To connect the miniature world of integrated circuits like a CPU with the outside world, a number of physical connections have to be made. Although this may seem straightforward, these I/O pads form a major risk to the chip’s functioning and integrity, in the form of electrostatic discharge (ESD), a type of short-circuit called a latch-up and metastability through factors like noise. Shielding the delicate ASIC from the cruel outside world is the task of the I/O circuitry, with [Ken Shirriff] recently taking an in-depth look at this circuity in Intel’s 386 CPU.
The 386 has a total of 141 of these I/O pads, each connected to a pin on the packaging with a delicate golden bond wire. ESD is on the top of the list of potential risks, as a surge of high voltage can literally blow a hole in the circuitry. The protective circuit for this can be seen in the above die shot, with its clamping diodes, current-limiting resistor and a third diode.
Latch-up is the second major issue, caused by the inadvertent creation of parasitic structures underneath the P- and NMOS transistors. These parasitic transistors are normally inactive, but if activated they can cause latch-up which best case causes a momentary failure, but worst case melts a part of the chip due to high currents.
To prevent I/O pads from triggering latch-up, the 386 implements ‘guard rings’ that should block unwanted current flow. Finally there is metastability, which as the name suggests isn’t necessarily harmful, but can seriously mess with the operation of the chip which expects clean binary signals. On the 386 two flip-flops per I/O pad are used to mostly resolve this.
Although the 386’s 1985-era circuitry was very chonky by today’s standards, it was still no match for these external influences, making it clear just how important these protective measures are for today’s ASICs with much smaller feature sizes.
