General question regarding unused IC pins...

Discussion in 'General Electronics Chat' started by JohnnyD, Jul 10, 2007.

  1. JohnnyD

    Thread Starter Well-Known Member

    Aug 29, 2006
    79
    0
    Hi,

    Sorry if this is a stupid question but, when there is a need to tie unused IC pins to either +ve or -ve, are they meant to be directly connected, or should they be connected via a current limiting resistor? I ask because I need to limit the current draw of a circuit I'm designing because it is powered by a battery and I want it to last as long as possible.

    For example, my circuit uses a CMOS 4013 Dual D-type flip-flop and I need to tie the SET and RESET pins to -ve. The datasheet says that the maximum input sink/source current is 10mA so I chose to limit the used inputs and outputs to around 5mA, but I'm not sure what to do about the unused ones.

    Another question arose from my ponderings about this - what is the minimum current I could use to reliably trigger the inputs of the 4013? Could I go down as far as 1mA... or even less? The datasheet only gives a maximum.

    Thanks,

    John.
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,152
    1,794
    For one of something you can tie directly to power or ground. The use of a resistor is mostly for automated test equipment which needs to force inputs high and low, which it can't do if they are tied to Vdd and Vss. CMOS inputs tied high or low do not source or sink any appreciable current. This is not true for TTL inputs which source current from the multi emitter transistor input.
     
  3. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    When it comes to CMOS logic gates you are pretty safe tying unused inputs directly to either power rail. Also with CMOS logic you need not include a series resistor when you tie control lines to the appropriate rail.

    The main advantage to using resistors is the convenience it provides if the board needs to be tested using automated test system. With resistors in series with the control lines, the test equipment can stimulate an otherwise static control line so that it can perform testing of the board that would not be possible with the line connected directly to the supply rail.

    hgmjr

    PS. Simultaneous replies, Great minds think alike.
     
  4. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    Athough there is some truth in your statement, the TTL inputs should not draw that much current. It is a good practice to connect unused inputs to ground or VCC without using a resistor, but in the case of entire gates being unused you can have inputs floating without interference, since TTL inputs always "float" towards Vcc.
     
  5. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    As was mentioned the use of resistors is useful for testing. The resistor also makes
    PCB modification easier. Rather than cutting a trace you could remove the resistor.

    (* jcl *)
     
  6. JohnnyD

    Thread Starter Well-Known Member

    Aug 29, 2006
    79
    0
    Thanks for the help on this. I re-breadboarded a circuit of mine that uses 2 CMOS ICs and it works great. I realised I had made a mistake when I first read the datasheet and tied all the unused outputs to the supply rails as well, which resulted in excessive current draw. I love the way CMOS chips use such a miniscule amount of current. It's very useful for this circuit because it's going to be put in my car once it's finished. At the moment it uses around 5mA, most of which is used up by the voltage regulator! It shouldn't run the battery out for years.
     
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