required decoder, transistors, and 3 7 segment leds way back when.
  • kwinnkwinn Posts: 8,697
    edited 2011-06-14 23:08
    With RDL2004 jogging my memory like that here are a few of the chips I used to drive multiplexed displays (in addition to the CD4553).

    Function:

    1 - Select values to display.

    2 - Store values to display.

    Details:

    '240/'244 – Fuction 1. Select 1 of 2 four bit values.

    '259 – Function 1. Eight bit addressable latch. Select one of eight values.

    '273/373/374 – Function 1 Store an 8 bit value for display. Function 2. Select one of 8 values for display.
  • RiJoRiRiJoRi Posts: 157
    edited 2011-06-16 08:00
    There was -- is? -- a 7-seg display with a built in latch. I think it was made by HP.
    Another solution might be a co-processor -- you send the bytes-to-display to the Co-P, and it converts them to 7-seg code, and does all the transmit-and-refresh stuff.

    --Rich
  • John A. ZoidbergJohn A. Zoidberg Posts: 514
    edited 2011-06-16 08:45
    RiJoRi wrote: »
    There was -- is? -- a 7-seg display with a built in latch. I think it was made by HP.
    Another solution might be a co-processor -- you send the bytes-to-display to the Co-P, and it converts them to 7-seg code, and does all the transmit-and-refresh stuff.

    --Rich

    About the co-processor, yeah - Sparkfun has made those as well. It's using a small Atmega168 there.

    I found some few websites which uses a CPLD instead, but I can't locate it anymore.
  • markaericmarkaeric Posts: 282
    edited 2011-06-16 09:19
    If speed is an issue, can't you just call your 7-segment methods/functions multiple times throughout your main loop cycle?
  • kwinnkwinn Posts: 8,697
    edited 2011-06-16 09:36
    markaeric wrote: »
    If speed is an issue, can't you just call your 7-segment methods/functions multiple times throughout your main loop cycle?

    That's the way it was usually done without interrupts. If it could be done with a 2MHz 8080 it should be trivial with the chips available now.
  • vaclav_salvaclav_sal Posts: 451
    edited 2011-06-16 11:17
    TIL311 - does include latch and display. However, check the price!
    Vaclav
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