11/7/2023 0 Comments Reverse engineering calculator![]() The "wiring" section is simply wiring between the upper half of the chip and the lower half, showing how much space is The "digit serialize" circuit converts a digit keypress into four serial bits. The "decimal point" circuitry deserializes the decimal point input "dp" and energizes the decimal point segment when The "digit scan" circuitry scans through the eight digit drive lines D1-D8. The diagram below shows the functional units in the keypad/display chip. Thus, as the keypad/display chip cycles through the digits of the display, it receives the binary value of each digit at the right time. The other chips to keep track of the current digit. The keypad/display chip cycles through the display digits, one digit every four clocks it transmits signals to The keypad/display chip produces additional timing signals keep everything synchronized.įirst, it divides the clock by 4, generating a "digit clock" signal that indicates each 4-bit digit. The basic timing comes from the 60-kilohertz clock chip one bit is sent each clock cycle. The four main chips communicate serially, sending each decimal digit as four BCD (binary-coded decimal) bits.Įach communication cycle consists of 8 digits plus a ninth unused spot, forming a 36-bit "packet". Photo courtesy of John Wolff's Web Museum © 2012. Note the special tube at the top that displays a minus sign and error dot. ![]() 2ĭisplay board for the Sharp EL-8 calculator. It does this by activating one display tube at a time and energizing the appropriate segments to produce the desired digit. Like most calculators, this calculator multiplexes the display it displays one digit at a time, repeated rapidly enough 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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