LTSpice logic models

How do I use the logic gates in LTSpice? They don't have any power supply connections!

Generally the logic models have a built-in fixed supply voltage. On some you can right-click on the icon and select the desired voltage.

throbscottle said:

How do I use the logic gates in LTSpice? They don't have any power supply connections!

Click to expand...

You talking about the 4000 CMOS lib, or the idealized ones built into LTSpice? If the latter, there is an example in the Educational subdirectory...

I meant the built in ones. In the example (160.asc) it just goes to 1v as logic 1. When right-clicking I get a box where I can set spice properties, but I don't know what to put in there. I want my sim to have other types of component in it so a consistent supply is neeeded. Guess I'll have to look for other libs like that 4000 one.

Gosh, I just found all the libs on the LTSpice Yahoo group...

The ones built-in are just generic gates, latches, and flip-flops. They are good for simulating general logic, and it is possible to run a simulation with hundreds of these, where a schematic containing 7400 or 4000 gates would bog down.... Spice is not a logic sim, but with the simplified models you can do quite complex things....

throbscottle said:

I meant the built in ones. In the example (160.asc) it just goes to 1v as logic 1. When right-clicking I get a box where I can set spice properties, but I don't know what to put in there. I want my sim to have other types of component in it so a consistent supply is neeeded. Guess I'll have to look for other libs like that 4000 one.

Click to expand...

Hi

160.asc is a very basic model of a 74160 using LTspice proprietary "A" devices.. Rise times, fall times impedences, prop delay, input/output currents/voltages, etc., are not modeled. 1v is the default logic voltage level. These devices are located in the "Digital" section of the component selector."Special Functions" in Help section describes parameters that can be set.

There is a CD4000_v lib and 74HC/HCT_v lib that have power supply input available. Its much easier to use one of these libraries.

eT

Hi folks, happily using 74HC lib now - thanks for all the help I'll remember to use the built in gates in the unlikely event I want to model something complex though.

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tv

Cheers tvtech, here is some ltspice logic stuff.....just right click them to see how to set them up. You can set what voltage you want to be your vout_hi and vout_low etc.....also you can set your rise and fall times, and delay times, and the trip voltage etc.