CMOS/TTL unused I/O pin terminations?

Discussion in 'General Electronics Chat' started by Rosbife, Feb 9, 2013.

  1. Rosbife

    Thread Starter New Member

    Jan 17, 2013
    9
    0
    Hello everyone, I've been trying to make full grasp of what to do with unused outputs and/or inputs in CMOS and TTL.

    I'm using FTDI's FT230XS chip (link below)

    (http://www.ftdichip.com/Support/Documents/DataSheets/ICs/DS_FT230X.pdf)

    , and not making use of the RESET#, CTS#/RTS# or any of the CBUSx pins, and trying to be sure of what I can leave unterminated (the datasheet only states RESET# can be left unconnected).

    It is my understanding CMOS outputs can be left unterminated and that TTL inputs need pull ups/downs. Now, from the datasheet I gather the chip already has internal pull-ups to VCCIO for the inputs. The outputs, being CMOS, don't require any pulling.

    Is this correct?

    Thank you a lot!
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    In general:

    CMOS inputs must be terminated (except in the rare instance that they are internally terminated).

    TTL inputs generally don't need to be terminated (they go high when un-terminated) but its good practice to do so to avoid any false triggering from noise.

    Neither CMOS or TTL outputs need to be terminated.
     
    Rosbife likes this.
  3. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    All the unused input pins need to set to Low or Hi.

    If All the unused input pins already settle down, and if any output pin is unused, you can left them behind, just don't care about them.
     
    Rosbife likes this.
  4. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    Rosbife likes this.
  5. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,365
    Leave all unused outputs alone.
    All inputs have internal 75kΩ pullups. Unused inputs can be left alone.
     
    Rosbife likes this.
  6. Rosbife

    Thread Starter New Member

    Jan 17, 2013
    9
    0
    Thanks to everyone.
     
  7. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    As you mentioned that the values of the resistors, they can be using in everywhere, sometimes if the power comsumption is considered when the device is using at home or a portable, then the values can be raising to 47K~100K, but if using in the industrial that is better not to this, just using the values as you mentioned.
     
Loading...