In electronics, glue logic is the custom electronic circuitry needed to achieve compatible interfaces between two (or more) different off-the-shelf integrated circuits. This is often achieved with a few cheap 7400- or 4000-components, or, in more complex cases, a CPLD or FPGA; the falling price of the latter two devices, combined with their reduced size and power drain compared to discrete elements, is making them common even for simple systems.
An example of glue logic is the address decoder which with older processors like the 6502 or Z80 had to be added externally to divide up the addressing space of the processor into RAM, ROM and I/O. Newer versions of the same processors (such as the WDC 65816 or Zilog eZ80) instead have internal address decoders so glueless interfacing to the most common external devices becomes possible.
Sometimes, glue logic is used to encrypt the proprietary electronics circuitry by the vendor and to prevent the product from being cloned or counterfeited.
Typically, glue logic chips perform routine digital summing or comparing duties (such as in the form of AND gates and OR gates), or the signals may need to be processed for data flow from one logic family (CMOS) to another (TTL). Inverters may also be part of a glue logic array.
The software equivalent of glue logic is called glue code.
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