What is parasitic extraction (PEX)?

PEX is the process of extracting the unwanted resistances and capacitances (parasitics) introduced by the physical wiring of a circuit. These parasitics are not known during design and must be extracted from the finished layout to accurately model the real circuit.

Why do parasitics matter in chip design?

Parasitics slow signals down and increase power dissipation. An ideal inverter inverts instantly, but in reality the gate and drain capacitances plus wiring resistance all add delay. Without accounting for these, timing analysis would be inaccurate.

How are parasitics used after extraction?

After extraction, the parasitics are annotated into the ideal circuit model to represent the real circuit. This annotated model can then be analysed for timing accuracy using simulation or STA.

PEX

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January 7, 2021

PEX stands for Parasitic Extraction (often called just ’extraction’). When we build a circuit on a chip we have an intention in mind, for example an ideal inverter inverts the incoming signal immediately and with no delay.

In reality the inverter will have capacitance on both the input (due to the gate) and the output (due to the diode in the drain) and the wiring of the circuit will have both (parallel) capacitance and (series) resistance. Typically, the modelling of the devices or standard cells takes into account the elements due to the transistors, but knows nothing about the wiring we add later.

These extra unwanted circuit elements are referred to as ‘parasitics’, and they slow signals down and increase power dissipation. By extracting the parasitics, we can annotate them into the ideal circuit to get a model of the real circuit, and we can then analyse its performance using simulation or STA.

At 22:35 of my #remoticon talk you can hear me explain about parasitic extraction and show a demo with magic.

Course feedback

I love the layout of the course, you did a perfect job Matt. Building up the foundation and then getting into the depths of it. Having everything lined up. I love the walkthroughs at the end of each section. I did the work and then I looked back, maybe there's a better way and in your videos you point out some other interesting things as well so that was really helpful. So yes I love the course.

Konrad Wilk (digital course)