Timing closure

The Timing closure in VLSI design and electronics engineering is the process by which a logic design of a clocked synchronous circuit consisting of primitive elements such as combinatorial logic gates (AND, OR, NOT, NAND, NOR, etc.) and sequential logic gates (flip flops, latches, memories) is modified to meet its timing requirements. Unlike in a computer program where there is no explicit delay to perform a calculation, logic circuits have intrinsic and well defined delays to propagate inputs to outputs. In simple cases, the user can compute the path delay between elements manually. If the design is more than a dozen or so elements this is impractical. For example, the time delay along a path from the output of a D-Flip Flop, through combinatorial logic gates, then into the next D-Flip Flop input must satisfy (be less than) the time period between synchronizing clock pulses to the two flip flops. When the delay through the elements is greater than the clock cycle time, the elements are said to be on the critical path. The circuit will not function when the path delay exceeds the clock cycle delay so modifying the circuit to remove the timing failure (and eliminate the critical path) is an important part of the logic design engineer's task. Critical path also defines the maximum delay in all the multiple register-to-register paths, and it must not be greater than the clock cycle time. After meeting the timing closure, one way to improve the circuit performance is to insert a register in between the combinational path of the critical path. This might improve the performance but increases the total latency (maximum number of registers from input to output path) of the circuit. Many times logic circuit changes are handled by user's EDA tools based on timing constraint directives prepared by a designer. The term is also used for the goal that is achieved, when such a design has reached the end of the flow and its timing requirements are satisfied. The main steps of the design flow, which may be involved in this process, are logic synthesis, placement, clock-tree synthesis and routing.