Method of finding a critical path in a circuit by considering the clock skew

• US 5,608,645 A
• Filed: 03/17/1994
• Issued: 03/04/1997
• Est. Priority Date: 03/17/1994
• Status: Expired due to Fees

• Current Assignee: VLSI Technology, Inc. (Koninklijke Philips N.V.)
• Original Assignee: VLSI Technology, Inc. (Koninklijke Philips N.V.)
• Inventors: Spyrou, Athanasius W.
• Primary Examiner(s): Teska, Kevin J.
• Assistant Examiner(s): Fiul, Dan
• Application Number:
  US08/214,482
• Time in Patent Office: 1,083 Days
• Field of Search: 364/488, 364/489, 364/490, 364/491, 327/232, 324/158, 368/113
• US Class Current: 716/113
• CPC Class Codes: G06F 1/10 Distribution of clock signals , e.g. skew
• CPC Class Codes: G06F 30/327 Logic synthesis; Behaviour synthesis, e.g. mapping logic, HDL to netlist, high-level language to RTL or netlist
• CPC Class Codes: G06F 30/3312 Timing analysis
• CPC Class Codes: G06G 7/122 for optimisation, e.g. least square fitting, linear programming, critical path analysis, gradient method

Abstract

The present invention is directed to a method of designing and fabricating a circuit layout which revolutionizes the manner by which critical weights of a circuit layout are assessed. In accordance with exemplary embodiments, a critical path is assessed on the basis of both a physical delay associated with a data propagation path and with respect to any clock skew which exists with respect to the data propagation path. A critical path can be a path having the shortest physical length from an input node to an output node if the clock skew along this path results in a high probability of a race condition. In accordance with exemplary embodiments, clock skew is assessed by determining the time differential between the arrival of a clock signal at a given data source instance and the arrival of a clock signal at a given data destination instance.

Claims

1. Method for producing a circuit layout having plural signal propagation paths, the method comprising the steps:

attributing a weight to each of said paths, said weight including clock skew information; and

comparing the weight of each of said paths to identify a critical path, said step of attributing a weight further including steps of;

weighting at least one instance using a clock signal path delay and a data path delay; and

subtracting the clock signal path delay of said at least one instance from said data path delay of said at least one instance to determine said weight of said at least one instance.

2. Method according to claim 1, wherein said step of attributing a weight includes the further steps of:

defining at least one node of the circuit layout as a clock source.

3. Method according to claim 2, further comprising the steps of:

determining a clock delay from each clock source to at least one instance located along each of said plural signal propagation paths.

4. Method according to claim 3, wherein said step of determining further includes the steps of:

accessing a netlist of the circuit layout to identify instances along a first clock signal path between the clock source and said at least one instance;

obtaining from memory a predetermined delay for each instance identified along the first clock signal path; and

adding the predetermined delays of all instances identified along the first clock signal path.

5. Method according to claim 4, wherein said step of determining further includes the steps of:

repeating the steps of accessing, obtaining and adding for each additional clock signal path between said clock source and said at least one instance; and

storing, for said at least one instance, an identity of a clock source and a delay determined for a clock signal path between the clock input and the at least one instance.

6. Method according to claim 4, further including the steps of:

determining said data path delay from a data source to a data input of at least one instance of the circuit layout.

7. Method according to claim 6, wherein said step of determining said data path delay further includes the steps of:

accessing said netlist of the circuit layout to identify instances along a first data path between a data source and said at least one instance;

obtaining from memory a predetermined delay for each instance identified along the first data path;

adding the predetermined delays of all instances identified along the first data path;

repeating the steps of accessing, obtaining and adding for each additional data path to said at least one instance; and

storing, for said at least one instance, an identity of a data source and a delay determined for a data path between the data source and the at least one instance.

8. Method according to claim 7, wherein said step of weighting further includes the steps of:

subtracting the clock signal path delay of said at least one instance from said data path delay of said at least one instance to determine said weight.

9. Method according to claim 7, further comprising the steps of:

weighting a data path for each of plural output nodes of the circuit layout;

comparing the weights determined for each of the plural output nodes; and

identifying a data path which has the smallest weight as a critical path of the circuit layout.

10. Method according to claim 9, further comprising the step of:

placing circuit components in the circuit layout to modify the weight of the critical path.

11. Method according to claim 1, further including the steps of:

determining a data path delay from a data source to a data input of at least one instance of the circuit layout.

12. Method according to claim 11, wherein said step of determining further includes the steps of:

accessing a netlist of the circuit layout to identify instances along a first data path between said data source and said at least one instance;

obtaining from memory a predetermined delay for each instance identified along the first data path; and

adding the predetermined delays of all instances identified along the first data path.

13. Method according to claim 12, wherein said step of determining further includes the steps of:

repeating the steps of accessing, obtaining and adding for each additional data path to said at least one instance; and

storing, for said at least one instance, an identity of a data source and a delay determined for a data path between the data source and the at least one instance.

14. Method according to claim 13, further including the steps of:

weighting said at least one instance using a clock signal path delay and said data path delay.

15. Method according to claim 14, wherein said step of weighting further includes the steps of:

subtracting the clock signal path delay of said at least one instance from said data path delay of said at least one instance to determine said weight.

16. Method for producing a circuit layout comprising the steps of:

determining a clock delay along a clock signal path from at least one clock source of the circuit layout to at least one instance of the circuit layout;

determining a data path delay along a data path from at least one data source of the circuit layout to said at least one instance; and

weighting said data path of said at least one instance using said clock delay and said data path delay, said step of weighting further including the step of;

subtracting the clock signal path delay of said at least one instance from said data path delay of said at least one instance to determine said data path weight.

17. Method according to claim 16, further comprising the steps of:

weighting a data path for each of plural output nodes of the circuit layout;

comparing the weights determined for each of the plural output nodes; and

identifying a data path which has the smallest weight as a critical path of the circuit layout.

18. Method according to claim 17, further comprising the step of:

placing circuit components in the circuit layout to modify the weight of the critical path.