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Switching Voltage — Parallax Forums

Switching Voltage

james.tjames.t Posts: 8
edited 2013-09-02 20:31 in General Discussion
I want to display multiple seven segment ic's. I figured i could do this by setting the first set of seg7 pins to the Propeller pins, then from the first seg7 to the second seg 7, then turn the voltage on and off to which ever seg7 i want to display.

My question is, can i do this programmatically, (using C propeller board) or do i need to use a NAND gate IC to switch the voltage to the desired seg7 cathode? Any assistance would be appreciated.

Thanks in advance..

Comments

  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2013-09-02 03:30
    You want to know if you can multipex a series of 7-segement displays all in Propeller software or if it would be best to off-load the task to external chips.

    It can be done both ways. The Propeller has 8 cogs, so one cog can handle the multiplexing of the display while other cogs do other things.

    Most of the time this is off-loaded because it ties up the one and only CPU unless one gets into time slicing and threading tasks... a bit more complicated level of programing. But the Propeller offers you the advantage of not having to...

    So as long as you have enough i/o in 28 pins for your project, do it all in software. If you run out of i/o, there are chips that just handle 7-segment display vial synchronous serial.

    There are other solutions to saving pins that have the 7-segment output as parallel data and select the active unit in sequence. Look at the 74LS47 chip or the 74LS48 chip

    You only save a few pins unless you have a heck of a lot of digits to display. For 2 to 4 digits, this approach may not be worthwhile -- you just save 1 to 3 i/o pins.

    More info in this link, and the end of the page .. http://www.tristantech.net/articles/led_display_multiplexing/3.php
  • james.tjames.t Posts: 8
    edited 2013-09-02 12:38
    Thanks, the solution using transistors to do the switching is more what i was looking for..

    Thanks again.
  • kwinnkwinn Posts: 8,697
    edited 2013-09-02 20:31
    The prop can drive several 7 segments displays directly, but at some point drivers will be needed or low brightness becomes an issue. How many digits do you want to display?
  • james.tjames.t Posts: 8
    I went and purchase a seven segment display driver chip...thanks for the advice...
  • kwinnkwinn Posts: 8,697
    @james.t

    From September 2013 to March 4 2017. Now that's what I call cautious and thorough deliberation ;-)
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