Identification and implementation of clock gating in the design of integrated circuits

• US 7,076,748 B2
• Filed: 08/01/2003
• Issued: 07/11/2006
• Est. Priority Date: 08/01/2003
• Status: Active Grant

• Current Assignee: Synopsys Incorporated
• Original Assignee: Atrenta Incorporated (Synopsys Incorporated)
• Inventors: Banerjee, Joy, Kapoor, Bhanu, Churiwala, Sanjay
• Primary Examiner(s): Thompson, A. M.
• Assistant Examiner(s): Rossoshek, Helen
• Application Number:
  US10/631,755
• Publication Number:
  US 20050028118A1
• Time in Patent Office: 1,075 Days
• Field of Search: 716/1, 716/2, 716 4- 7, 716/10, 716/12, 716/17, 716/18, 715/517, 713/400, 713/300, 703/19, 703/14
• US Class Current: 716/103
• CPC Class Codes: G06F 2117/04 Clock gating
• 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 30/396 Clock trees

Abstract

Identification and implementation of clock gating in the design of an integrated circuit (IC) is performed with automated assistance. Electrical power consumption is reduced by clock gating. The automated assistance identifies registers that are candidates for clock gating, and highlights, in the IC design, registers associated with a gated clock domain and the logic blocks driven by these registers.

Claims

1. An integrated circuit (IC) design method, intended for reducing IC power consumption by the efficient identification and implementation of register clock gating in an IC design, comprising:

identifying candidate registers to be clock gated; and

for each of the candidate registers;

determining an output function of the candidate register;

determining at least one gating condition for the candidate register;

determining at least one hold expression for the candidate register;

performing a feedback loop identification step for the candidate register, including identifying a feedback loop that is free of combinational logic elements, except for combinational logic elements having a function that becomes equivalent to a buffer as a result of setting a control value; and

based on the result of the determining of the output function, the gating condition, and the hold expression, and the identification of the feedback loop, clock gating the candidate register.

2. The IC design method as set forth in claim 1, further comprising:

identifying gated clock domains in the IC design; and

highlighting, in a visual display of the IC design, the gated clock domains.

3. The IC design method as set forth in claim 2, wherein highlighting the gated clock domains further comprises highlighting the candidate registers and logic units connected to the candidate registers.

4. The IC design method as set forth in claim 3, wherein the highlighting of the candidate registers and the logic units:

for each gated clock domain, highlighting registers connected to the gated clock;

tracing forward from each highlighted register to detect the logic units affected by clock gating the highlighted register; and

, highlighting the affected logic units.

5. The IC design method as set forth in claim 4, wherein the affected logic units are the logic units in a path between:

the highlighted register, and a next set of registers belonging to a different gated clock domain.

6. The IC design method as set forth in claim 1, wherein the identifying, of the candidate registers to be clock gated, includes selecting a register with a fanout higher than a predetermined threshold.

7. The IC design method as set forth in claim 1, wherein the combinational logic elements include AND gates, OR gates, NAND gates, NOR gates, NOT gates, XOR gates, and multiplexers.

8. The IC design method as set forth in claim 1, wherein the hold expression defines logic conditions for propagating data to the candidate register.

9. The IC design method as set forth in claim 1, wherein the gating condition controls data loading to the candidate register.

10. The IC design method as set forth in claim 9, wherein the step of determining of the at least one gating condition for the candidate register comprises:

identifying control signals controlling the load of data to the candidate register;

for each of the identified control signals;

setting the control signal to a first binary value;

performing a feedback loop determination with the control signal having the first binary value;

when the feedback loop determination indicates a feedback loop, determining that the control signal with the first binary value provides the gating condition;

when the feedback loop determination does not indicate a feedback loop, setting the control signal to the opposite binary value of said first binary value;

performing a subsequent feedback loop determination; and

when the subsequent feedback loop determination indicates a feedback loop, determining that the control signal with the opposite binary value provides the gating condition.

11. The IC design method for claim 1, wherein the identifying of the candidate registers comprises, for each one of the registers in the IC design:

creating a list of logic nets connected to the fanouts of the register;

searching for a feedback loop connected between the fanouts and the input pins of the register; and

when the result of the searching indicates the finding of the feedback loop, determining the register to be a candidate register.

12. The IC design method as set forth in claim 11, wherein the searching for the feedback loop comprises, for each logic net of the list of logic nets:

performing a combinational logic element connection check of the logic net;

when the combinational logic element connection check indicates the logic net is connected to combinational logic, adding the outputs of the combinational logic to the list of logic nets;

performing an input pin identity check for the logic net and the input pins of the register; and

when the input pin identity check result indicates no feedback loop is present for the logic net, removing the logic net from the list of logic nets.

13. The IC design method as set forth in claim 1, wherein the clock gating of the candidate register comprises:

forming a first combinational logic net based on the gating condition and the hold expression;

forming a second combinational logic net based on the output function;

using the first combinational logic net and the clock signal as inputs to a logical AND function;

providing the output of the logical AND to the candidate register; and

connecting the second combinational logic net to the candidate register.

14. The IC design method as set forth in claim 13, wherein the clock signal is used to clock the candidate register.

15. The IC design method as set forth in claim 1, implemented in one of a computer aided design (CAD) system and a CAD program.

16. The method as set forth in claim 1, wherein the identifying of the candidate registers to be clock gated comprises:

creating a list of logic nets connected to the fanouts of the register;

searching for a feedback loop connected between the fanouts and the input pins of the register; and

when the result of the searching indicates the finding of the feedback loop, determining the register to be a candidate register.

17. The method as set forth in claim 16, wherein the searching for the feedback loop comprises, for each logic net of the list of logic nets:

performing a combinational logic element connection check of the logic net;

when the combinational logic element connection check indicates the logic net is connected to combinational logic, adding the outputs of the combinational logic to the list of logic nets;

performing an input pin identity check for the logic net and the input pins of the register; and

when the input pin identity check result indicates no feedback loop is present for the logic net, removing the logic net from the list of logic nets.

18. The method as set forth in claim 17, wherein the feedback loop is free of combinational logic elements, the combinational logic elements include AND gates, OR gates, NAND gates, NOR gates, NOT gates, XOR gates, and multiplexers.

19. The method as set forth in claim 17, wherein highlighting the gated clock domains further comprises highlighting the candidate registers and logic units connected to the candidate registers.

20. A computer program product for enabling a computer system to perform operations for an integrated circuit (IC) design method, intended for reducing IC power consumption by the efficient identification and implementation of register clock gating in an IC design, the computer program product having computer instructions on a computer readable medium, the operations comprising:

identifying candidate registers to be clock gated; and

for each of the candidate registers;

determining an output function of the candidate register;

determining at least one gating condition for the candidate register;

determining at least one hold expression for the candidate register;

performing a feedback loop identification step for the candidate register, including identifying a feedback loop that is free of combinational logic elements, except for combinational logic elements having a function that becomes equivalent to a buffer as a result of setting a control value; and

based on the result of the determining of the output function, the gating condition, and the hold expression, clock gating the candidate register.

21. The computer program product as set forth in claim 20, wherein the operations further comprise:

identifying gated clock domains in the IC design; and

highlighting, in a visual display of the IC design, the gated clock domains.

22. The computer program product as set forth in claim 21, wherein highlighting the gated clock domains further comprises highlighting the candidate registers and logic units connected to the candidate registers.

23. The computer program product as set forth in claim 22, wherein the highlighting of the candidate registers and the logic units:

for each gated clock domain, highlighting registers connected to the gated clock;

tracing forward from each highlighted register to detect the logic units affected by clock gating the highlighted register; and

, highlighting the affected logic units.

24. The computer program product as set forth in claim 23, wherein the affected logic units are the logic units in a path between:

the highlighted register, and a next set of registers belonging to a different gated clock domain.

25. The computer program product as set forth in claim 20, wherein the identifying, of the candidate registers to be clock gated, includes selecting a register with a fanout higher than a predetermined threshold.

26. The computer program product as set forth in claim 20, wherein the combinational logic elements include AND gates, OR gates, NAND gates, NOR gates, NOT gates, XOR gates, and multiplexers.

27. The computer program product as set forth in claim 20, wherein the hold expression defines logic conditions for propagating data to the candidate register.

28. The computer program product as set forth in claim 20, wherein the gating condition controls data loading to the candidate register.

29. The computer program product as set forth in claim 28, wherein the step of determining of the at least one gating condition for the candidate register comprises:

identifying control signals controlling the load of data to the candidate register;

for each of the identified control signals;

setting the control signal to a first binary value;

performing a feedback loop determination with the control signal having the first binary value;

when the feedback loop determination indicates a feedback loop, determining that the control signal with the first binary value provides the gating condition;

when the feedback loop determination does not indicate a feedback loop, setting the control signal to the opposite binary value of said first binary value;

performing a subsequent feedback loop determination; and

when the subsequent feedback loop determination indicates a feedback loop, determining that the control signal with the opposite binary value provides the gating condition.

30. The computer program product for claim 20, wherein the identifying of the candidate registers comprises, for each one of the registers in the IC design:

creating a list of logic nets connected to the fanouts of the register;

searching for a feedback loop connected between the fanouts and the input pins of the register; and

when the result of the searching indicates the finding of the feedback loop, determining the register to be a candidate register.

31. The computer program product as set forth in claim 30, wherein the searching for the feedback loop comprises, for each logic net of the list of logic nets:

performing a combinational logic element connection check of the logic net;

when the combinational logic element connection check indicates the logic net is connected to combinational logic, adding the outputs of the combinational logic to the list of logic nets;

performing an input pin identity check for the logic net and the input pins of the register; and

when the input pin identity check result indicates no feedback loop is present for the logic net, removing the logic net from the list of logic nets.

32. The computer program product as set forth in claim 20, wherein the clock gating of the candidate register comprises:

forming a first combinational logic net based on the gating condition and the hold expression;

forming a second combinational logic net based on the output function;

using the first combinational logic net and the clock signal as inputs to a logical AND function;

providing the output of the logical AND to the candidate register; and

connecting the second combinational logic net to the candidate register.

33. The computer program product as set forth in claim 32, wherein the clock signal is used to clock the candidate register.

34. The computer program product as set forth in claim 20, implemented in one of a computer aided design (CAD) system and a CAD program.

35. A computer system for performing an integrated circuit (IC) design method, intended for reducing IC power consumption by the efficient identification and implementation of register clock gating in an IC design, the computer system having a processor and a memory under control of the processor, the memory including software instructions for performing operations comprising:

identifying candidate registers to be clock gated; and

for each of the candidate registers;

determining an output function of the candidate register;

determining at least one gating condition for the candidate register;

determining at least one hold expression for the candidate register; and

performing a feedback loop identification step for the candidate register, including identifying a feedback loop that is free of combinational logic elements, except for combinational logic elements having a function that becomes equivalent to a buffer as a result of setting a control value; and

based on the result of the determining of the output function, the gating condition, and the hold expression, and the identification of the feedback loop, clock gating the candidate register.

36. The computer system as set forth in claim 35, wherein the identifying, of the candidate registers to be clock gated, includes selecting a register with a fanout higher than a predetermined threshold.

37. The computer system as set forth in claim 35, wherein the gating condition controls data loading to the candidate register.

38. The computer system as set forth in claim 35, wherein the step of determining of the at least one gating condition for the candidate register comprises:

identifying control signals controlling the load of data to the candidate register;

for each of the identified control signals;

setting the control signal to a first binary value;

performing a feedback loop determination with the control signal having the first binary value;

when the feedback loop determination indicates a feedback loop, determining that the control signal with the first binary value provides the gating condition;

when the feedback loop determination does not indicate a feedback loop, setting the control signal to the opposite binary value of said first binary value;

performing a subsequent feedback loop determination with; and

when the subsequent feedback loop determination indicates a feedback loop, determining that the control signal with the opposite binary value provides the gating condition.

39. The computer system for claim 35, wherein the identifying of the candidate registers comprises, for each one of the registers in the IC design:

creating a list of logic nets connected to the fanouts of the register;

searching for a feedback loop connected between the fanouts and the input pins of the register; and

when the result of the searching indicates the finding of the feedback loop, determining the register to be a candidate register.

40. The computer system as set forth in claim 39, wherein the searching for the feedback loop comprises, for each logic net of the list of logic nets:

performing a combinational logic element connection check of the logic net;

when the combinational logic element connection check indicates the logic net is connected to combinational logic, adding the outputs of the combinational logic to the list of logic nets;

performing an input pin identity check for the logic net and the input pins of the register; and

when the input pin identity check result indicates no feedback loop is present for the logic net, removing the logic net from the list of logic nets.

41. The computer system as set forth in claim 35, wherein the clock gating of the candidate register comprises:

forming a first combinational logic net based on the gating condition and the hold expression;

forming a second combinational logic net based on the output function;

using the first combinational logic net and the clock signal as inputs to a logical AND function;

providing the output of the logical AND to the candidate register; and

connecting the second combinational logic net to the candidate register.

42. The computer system as set forth in claim 41, wherein the clock signal is used to clock the candidate register.

43. The computer system as set forth in claim 35, implemented in one of a computer aided design (CAD) system and a CAD program.

44. An integrated circuit visualization tool adapted to implement a method for highlighting gated clock domains and gated registers in a design of an integrated circuit (IC), the visualization tool comprising:

means for displaying the design of the IC;

processing means controlling the means for displaying; and

,

a memory, under control of the processing means, including software instructions adapted to enable the processing means to perform the steps of;

identifying the clock gated domains in the IC'"'"'s design;

for each the gated clock domain, highlighting registers that are connected to the gated clock;

tracing forward from each of the highlighted register to detect logic units affected by clock gating of the highlighted register; and

highlighting the affected logic units;

wherein the memory further comprises software instructions adapted to enable the processing means to identify candidate registers to be clock gated, and, for each of the candidate registers;

determining an output function of the candidate register;

determining at least one gating condition for the candidate register;

determining at least one hold expression for the candidate register;

performing a feedback loop identification step for the candidate register, including identifying a feedback loop that is free of combinational logic elements, except for combinational logic elements having a function that becomes equivalent to a buffer as a result of setting a control value; and

based on the result of the determining of the output function, the gating condition, and the hold expression, and the identification of the feedback loop, clock gating the candidate register.

45. The visualization tool of claim 44, wherein the affected logic units are the logic units in a path between the highlighted register and a next set of registers belonging to a domain different than the gated clock domain.

46. The visualization tool of claim 44, implemented as at least part of a computer aided design (CAD) system and a CAD program.