In electronics

Electronics

Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...

, glue logic is the custom logic circuitry

Digital circuit

Digital electronics represent signals by discrete bands of analog levels, rather than by a continuous range. All levels within a band represent the same signal state...

 used to interface a number of off-the-shelf

Commercial off-the-shelf

In the United States, Commercially available Off-The-Shelf is a Federal Acquisition Regulation term defining a nondevelopmental item of supply that is both commercial and sold in substantial quantities in the commercial marketplace, and that can be procured or utilized under government contract...

 integrated circuits.

This is often achieved using ordinary (cheap) 7400

7400 series

The 7400 series of transistor-transistor logic integrated circuits are historically important as the first widespread family of TTL integrated circuit logic. It was used to build the mini and mainframe computers of the 1960s and 1970s...

- or 4000

4000 series

The 4000 series is a family of industry standard integrated circuits which implement a variety of logic functions using Complementary Metal–Oxide–Semiconductor technology, and are still in use today. They were introduced by RCA as CD4000 COS/MOS series in 1968, as a lower power and more versatile...

-series components. In more complex cases, programmable logic devices like a CPLD or FPGA

Field-programmable gate array

A field-programmable gate array is an integrated circuit designed to be configured by the customer or designer after manufacturing—hence "field-programmable"...

 might be used. The falling price of programmable logic devices, combined with their reduced size and power consumption (compared to discrete components), is making them common even for simple systems.

Typical functions of glue logic may include (but not limited to):

• Simple logic functions.
• Address decoding circuitry
  Address decoder
  In digital electronics, an address decoder is a decoder circuit that has two or more bits of an address bus as inputs and that has one or more device selection lines as outputs. When the address for a particular device appears on the address bus, the address decoder asserts the selection line for...
  
   used with older processors like the 6502
  MOS Technology 6502
  The MOS Technology 6502 is an 8-bit microprocessor that was designed by Chuck Peddle and Bill Mensch for MOS Technology in 1975. When it was introduced, it was the least expensive full-featured microprocessor on the market by a considerable margin, costing less than one-sixth the price of...
  
   or Z80
  Zilog Z80
  The Zilog Z80 is an 8-bit microprocessor designed by Zilog and sold from July 1976 onwards. It was widely used both in desktop and embedded computer designs as well as for military purposes...
  
   to divide up the processor's address space into RAM, ROM and I/O. Newer versions of these processors (such as 65816 or eZ80
  Zilog eZ80
  The Zilog eZ80 is an 8-bit microprocessor which is essentially an updated version of the company's earlier Z80 8-bit microprocessor.The eZ80 is binary compatible with the Z80 and Z180, but almost four times as fast as the original Z80 chip at the same clock frequency...
  
  ), may add features that enable glueless interfacing to external devices.
• Buffers to protect outputs from overload, or protect sensitive inputs from ESD
  Electrostatic discharge
  Electrostatic discharge is a serious issue in solid state electronics, such as integrated circuits. Integrated circuits are made from semiconductor materials such as silicon and insulating materials such as silicon dioxide...
  
   damage.
• Voltage level conversion, e.g. when interfacing one logic family (CMOS
  CMOS
  Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...
  
  ) to another (TTL
  Transistor-transistor logic
  Transistor–transistor logic is a class of digital circuits built from bipolar junction transistors and resistors. It is called transistor–transistor logic because both the logic gating function and the amplifying function are performed by transistors .TTL is notable for being a widespread...
  
  ).
• Programmable logic can be used to hide the exact function of a circuit, in order to prevent a product from being cloned or counterfeited (see: reverse engineering
  Reverse engineering
  Reverse engineering is the process of discovering the technological principles of a device, object, or system through analysis of its structure, function, and operation...
  
  ).

The software equivalent of glue logic is called glue code

Glue code

In programming, glue code is code that does not contribute any functionality towards meeting the program's requirements, but instead serves solely to "glue together" different parts of code that would not otherwise be compatible...

.