Is LTSpice an appropirate tool to use to model digital circuits?

[JoeN]

By digital circuits, I mean transistor circuits working as digital circuits where the circuit is modeled up from transistors up to gates, more complex subcircuits, and finally state machines.  Right now I modeled a RS flip flop and built a test circuit to use it as a frequency divider, just as a test.  I think it is quite interesting how as an analog simulation it shows you the slew of the transistors (especially when it is run at a faster clock), voltage excursions, etc.  Is this the way to do it?  Or is this insane?  How well is this going to perform if there are a few thousand transistors in the simulation?

Comments or advice would be appreciated.

Basic gates:

Inverter



2-NAND



3-NAND



The RS flip flop:



The testbench circuit:



The results:

[bobaruni]

Looks good and if it works for you, why not!
Simulations will start to slow down the more components and signal paths you have.
Also, there are a few digital gate components built into LTSpice's shipped library.
I just wonder though, if you are going to design something using thousands of transistor, it might be time to look at something like CPLD/FPGA.

[rstofer]

Or at least use the toolchain for the CPLD/FPGAs.  Most have schematic entry and many have simulators.  There's a bit of a learning curve to using the toolchain and describing circuits but they can be described at the gate level.

Try the free version of Xilinx Vivado.