XOR chip is leaking

Thread Starter

DK3250

Joined Jul 15, 2020
12
Hi,
I'm new on this site and I'm an absolute beginner in electronics.
My target is to build an adder - inspired by this Ben Eater video:

I use an XOR chip from TI: SN74HC86N, 5V on pin 14 and Ground on pin 7.
The problem is that I get an output on pin3 despite no input on pin 1 or 2.
I measure just under 1V on pin 1 and 2 - I was expecting 0 V, and I don't understand why pin 1 and 2 are charged.
I have tried installing a pull-down resistor, 10k, and it worked in a small test installation.
The final installation, however needs two XOR chips with a total of 16 inputs - it's going to be a lot of pull-downs and quite messy.
And most importantly: The samples I've seen on the internet don't need this (ok, Ben uses a chip from Fairchild, but they should be the same..?)

Any comments or input?
 

dl324

Joined Mar 30, 2015
10,944
Welcome to AAC!
Any comments or input?
Post a schematic of your circuit. You don't require pull up/down resistors if the inputs are always driven.

The gate probably isn't "leaking"; whatever that means.

I didn't watch the video. It's too long.
 

Papabravo

Joined Feb 24, 2006
13,926
Unconnected inputs are either the gates of MOS transistors or the emitters of a BJT (Bipolar Junction Transistor). As such they will "float" to an undefined voltage and the associated output will do what it "thinks" the inputs are telling it. You can look in the datasheets for the part and it will tell you what the defined voltage levels are for a zero and a one.
 

Thread Starter

DK3250

Joined Jul 15, 2020
12
Thank you, dl324 and papabravo.

I'd like to show the circuit - but as new in this I still don't know a tool for drawing the circuits. Any suggestons?

Yeah, this 'floating' is probably the problem. I use a simple set of switches to control the input - maybe I can find some with 'build-in' pull-down resistors...?
 

dl324

Joined Mar 30, 2015
10,944
I'd like to show the circuit - but as new in this I still don't know a tool for drawing the circuits. Any suggestons?
You can draw by hand and take a picture, use a drawing program, etc.

maybe I can find some with 'build-in' pull-down resistors...?
I doubt that they exist. When we design circuits, we don't let any inputs float. Even unused gates need to be tied off so they don't increase power dissipation or oscillate and cause spikes on the power supply.
 

Thread Starter

DK3250

Joined Jul 15, 2020
12
Thanks again, - I'll install the pull-down resistors.
For now I consider the problem solved.
Great site !! - I'll definitely come back here with more 'stupid' questions.
 

dl324

Joined Mar 30, 2015
10,944
I'll definitely come back here with more 'stupid' questions.
It doesn't help if you're using stupid videos to design circuits.

From the looks of the video you're watching, the switches let inputs float (which is bad), and some outputs are driving LEDs without current limiting resistors (also bad). If those outputs are also used as inputs to other gates, loading them with LEDs is also a bad idea (unless the load isn't enough to affect gate fanout).

When discussing circuits, it's best to use schematics. A definite no-no is to use videos. Some of us don't have the patience to watch videos that take forever to get to the point.
 

Papabravo

Joined Feb 24, 2006
13,926
LTspice is a circuit simulator, which has an excellent and easy to use schematic entry program. It is also a free download. The schematic is rendered as a text file. You can also export the schematic image to MSPaint and save it as a .PNG file. At some point you can learn to use the simulation feature, but that is not necessary at the beginning.
 

jpanhalt

Joined Jan 18, 2008
9,706
You can connect all unused inputs together and tie to ground. Conversely, each can be tied to ground separately. You do not need a resistor. If you feel you must, then use one resistor for all unused inputs.

The individual gates in the device are separate, except for power and ground, and of course, you are using a common supply for each chip.
 

MrChips

Joined Oct 2, 2009
21,326
Good thing that you learned a few things:

1) Chips don't leak. You'd think I should know that.

2) Leaving input pins not connected is not the same as giving it a 0 input.

3) You don't leave input pins not connected to a valid logic level. In other words, you don't leave input pins floating.
 

jpanhalt

Joined Jan 18, 2008
9,706
My reading of the relevant Application Note on those devices (i.e., TI SCBA004) is that pull-up/pull-down resistors are required when the inputs are floating (i.e,tristated) and on a bus. If the input will never be pulled high or low, resistors are not needed.
 

Jolly13

Joined Mar 25, 2018
1
Ben’s videos are great and you will learn a lot, .. but he uses LS type chips which are much more forgiving to floating inputs. With the HC series like you are using you must have all inputs pulled up or down.

Later in his video you will be tying the OR gates to the B register and a control signal to invert the B inputs ( add/Subtraction spoiler maybe) ..

So add a test jumpers for the 8 data inputs and 1 for the invert signals and all will be good.

HC is a good choice , so Google up on how to deal with unused pins, is a fun project.
 

sparky 1

Joined Nov 3, 2018
260
There is a digital I/O module should not be leaking and it can be tested for leaks p.3 figure 2.
http://www.engr.newpaltz.edu/~bai/EGC221/Lab 2 - Basic Logic Gate Physical Verification.pdf

I recall some time ago reading about avoiding the intermediate zone and then there is Forrest Mims again.
https://www.electronics-tutorials.ws/logic/logic_1.html
http://thelukens.net/science/electronics/Engineer's Mini-Notebooks/Digital Logic Circuits.pdf
Determination and believing there are just a few more things to get it working, a few possibilities.
Maybe developing a test where a potentiometer as a voltage divider could be used to find the intermediate zone.
Having the right resistor value for your 74HC86 or evaluate using a different model.
 
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