Reverse Engineering Silicon From The First Pocket Calculator

We’ve seen so many explorations of older semiconductors at the hands of [Ken Shirriff], that we know enough to expect a good read when he releases a new one. His latest doesn’t disappoint, as he delves into the workings of one of the first hand-held electronic calculators. The Sharp EL-8 from 1969 had five MOS ICs at its heart, and among them the NRD2256 keyboard/display chip is getting the [Shirriff] treatment with a decapping and thorough reverse engineering.

The basic functions of the chip are explained more easily than might be expected since this is a relatively simple device by later standards. The fascinating part of the dissection comes in the explanation of the technology, first in introducing the reader to PMOS FETs which required a relatively high negative voltage to operate, and then in explaining its use of four-phase logic. We’re used to static logic that holds a state depending upon its inputs, but the technologies of the day all called for an output transistor that would pull unacceptable current for a calculator. Four phase logic solved this by creating dynamic gates using a four-phase clock signal, relying on the an output capacitor in the gate to hold the value. It’s a technology that lose out in the 1970s as later TTL and CMOS variants arrived that did not have the output current drain. Fascinating stuff!

[Ken] gave a talk at the Hackaday Superconference a couple of years ago, if you’ve not seen it then it’s worth a watch.

4 thoughts on “Reverse Engineering Silicon From The First Pocket Calculator

  1. The TI “Speak n Spell”(?) toys needed a high negative voltage (-22?) to work.
    I recall an article in Radio-Electronics adapting one for a speech synthesizer. I picked up one of those toys cheap at a garage sale, but figured it was too much trouble to build from it, partly because of the negative voltage.

        1. Yah, glowy elephants in a tube whatever, still needs about 30V potential difference somewhere. I keep thinking 1st gen S&S had bubble mem in because original conception was for a mass market application of bubble mem.

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