Connect with us

5V Logic Hi question

Discussion in 'Electronic Basics' started by Jay, Apr 3, 2007.

Scroll to continue with content
  1. Jay

    Jay Guest

    How close to 5V should a logic HI be? Is there any plus or minus
    deviation from 5 volts allowed?
     
  2. Depends upon the parts family. CMOS want .7 * Vcc (3.5V), Schmidt inputs
    often want to see .8 * Vcc (4V). TTL will let you get away with as little
    as 2V. Your best bet is to consult the datasheet(s). This seems pretty
    close:

    http://www.interfacebus.com/voltage_threshold.html
     
  3. But generally you have an easy "hi" in the voltage supply to
    the IC.

    If you need something set "hi" permanently, you tie it to the supply
    voltage.

    With TTL, it's suggested that a current limiting resistor be used,
    but a lot got away without them.

    And if this is connected to some external point, you'd want to use
    a resistor so when that point gets switched to ground it doesn't
    short out the supply voltage.

    But the resistors are there not to fiddle with voltage, but to
    limit current at the input of the device.

    You need to be specific if you have something else in mind. Because
    if you aren't using the supply voltage as your "hi", then likely you
    are talking about some external source, and in those cases you likely
    don't want to just apply that voltage directly into logic IC inputs.

    Michael
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-