Digital Electronics: Principles, Devices and Applications

Programmable Logic Devices 351I/O BlocksLogicBlockRowsRoutingChannelsI/O BlocksI/O BlocksI/O Blocks(a)InputMultiplexer-basedCircuit BlockOutputInputs(b)Figure 9.41Actel FPGA.of traditional gate arrays comprising logic blocks arranged in horizontal rows with horizontal routingchannels between adjacent rows, as shown in Fig. 9.41(a). Actel chips also have vertical wires thatoverlay the logic blocks to provide signal paths that span multiple rows. These are not shown in Fig.9.41(a). The logic block is not LUT based. Instead, it comprises an AND gate and an OR gate feedinga multiplexer circuit block, as shown in Fig. 9.41(b). The multiplexer circuit, along with the two gates,can realize a large range of logic functions. In the case of Act-3 FPGAs, 50 % of the logic blocks alsocontain a flip-flop.