New Gate Design Using LTspice/SwitcherCAD III

Ron Fredericks writes: recently I discovered that I was going to have to create my own IC component and symbol for my on-going digital volume control circuit simulation. My first step was to check in with the LTspice forum on yahoo groups. On the forum I requested any previous design for the IC I needed. I also asked the group’s readership if they thought it was correct to build a new IC from existing low level digital gates – gates that are already supplied with the LTspice distribution by Linear Technologies.

Well, I did not find a previous inventor for my IC simulation, but I did get confirmation that the gate build-up was a common strategy. And, this same forum engineer supplied me with a copy of an IC simulation of his own – one very similar to my IC requirement – he supplied a symbol and sample test bed to accelerate my learning curve. Here is a link to my support dialog.

I would like to thank Helmut Sennewald for his time and excellent service to the LTspice yahoo forum. It is his effort and many others who make this forum such a valuable community resource. This forum in turn, has made the LTspice/SwitcherCAD III circuit capture and spice tool a viable design tool for many embedded component users and EE designers.

Introducing the T S-R Flip-Flop

To build my new IC, I had to build a new digital logic block. This component is a Toggle Flip-Flop with Set and Reset functions added. In this blog post I introduce my readers to this new component and share the simulation circuit for others to use and learn from.

See the figure below for an initial design of the T S-R Flip-Flop, including a truth table in the form of a waveform diagram, the circuit, a pulse detector sub-circuit and their related assemblies. This circuit is just an initial design because it uses an S-R Flip-Flop and a simple pulse detector sub-circuit for its clock.

T S-R Flip-Flop and releated sub-circuits and assemblies

View larger image>

Final Design for the T S-R Flip-Flop

This section of my post is an update, thanks to a review by Helmut Sennewald. See figure below for my final design of the Toggle S-R Flip-Flop. This design overcomes two problems in my initial design, both resolved by starting with the D Flip-Flop with its built-in clock. The reuse of this more full-featured LT supplied component in my design eliminated the home-brew pulse maker sub-circuit. And in so doing, the slower S-R Flip-Flop. Slower because I had to set the SpiceLine time delay to a minimum of 20 nanoseconds (or td >= 2x the gate time delay) to support the simulation of my simple pulse maker sub-circuit. The D Flip-Flop has an internal clock so I could eliminate the pulse maker sub-circuit. End result: one less sub-circuit and faster Flip-Flop simulation using a time delay set to a minimum of 10 nanoseconds (or td >= 1x the gate time delay).

T S-R Flip-Flop (final design)

View larger image>


To test my knowledge of digital design using the LTspice tool, I created a number of similar flip-flop components which are included in the download:

  1. S-R Flip-Flop test circuit
  2. S-R Flip-Flop with Enable gate and test circuit
  3. S-R Flip-Flop with rising edge clock and test circuit
  4. J-K Flip-Flop with rising edge clock and test circuit
  5. D Flip-Flop with Enable gate and test circuit
  6. T S-R Flip-Flop from S-R Flip-Flop and test circuit (initial design)
  7. Rising Edge Pulse Detector (not high performance design)
  8. T S-R Flip-Flop from D Flip-Flop and test circuit (final design)

Download the components listed above for your LTspice designs all in one zipped directory.

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8 Responses to “New Gate Design Using LTspice/SwitcherCAD III”

  1. Helmut Sennewald Says:


    It’s more safe to make JK- and T-flipflops based on the A-device dflop (D-flipflop). This dflop is already only edge sensitive as required.

    You can find the examples in the Files-section of the LTspice group.
    Files > Lib > JK-Flipflop and T-Flipflop

    Best regards,

  2. Ron Fredericks Says:

    Thank you Helmut. I have updated this blog post to include an updated design based on the LT supplied D Flip-Flop. By making this update, I hope my new IC built from this work will be all that much better.

    Best regards,

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  4. Allen Kelly Says:

    I don’t know why but plain SR and DFLOP components from today’s download 10/09/2008 only work with the most simple of circuits.

    Embedded in a mixed analog and power model these logic functions fail to perform. I did a side-by-side with the SR Flop for example, and given the same input and output characteristics with a very, very simple demo circuit and my more complex application.. the SR Flop in the more complex application failed. Any Ideas???

    Allen Kelly, Northrop Grumman

  5. telefon dinleme Says:

    thank you for your article. i find it after to 1 month… good evening.

  6. Rajesh Autee Says:

    Nice design, Now I am looking for simulaion of Switched Reluctance Motor and its Driver

    Good Night

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