RESEARCH | February 23, 2021

Probing and Signal Integrity Fundamentals for the Hardware Hacker, part 2: Transmission Lines, Impedance, and Stubs

This is the second post in my ongoing series on the troubles posed by high-speed signals in the hardware security lab.

What is a High-speed Signal?

Let’s start by defining “high-speed” a bit more formally:

A signal traveling through a conductor is high-speed if transmission line effects are non-negligible.

That’s nice, but what is a transmission line? In simple terms:

A transmission line is a wire of sufficient length that there is nontrivial delay between signal changes from one end of the cable to the other.

You may also see this referred to as the wire being “electrically long.”

RESEARCH | January 28, 2021

Probing and Signal Integrity Fundamentals for the Hardware Hacker

The latest new widget just showed up on your desk. You excitedly crack open the case, look around a bit, and find a signal that looks interesting. You fire up your oscilloscope, touch a probe to the signal, and… the widget won’t boot! Or maybe it works fine, but you see garbage on the scope screen that looks nothing like a useful digital waveform.

It’s a problem that’s becoming all too familiar to hardware hackers. As technology advances, signals become faster, which makes them more sensitive and less tolerant to the sloppy wiring and probing techniques commonly used in reverse engineering. Even cheap SPI flash can run at 100+ MHz in modern designs.