The present invention relates generally to logic synthesis and timing analysis of integrated circuit (IC) design, and more particularly to a technology for generation of timing exceptions.
In recent years, the size of integrated circuits (ICs) has dramatically increased in both size and number of gates, requiring designers to spend time and effort to meet timing closure for the IC design. Moreover, complexity, speed and deep-submicron effects make timing closure of IC designs a more critical task. In order to enable a designer to achieve accurate timing closure, static timing analyzers and other timing optimization tools are utilized.
In IC design, every path, that originates from either an input port or a register clock pin, must be properly constrained to obtain correct implementation of the register transfer level (RTL hereafter) description. Typically, timing constraints are applied mainly to achieve the following: 1) describing the different attributes of clock signals, such as clock frequency, duty cycle, clock skew, and clock latency; 2) specifying input and output delay requirements of ports relative to a clock transition; and, 3) setting up timing exceptions. Different types of timing exceptions are possible. Some examples of timing exceptions include: set minimum delay, set maximum delay, set disable arc, set false path, set multi-cycle path, and so on.
False paths and multi-cycle paths are timing exceptions which, if not specified, or if not handled correctly, certainly result in not achieving timing closure. False paths are logic paths which cannot be sensitized (1) because they are functionally blocked, or (2) because of delays in reconvergent logic, or (3) because of disabled arcs. As an example,
Multi-cycle paths are paths which intentionally require more than one clock cycle to propagate data. Since this information cannot possibly be inferred by a timing analyzer, multi-cycles paths need to be specified by the designer.
MUX 220-1 selects input 250 when the transition of the gray-code counter is (0, 0), i.e., (FF 210-3, FF 210-4)=(0, 0). Then, flip-flop 210-1 is set to the value at input 250 when (FF 210-3, FF 210-4)=(0, 1). On the other hand, MUX 220-2 selects the output of combinational logic 230 when (FF 210-3, FF 210-4)=(1, 0). Flip-flop 210-2 is then set to the input's value when (FF 210-3, FF 210-4)=(0, 0). Three clocks are required to go from state (0, 1) to state (0, 0). Thus, the path from flip-flop 210-1 to flip-flop 210-2 is a multi-cycle path that uses three clocks cycle to propagate signals. Consequently, the timing constraint of these paths can be relaxed from a single clock cycle to three clock cycles.
In the related art, several techniques are disclosed to perform timing analysis of time exceptions. Examples for such techniques can be found, e.g., in U.S. Pat. Nos. 6,327,692, 6,438,731, 6,532,577, and 6,845,494 and in U.S. application Ser. Nos. 10/166,944, 11/006,349 and 11/063,773 incorporated herein by reference for their useful understanding of the background of the invention, and in particular with respect to the sections of those documents relating to timing analysis of time exceptions. The drawback of these techniques is that timing exceptions (i.e., false and multi-cycle paths) must be manually defined by the designer. As the complexity of digital circuits continues to increase, this approach of having the designer manually define timing exceptions is seen as too time consuming and error-prone. Furthermore, these above-identified prior techniques cannot automatically generate timing exceptions from an RTL description and, thus, such exceptions cannot be verified as early in the design cycle as would be preferred.
It is therefore one object of the invention, among others that will become apparent to the reader, to provide a solution for automatically generating timing exceptions from a RTL level description of an IC design.
Now disclosed, by of a detailed description of some representative simplified examples, is an automated method for generating timing exceptions for integrated circuit (IC) designs. The method includes synthesizing an input register transfer level (RTL) description into a gate-level netlist mapped to a technology library; detection of timing critical paths in the netlist; and determining for each detected timing critical path whether it induces timing exceptions. The timing exceptions generated by the disclosed method include, but are not limited to, multi-cycle paths, clock domain crossing false paths, asynchronous false paths, functional false paths, combinational false paths, sequential false paths, timing false paths, and the like.
At S340, a synthesized netlist is produced by an IC synthesis tool (this may also be referred to as generating a structural netlist). Synthesis tools produce a gate level netlist based on a RTL representation (i.e., the RTL description received in step S310), a timing constraint file (from S320), and a technology library (from S330). A netlist generally includes logic gates such as AND, NAND, NOR, OR, XOR, NXOR, NOT, IC blocks, multipliers, adders, memories, and so on. In addition a netlist includes the interconnection between the logical gates and different blocks. One such synthesis tool is disclosed in a U.S. Pat. No. 6,993,733 entitled “Apparatus and Method for Handling of Multi-Level Circuit Design Data”, assigned to the common assignee and hereby incorporated by reference for all that it contains, and in particular the section dealing with generating a structural netlist. At S350, a process for performing timing optimization is executed.
Referring to
At S430, a static timing analyzer is executed so timing critical paths in the design are detected. The timing critical paths are paths with negative slack. Slack measures how closely a timing constraint is satisfied. Positive slack indicates that a constraint is satisfied with a safety margin equal to the slack value. Circuits with positive slack are usually considered to be over-designed, since the slack indicates that the circuit could either be operated at a higher speed or redesigned to operate at the same speed using less area and/or power. Negative slack indicates that a constraint is unsatisfied and cannot be satisfied unless delays in the circuit are modified by the amount of the slack. At S440, a timing report that includes all timing critical paths detected in the netlist is output. At S450, a file that includes all modifications made in the netlist during timing optimization is generated (i.e., a final mapped netlist).
Referring back to
What now follows is a non-limiting example for the operation of the method for generating timing exceptions.
The solver simulates conditions to determine if a signal can be propagated on P1 and P2. That is, if for a set of input values the values at outputs are changed. It can be noticed that path P1 is active if the input ‘1’ of MUX 520-1 is selected. On the other hand, to propagate a signal on P2, the inputs ‘1’ of both MUXes 520 and 530 must be selected. As defined in the RTL code provided in
Many variations to the above-identified embodiments are possible without departing from the scope and spirit of the invention. Possible variations have been presented throughout the foregoing discussion. Combinations and subcombinations of the various embodiments described above will occur to those familiar with this field, without departing from the scope and spirit of the invention.