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WHAT'S YOUR ENGINEERING QUOTIENT?

Problem 8—In high-speed digital logic ICs, what is the phenomenon known as "ground bounce"?

When the output of a digital IC switches high-to-low, the charge stored in the distributed capacitances of the output circuitry must be dumped to ground. If the IC is very fast, this results in a very narrow, but very large current pulse in the pins that connect the ground of the chip itself to the ground of the printed circuit board. Because these pins have a tiny but nonzero impedance (resistance and inductance), this causes the chip ground to become a higher voltage than the PCB ground for a brief time. You can see this on an oscilloscope if you connect it to another output that's already in the low state at the same time an output is switching. The pulse will be bigger if several outputs are switching simultaneously.

This causes problems for the inputs on the chip, because their switching thresholds are relative to the chip ground level. When the ground bounces up, the switching threshold bounces up as well, and it can make an input that's supposed to be high appear low to the chip for the duration of the bounce. This is particularly bad if the input in question is a clock input to the chip; you can get double- or triple-clocking of internal state machines on every "real" clock edge you see going in.

Contributor: Dave Tweed

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