The applets in this chapter demonstrate three different topics of programmable logic devices. Given the ever-higher integration of semiconductor devices, the high development costs of full-custom designs, and factors like intellectual property protection, programmable logic devices are often the best solution to implement custom logic functions. The following applets demonstrate the basic architecture of the still popular PLA and GAL devices, and the logic cell underlying multiplexer-based FPGAs. Unfortunately, the planned applets on lookup-table or SRAM-based FPGAs are not ready yet.

The first applet demonstrates the structure and organization of the so-called programmable logic array. A PLA allows creating a user-defined function based on a two-level AND-OR expansion, with the connections between input and output terms via user-programmable connections. In the applet, a fixed example connection is used.

The next five applets demonstrate the so-called generic array logic or GAL architecture. A GAL device consists of several basic cells, each of which include a programmable PLA matrix and a programmable output-cell with a flipflop. As the output of the flipflop is fed back into the PLA matrix, a GAL can be used to implement combinatorical circuits (flipflops disabled), registers, and general sequential circuits. In the applets, click on the special 'interactive transistors' to enable or disable the corresponding connection. This allows you to change the logic functions of the GALs and to create your own circuits.

The final group of four applets demonstrates the multiplexer-based logic cell used in the ACT1 family of programmable devices.

	PLA (programmable logic array)	applet	webstart	print
	GAL output cell	applet	webstart	print
	GAL demonstration: basic logic functions	applet	webstart	print
	GAL demonstration: complex functions	applet	webstart	print
	GAL demonstration: counter	applet	webstart	print
	GAL demonstration: traffic light	applet	webstart	print
	ACTEL ACT1 mux-based logic cell	applet	webstart	print
	ACTEL ACT1 mux-based AND3 gate	applet	webstart	print
	ACTEL ACT1 mux-based XOR2 gate	applet	webstart	print
	ACTEL ACT1 mux-based SR-flipflop	applet	webstart	print