Here be dragons: Preventing static damage, latchup, and metastability in the 386

Summary

I've been reverse-engineering the Intel 386 processor (from 1985), and I've come across some interesting circuits for the chip's input/output (I/O) pins. Since these pins communicate with the outside world, they face special dangers: static electricity and latchup can destroy the chip, while metastability can cause serious malfunctions. These I/O circuits are completely different from the logic circuits in the 386, and I've come across a previously-undescribed flip-flop circuit, so I'm venturing into uncharted territory. In this article, I take a close look at how the I/O circuitry protects the 386 from the "dragons" that can destroy it. The 386 die, zooming in on some of the bond pad circuits. The colors change due to the effects of different microscope lenses. Click this image (or any other) for a larger version. The photo above shows the die of the 386 under a microscope. The dark, complex patterns arranged in rectangular regions arise from the two layers of metal that connect the circuits on the 386 chip. Not visible are the transistors, formed from silicon and polysilicon and hidden beneath the metal. Around the perimeter of this fingernail-sized silicon die, 141 square bond pads provide the connections between the chip and the outside world; tiny gold bond wires connect the bond pads to the package. Next to each I/O pad, specialized circuitry provides the electrical interface between the chip and the external components while protecting the chip. I've zoomed in on three groups of these bond pads along with the associated I/O circuits. The circuits at the top (for data pins) and the left (for address pins) are completely different from the control pin circuits at the bottom, showing how the circuitry varies with the pin's function. Static electricity The first dragon that threatens the 386 is static electricity, able to burn a hole in the chip. MOS transistors are constructed with a thin insulating oxide layer underneath the transistor's gate. In the 386, this ...