Digital Electronics: Principles, Devices and Applications

Programmable Logic Devices 317Table 9.2 Function table for example 9.3.A 1 A 0 B 1 B 0 Output 1 Output 2 Output 3 Output 40 0 0 0 1 0 0 00 0 0 1 0 1 1 00 0 1 0 0 1 1 00 0 1 1 0 1 1 00 1 0 0 0 1 0 10 1 0 1 1 0 0 00 1 1 0 0 1 1 00 1 1 1 0 1 1 01 0 0 0 0 1 0 11 0 0 1 0 1 0 11 0 1 0 1 0 0 01 0 1 1 0 1 1 01 1 0 0 0 1 0 11 1 0 1 0 1 0 11 1 1 0 0 1 0 11 1 1 1 1 0 0 0Output 3less than = A 1 B 1 + A 1 A 0 B 0 + A 0 B 1 B 0 (9.11)Output 4Greater than = A 1 B 1 + A 1 A 0 B 0 + A 0 B 1 B 0 (9.12)Examination of minimized Boolean expressions (9.9) to (9.12) reveals that there are 12 differentproduct terms to be accounted for. Therefore, a PLA device with 12 AND gates will meet therequirement. Also, since there are four outputs, we need to have four OR gates at the output. Figure9.17 shows the programmed PLA device. Note that, in the programmed PLA device, an unprogrammedinterconnection indicated by a cross (× is a ‘make’ connection.9.5 Programmable Array LogicThe programmable array logic (PAL) device is a variant of the PLA device. As outlined in Section9.2, it has a programmable AND gate array at the input and a fixed OR gate array at the output.The idea to have a fixed OR gate array at the output and make the device less complex originatedfrom the fact that there were many applications where the product-term sharing capability of the PLAwas not fully utilized and thus wasted. The PAL device is a trademark of Advanced Micro DevicesInc. PAL devices are however less flexible than PLA devices. The flexibility of a PAL device can beenhanced by having different output logic configurations including the availability of both OR (alsocalled active HIGH) and NOR (also called active LOW) outputs and bidirectional pins that can actboth as inputs and outputs, having clocked flip-flops at the outputs to provide what is called registeredoutputs. These features allow the device to be used in a wider range of applications than would bepossible with a device with fixed input and output allocations. The mask-programmed version of PALis known as the HAL (Hard Array Logic) device. A HAL device is pin-to-pin compatible with its PALcounterpart.