NOT gate in lt spice, what is the 3rd terminal?

Thread Starter

MikeJacobs

Joined Dec 7, 2019
94
Trying to build something in LT spice but for some odd reason the not gate has a 3rd terminal.
There is no part number it should just be an idea case I would of assumed.

When I tie the 3rd terminal to say 5V I loose the inversion. With the 3rd terminal open I get no current flow?

Any ideas?
Thanks
 

djsfantasi

Joined Apr 11, 2010
5,841
Trying to build something in LT spice but for some odd reason the not gate has a 3rd terminal.
There is no part number it should just be an idea case I would of assumed.

When I tie the 3rd terminal to say 5V I loose the inversion. With the 3rd terminal open I get no current flow?

Any ideas?
Thanks
And if you ground the third terminal, then what happens?
 

Papabravo

Joined Feb 24, 2006
12,697
Doesn't sound like an inverter, but rather something else. Tell me how you selected and placed the device on your schematic -- what library dis it come from?
 

djsfantasi

Joined Apr 11, 2010
5,841
I don’t think (I am probably wrong), that the default LTSpice libraries contain logic gates. So what library does your NOT gate come from?
 

Papabravo

Joined Feb 24, 2006
12,697
I don’t think (I am probably wrong), that the default LTSpice libraries contain logic gates. So what library does your NOT gate come from?
The "inv" part in the standard library is a behavioral inverter. It has 3 pins. The input is pin 1 loacated in the middle of the left side of the symbol and is the on;y input. Pin 6, the output is located to the right of the output bubble. Pin 8, in the lower left corner is the Common Pin. Read the help section for how to set your logic threshold in the help sections for "Special Functions". Post again if you still don't understand what is happening.
 

Thread Starter

MikeJacobs

Joined Dec 7, 2019
94
what is the difference between a behavior inverter and a not gate?
I do see the inversion one I ground the pin but also a massive voltage drop like 60%
 

Papabravo

Joined Feb 24, 2006
12,697
what is the difference between a behavior inverter and a not gate?
I do see the inversion one I ground the pin but also a massive voltage drop like 60%
A behavioral inverter does not "simulate" a specific realizable part. If you do not connect the 3rd terminal in the corner of the symbol LTSPice will connect it to global ground. In order to get things to work you need to right click on the part and edit the "SPICE Line" to contain values defining:
  1. The high voltage level. e.g. Vhigh=5V
  2. The rise time of the output e.g. Trise=20e-9
  3. The gate delay e.g. Td=5e-9
In short I get a working gate with the following SPICE Line --> Vhigh=5V,Td=5e-9,Trise=20e-9

Here is a relaxation oscillator with a Schmidt Inverter with a frequency of about 100 Hz with the default threshold parameters. Of course you can affect the frequency by playing with parameters Vt and Vh. See the LTSpice Help file for details.
 

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Papabravo

Joined Feb 24, 2006
12,697
All of my digital parts have a left bottom corner pin that must be connected to ground.
Yes, but the LTSpice help file specifically states that you do not need to explicitly connect that node to Ground. The actual text is:

The schematic capture aspect of LTspice netlists symbols for these devices in a special manner. All unconnected terminals are automatically connected to terminal 8. Also, if terminal 8 is unconnected, then it is connected to node 0.

Node 0 is understood to be Ground.
 

eetech00

Joined Jun 8, 2013
1,740
Trying to build something in LT spice but for some odd reason the not gate has a 3rd terminal.
There is no part number it should just be an idea case I would of assumed.

When I tie the 3rd terminal to say 5V I loose the inversion. With the 3rd terminal open I get no current flow?

Any ideas?
Thanks
When an input is connected directly to ground (like with an LTspice ground symbol), LTspice treats it as an unused pin and eliminates the pin from the circuit. So its basically ignored. Unused pins should be left unconnected. If you need an input to be active, pull it high, or low (with a resistor), or connect it to another component. The A digital devices aren't really intended to be used in their bare form, but to be encapsulated with other devices or circuits to form a complete logic function.
Normally, I don't use the com pin and leave it disconnected.

Some of this is described in the LTspice Help.

Hope that helps...

eT
 

Papabravo

Joined Feb 24, 2006
12,697
When an input is connected directly to ground (like with an LTspice ground symbol), LTspice treats it as an unused pin and eliminates the pin from the circuit. So its basically ignored. Unused pins should be left unconnected. If you need an input to be active, pull it high, or low (with a resistor), or connect it to another component. The A digital devices aren't really intended to be used in their bare form, but to be encapsulated with other devices or circuits to form a complete logic function.
Normally, I don't use the com pin and leave it disconnected.

Some of this is described in the LTspice Help.

Hope that helps...

eT
Right -- see post #11 with a quote from the help file about what happen whens pin 8 is left open.
 

ci139

Joined Jul 11, 2016
1,010
the Spice is good analog simulator
the Spice is not too efficient nor too stable digital simulator - i don't do large scale digital stuff . . . otherwise using specially designed digital simulators would be justified!

the help says it's a common return (if i remember right) . . . see "terminal 8" http://ltwiki.org/index.php?title=A_Special_functions
/// for below : Vhigh = default = 1V // Vlow = default = 0V // . . . all = defaults

Basic - Spice - Digital - Test - 010-aa.png
 
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eetech00

Joined Jun 8, 2013
1,740
the Spice is good analog simulator
the Spice is not too efficient nor too stable digital simulator - i don't do large scale digital stuff . . . otherwise using specially designed digital simulators would be justified!

the help says it's a common return (if i remember right) . . . see "terminal 8" http://ltwiki.org/index.php?title=A_Special_functions
/// for below : Vhigh = default = 1V // Vlow = default = 0V // . . . all = defaults

View attachment 194358
Yes...but it is almost never used. And for what the TS is doing they don’t need to connect it

eT
 
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