Maker Pro
Maker Pro

why 74hc inputs are high by default?

maker_0dwedp6d_1663786671

Sep 21, 2022
3
Joined
Sep 21, 2022
Messages
3
I was working with 74hc ic and their input pins are high by default, why?

Is there a good reason for such a design and also is there a way to connect them them so they can be used as normal without using a bunch of pull down resistors or inverters

I am a beginner so if I am missing something obvious plz consider that
 

kellys_eye

Jun 25, 2010
6,514
Joined
Jun 25, 2010
Messages
6,514
High impedance inputs are susceptible to noise triggering on a 'pulled-low' basis.
 

AnalogKid

Jun 10, 2015
2,884
Joined
Jun 10, 2015
Messages
2,884
I looked at several HC datasheets, and don't see anything that indicates that the inputs have a default state. Given the extremely high input impedance, you might be seeing the effect of a charge on the input capacitance.

In any case, it is .n.e.v.e.r. ok to leave logic circuit inputs unterminated in a firm 1 or 0 state. Leaving a logic gate input floating makes it susceptible to radiated noise. It is particularly bad for CMOS devices because stray charge and capacitance can bias the gate into a linear amplifier state, greatly increasing power dissipation. Hysteretic inputs are a special case, but the vast majority of CMOS devices do not have such inputs.

ak
 

Alec_t

Jul 7, 2015
3,587
Joined
Jul 7, 2015
Messages
3,587
I looked at several HC datasheets, and don't see anything that indicates that the inputs have a default state.
Ditto. I don't know how accurate the 74HC Spice models are, but simulation with LTspice suggests the simple gate inputs are actually internally pulled somewhat below mid-rail voltage.
 

crutschow

May 7, 2021
839
Joined
May 7, 2021
Messages
839
is there a way to connect them them so they can be used as normal without using a bunch of pull down resistors or inverters
Normal operation is to have the input driven high or low by an external signal, no pull-down resistors or inverters needed, so I don't understand your question.
What's your definition of "normal"?

If the input is not being used, than just tie it directly to ground or V+.
I don't know how accurate the 74HC Spice models are, but simulation with LTspice suggests the simple gate inputs are actually internally pulled somewhat below mid-rail voltage
The LTspice models apparently do that, but the real components have no fixed input states.
They must always be pulled low or high by some external means.
 
Last edited:
Top