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CMOS logic level range

Discussion in 'Electronic Design' started by Robert Harris, Mar 8, 2007.

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  1. I am designing a circuit using all CMOS 4000 series IC's.

    It is to be powered by four AA cells, either alkaline or NiCad's.

    What is the minimum and maximum voltage that will work as logic level

    IOW how far off 5V can it go either way?

    Thank you,

    Rob Harris
  2. D from BC

    D from BC Guest

    Isn't this a datasheet thing?
    D from BC
  3. Dotinho

    Dotinho Guest

    level 1 is 4.5V to Vcc 0.5V, okay?
  4. Jim Thompson

    Jim Thompson Guest

    The "official" specification is usually...

    "0" <= 0.3*VDD

    "1" >= 0.7*VDD

    ...Jim Thompson
  5. Joerg

    Joerg Guest

    The not-so-official specification is usually...

    "0" <= 0.3*VDD

    "1" >= 0.7*VDD

    "linear" = 0.5*VDD

  6. Jim Thompson

    Jim Thompson Guest

    Sane people don't do "linear" with off-the-shelf CMOS ;-)

    ...Jim Thompson
  7. Joerg

    Joerg Guest

    Good. So if someone wants to sue me I could then claim immunity based on
    the grounds of insanity ;-)
  8. John Larkin

    John Larkin Guest

    That will certainly work, but if any logic level is very far off the
    rails (a volt maybe? less?) the chips will draw excess current and run
    down your battery.

  9. Rich Grise

    Rich Grise Guest

    So, I guess the HCU04 isn't "off the shelf"? ;-)

    and, BTW, I had thought that 0 <= 1/3 Vdd (.33) and 1 >= 2/3 Vdd. (.67)

  10. Joerg

    Joerg Guest

    That doesn't include taxes ;-)
  11. jasen

    jasen Guest

    typically cmos high is 2/3 of VCC and above, read a few datasheets.
    with a 15V supply 5V would be low :)

  12. Lionel

    Lionel Guest

    Depends on the logic family. Traditionally (eg the old CD4000 series
    chips), you can power them from anywhere from 3V to 15V, & a logic '1'
    is anything above 1/2 the supply voltage, '0' is anything below that.
    For newer CMOS logic families, they specified for all sorts of weird
    voltage ranges. You can find out for sure from the data sheet.

    But as a general rule of thumb for any CMOS logic chip powered by 5V,
    you can assume that any signal above 2.5V will be treated as a '1', &
    that anything below about 1V will be considered a '0'.

    Again, if you're doing anything critical, you need to look up the data
  13. I believe 2.5V is wayyyyy too low for a Vcc of 5V unless you're warming up
    the parts. ;-) I'd keep it at .7Vcc (.8 *Vcc for Schmitt trigger inputs)
    to be safe. IOW, 3.5V is the minimum.
  14. Lionel

    Lionel Guest

    You've obviously grown up with TTL compatible CMOS parts. ;) Go check
    the 4000 series spec's.
  15. Jim Thompson

    Jim Thompson Guest

    Lionel, YOU need to re-read the data sheets.

    ...Jim Thompson
  16. You must think they all are. ;-) I'm not trying to be rude, but maybe you
    should look again. Vih is usually at least 3.5V on CMOS, anything below
    that and you are gambling. 2.5V is waaaayyyyyyy toooooooo looooooowwww for
    CMOS inputs. ;-) PIC chips want 4V minimun on 5V Vcc (.8 * Vcc).
  17. I will add that for the PICs, the pins that have TTL input buffers (most of
    them) only require 2.0V (4.5V < Vcc < 5.5V). The official formula is (1.25
    * Vcc) + .8V, but they make a special case for 5V operation. But I believe
    this is a special case and not consistant with normal CMOS specifications.
    The Schmitt trigger inputs want .8 * Vcc.
  18. Lionel

    Lionel Guest

    I do? I'll admit that it's been at least 15 years since I last looked
    at a 4000 series data sheet...

    <goes & checks>

    Well, according to Motorola
    at a Vcc of 5V, (@25c), a logic '0' = 0-2.25V, & a logic '1' = 2.75V -
    5V. That'd make the swicthing point 50% of Vcc, wouldn't it?
    Seems to me that I was dead on the money.

    Maybe your memory's getting a little rusty too, eh Jim? ;^)
  19. That's the Typical spec and certainly wouldn't be something to recommend as
    a rule of thumb IMO. I'm going to stick with the value in the column next
    to that one that says (Min 3.5V).
  20. Lionel

    Lionel Guest

    Sure, but they're designed to switch at Vcc/2.
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