As an example of the configurable logic blocks (CLBs) of which FPGAs are made, xilinx s FPGA technology is exquisitely documented in their white papers and application notes.14

CLBs implement most of the logic in an FPGA. The principal CLB elements are shown in Figure 13-1. In this example of the Xilinx XC4000E and XC4000X series, two 4-input function generators (F and G) offer unrestricted versatility. Most combinatorial logic functions need four or fewer inputs. However, a third function generator (H) is also provided. The H function generator has three inputs. Either zero, one, or two of these inputs can be the outputs of F and G; the other input(s) are from outside the CLB. The CLB can therefore implement certain functions of up to nine variables, like parity check or expandable-identity comparison of two sets of four inputs. Each CLB contains two storage elements that can be used to store the function generator outputs. However, the storage elements and function generators also can be used independently. In specific FPGA device models these storage elements can be configured as flip-flops and/or optionally as latches. DIN can be used as a direct input to either of the two storage elements. H1 can drive the other through the H function generator. Function generator outputs can also drive two outputs independently of the storage element outputs. This versatility increases logic capacity and simplifies routing.

In this device example, thirteen CLB inputs and four CLB outputs provide access to the...