Continuous assignment is one of the main constructs used in dataflow modeling. A continuous assignment is always active and the assignment expression is evaluated | Summary Continuous assignment is one of the main constructs used in dataflow modeling. A continuous assignment is always active and the assignment expression is evaluated as soon as one of the right-hand-side variables changes. The left-hand side of a continuous assignment must be a net. Any logic function can be realized with continuous assignments. Delay values control the time between the change in a right-hand-side variable and when the new value is assigned to the left-hand side. Delays on a net can be defined in the assign statement implicit continuous assignment or net declaration. Assignment statements contain expressions operators and operands. The operator types are arithmetic logical relational equality bitwise reduction shift concatenation replication and conditional. Unary operators require one operand binary operators require two operands and ternary require three operands. The concatenation operator can take any number of operands. The conditional operator behaves like a multiplexer in hardware or like the if-then-else statement in programming languages. Dataflow description of a circuit is more concise than a gate-level description. The 4-to-1 multiplexer and the 4-bit full adder discussed in the gate-level modeling chapter can also be designed by use of dataflow statements. Two dataflow implementations for both circuits were discussed. A 4-bit ripple counter using negative edge-triggered D-flipflops was designed. Team LiB Team LiB Exercises 1 A full subtractor has three 1-bit inputs x y and z previous borrow and two 1-bit outputs D difference and B borrow . The logic equations for D and B are as follows D x .y .z x . .z B x .y x .z Write the full Verilog description for the full subtractor module including I O ports Remember that in logic equations corresponds to a logical or operator in dataflow . Instantiate the subtractor inside a stimulus block and test all eight possible combinations of x y and z given in the following
