How to code this design ?

[Xu Xian]

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[WBahn]

You can't. This HDL simulator for Nand-2-Tetris can't handle unclocked feedback circuits, which is why the course provides the DFF with the hidden clock as a primitive element.

[Xu Xian]

so,is it not possible to implement the above design?

[WBahn]

Xu Xian wrote

so,we can't implement the above design ?

You can create the design, but you can't simulate it with the provided simulator.

Even if you could, you have issues that you can't adequately model that are necessary to get this type of circuit to work. You are hoping that when you switch the Mux that it holds the output from the Not gate that is being fed back to one input of the Mux. But what happens during the switch? The output of the Mux does change instantly from one input to the other, so in the meantime the output is not defined and therefore the input to the Not gate is not defined, so what is it's output under those conditions?

Simulating these kinds of circuits requires a proper analog circuit simulator or, at the very least, models for the gate elements that realistically reflect transition behaviors, including slew rates, input thresholds, and propagation delates. The N2T tools are simply not intended for implementing and simulating such circuits.