Digital Electronics: Principles, Devices and Applications

Programmable Logic Devices 327Input-1Input-2OLMCInput/Output-1E 2 CMOSProgrammableAND-ArrayOLMCInput/Output-2Input-nOLMCInput/Output-3(a)FromProgrammableAND-ArrayDFFQQ4-to-1MUXI/OToProgrammableAND-Array2-to-1MUXS1S0S1(b)Figure 9.23(a) Generic array logic generalized block schematic and (b) architecture of an OLMC.array is not just programmable as is the case in a PAL device but is reprogrammable. That is, itcan be reprogrammed any number of times. This has been made possible by the use of electricallyerasable PROM cells for storing the programming pattern. The other difference is in the use ofreprogrammable output logic, which provides more flexibility to the designer. GAL devices employoutput logic macrocells (OLMCs) at the output, which allows the designer to configure the outputseither as combinational outputs or registered outputs.