Programmable Logic Device - FPGAs

FPGAs

While PALs were busy developing into GALs and CPLDs (all discussed above), a separate stream of development was happening. This type of device is based on gate array technology and is called the field-programmable gate array (FPGA). Early examples of FPGAs are the 82s100 array, and 82S105 sequencer, by Signetics, introduced in the late 1970s. The 82S100 was an array of AND terms. The 82S105 also had flip flop functions.

FPGAs use a grid of logic gates, and once stored, the data doesn't change, similar to that of an ordinary gate array. The term "field-programmable" means the device is programmed by the customer, not the manufacturer.

FPGAs are usually programmed after being soldered down to the circuit board, in a manner similar to that of larger CPLDs. In most larger FPGAs the configuration is volatile, and must be re-loaded into the device whenever power is applied or different functionality is required. Configuration is typically stored in a configuration PROM or EEPROM. EEPROM versions may be in-system programmable (typically via JTAG).

The difference between FPGAs and CPLDs is that FPGAs are internally based on Look-up tables (LUTs) whereas CPLDs form the logic functions with sea-of-gates (e.g. sum of products). CPLDs are meant for simpler designs while FPGAs are meant for more complex designs. In general, CPLDs are a good choice for wide combinational logic applications, whereas FPGAs are more suitable for large state machines (i.e. microprocessors).