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

If you've got any interest in how the sausage is made you should get on the course and you should dig in and find out more. I mean this was the work of secret witches and wizards in mysterious cloaks casting strange incantations over a cauldron! This was all secret stuff and I love that this project is trying to do to silicon design what the open source community has been trying to do with software for the last 30 years. This feels like the next logical step and I think we'll look back and say well of course you can make your own chips that's just a thing and it will just be obvious and commonplace and I look forward to that.

Jonathan Pallant (digital course)