How The 8087 Coprocessor Got Its Bias

Most of us have been there. You build a device but realize you need two or more voltages. You could hook up multiple power supplies but that can be inconvenient and just not elegant. Alternatively, you can do something in the device itself to create the extra voltages starting with just one. When [Ken Shirriff] decapped an 8087 coprocessor to begin exploring it, he found it had that very problem. It needed: +5 V, a ground, and an additional -5 V.

His exploration starts with a smoking gun. After decapping the chip and counting out all the bond wires going to the various pads, he saw there was one too many. It wasn’t hard to see that the extra wire went to the chip’s substrate itself. This was for providing a negative bias to the substrate, something done in some high-performance chips to get increased speed, a more predictable transistor threshold voltage, and to reduce leakage current. Examining where the bond wire went to in the circuitry he found the two charge pump circuits shown in the banner image. Those worked in alternating fashion to supply a -5 V bias to the substrate, or rather around -3 V when you take into account voltage drops. Of course, he also explains the circuits and dives in deeper, including showing how the oscillations are provided to make the charge pumps work.

If this is anything like [Ken’s] previous explorations, it’ll be the first of a series of posts exploring the 8087. At least that’s what we hope given how he’d previously delighted us with a reverse engineering of the 76477 sound effects chip used in Space Invaders and then went deeper to talk about¬†integrated injection logic (I2L)¬†as used in parts of the chip.