![]() Its second main function is to display digits on the display. (Unexpectedly, non-digit keypresses are handled by other chips.) One function of the keypad/display chip is to handle keypresses, converting a digit key into a 4-bit serial binary value. The system clock runs at about 60 kilohertz, very slow by microprocessor standards, but fast enough for a calculator In the small metal can that provides the four-phase timing pulses. This blog post focuses on the keypad/display chip (NRD2256) in the upper left. These four chips have different functions:Īn arithmetic chip, a decimal point chip, a keypad/display chip, and a control chip. The clock IC is the small metal-can package in the middle. The circuit board for the Sharp EL-8 calculator. ![]() Unlike modern printed circuit boards, the traces on this board are curved, showing its hand-drawn layout. These integrated circuits were packaged as 42-pin ceramic ICs with staggered pins,Īn arrangement that provided more room for the PCB traces. ![]() Integrated circuits with circular golden lids. The photo below shows the circuit board inside the calculator. The chip contains roughly 500 transistors implementing 100 logic gates.Ĭhip is absurdly low by modern standards, it illustrates the progress of MOS integrated circuits in the late 1960s. Around the edges, thin bond wires connect the die to the 42 external pins. The silicon substrate has a purple tint while the doped, conductive silicon is green. This photo shows the tiny silicon die under a microscope. In this blog post, I reverse-engineer the keypad/display chip shown above. Die photos courtesy of François Gueissaz. Die photo of the NRD2256 keypad/display chip.
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