Glue logic

In electronics, glue logic is the custom logic circuitry used to interface a number of off-the-shelf integrated circuits.^[1]

This is often achieved using ordinary (cheap) 7400- or 4000-series components. In more complex cases, programmable logic devices like a CPLD or FPGA 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. In addition, 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).

Usage

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

• Simple logic functions.
• Address decoding circuitry used with older processors like the 6502 or Z80 to divide up the processor's address space into RAM, ROM and I/O. Newer versions of these processors (such as 65816 or eZ80), may add features that enable glueless interfacing to external devices.
• Buffers to protect outputs from overload, or protect sensitive inputs from ESD damage.
• Voltage level conversion, e.g. when interfacing one logic family (CMOS) to another (TTL).

The software equivalent of glue logic is called glue code.

References