# Patent

## Title
Dynamic critical path detector for digital logic circuit paths

## Abstract

Method for correcting timing failures in an integrated circuit and device for monitoring an integrated circuit. The method includes placing a first and second latch near a critical path. The first latch has an input comprising a data value on the critical path. The method further includes generating a delayed data value from the data value, latching the delayed data value in the second latch, comparing the data value with the delayed data value to determine whether the critical path comprises a timing failure condition, and executing a predetermined corrective measure for the critical path.

## Background
FIELD OF THE INVENTION
The invention generally relates to methods and devices to monitor an integrated circuit, and in particular to monitor and correct for process variation in semiconductor chips, e.g., at worse case conditions.
BACKGROUND OF THE INVENTION
Semiconductor integrated circuit chips are normally designed in view of process variations in forming the circuits. Specifically, process variations are presumed, and semiconductor integrated circuit chips are designed such that they will operate reliably for desired performance within the presumed range of process variation. However, since it is difficult to presume device performance variations, the period of time required to design semiconductor integrated circuit chips is increased, and it is necessary to give timing margins to allow semiconductor integrated circuits to operate in worst-cases, the semiconductor integrated circuit chips thus designed tend to suffer performance reductions.
In view of this process variation in today's technologies, additional design time is likewise required in order to close timing at worst-case conditions, e.g., in the classic four timing corners, on integrated circuit chips. In the classic four timing corners, the four corners are worst-case process and worst-case temperature and voltage; worst-case process and best-case temperature and voltage; best-case process and worst-case temperature and voltage; and best-case process and best-case temperature and voltage. The best-case process with high voltage and low temperature yields fast switching circuits, while worst-case process with low voltage and high temperature yields slow switching circuits. While very few chips are ever produced or operated in these worst-case extremes, designers design for the rare event in which the chips are produced or operated in these extremes. Moreover, because many of the gates formed on the integrated circuit chips are built with larger more powerful FETs to ensure the chips will close timing at worst-case conditions, additional power is required on the chips.
Today, there are several different methods to maintain integrated circuits as operational when process variation causes the chips to be operated out of their specifications, i.e., at worst-case condition. These methods include, but are not limited to, raising the voltage, reducing the frequency, back bias, etc. These methods can be applied for the life of the chip or only after a specified period of time has elapsed.
Other solutions in the marketplace today monitor the chip or areas of the chip performance in order to minimize the chip power. These solutions use performance scan-ring oscillators (PSROs) to monitor performance and to insure the power on the chip stays below a predetermined level. However, as it is the PSRO on the chip being monitored rather than the critical paths themselves, this method results in a very coarse measurement. Further, as other solutions monitor how much margin is in the path for sorting purposes, the path is not continuously monitored.
As a result of the above-noted methods, the chips designed to address production or operation in the worse case extremes needlessly waste power, area and time.
SUMMARY OF THE INVENTION
The invention is directed to a method for correcting timing failures in an integrated circuit. The method includes placing a first and second latch near a critical path. The first latch has an input including a data value on the critical path. The method further includes generating a delayed data value from the data value, latching the delayed data value in the second latch, comparing the data value with the delayed data value to determine whether the critical path includes a timing failure condition, and executing a predetermined corrective measure for the critical path.
According to aspects of the invention, a device for monitoring an integrated circuit includes a first latch arranged in a region of a critical path and structured to receive a data signal, a second latch arranged in a region of the critical path, a delay element structured and arranged to couple a delayed version of the data signal to the second latch, and a comparator device structured and arranged to compare outputs of the first and second latches. A miscompare from the comparator device is indicative of an approaching timing failing condition.
According to other aspects of the invention, a method for monitoring an integrated circuit includes applying a data signal to a first latch, applying a delayed version of the data signal to a second latch, and comparing outputs of the first and second latches. A miscompare from the comparing of outputs is indicative of an approaching timing failing condition in a path in a region of the first and second latches.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a circuit according to the invention for monitoring whether paths are near failing;
FIG. 2 illustrates timing diagrams for a passing path;
FIG. 3 illustrates timing diagrams for a failing path;
FIG. 4 illustrates a chip-level view of an integrated circuit chip according to the invention;
FIG. 5 illustrates an exemplary flow diagram for determining whether a chip is operational;
FIG. 6 illustrates an exemplary flow diagram for a calibration process; and
FIG. 7 illustrates an exemplary flow diagram for a monitoring section.



DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
According to the invention, the integrated circuit chip can check its own critical paths and make adjustments to ensure and/or maintain proper operation of the chip. By way of example, timing the integrated circuits closer to the nominal point will produce a less power hungry chip and will decrease the time to market. In order to account for the chips that end up in the worse case conditions, the critical paths can be monitored so any corrective action can be taken before the chip fails. This can ensure the chip is able to run at its desired cycle time. Further, the chip may consume less power and may require less time to get into manufacturing.
In order to time chips closer to nominal conditions, the invention monitors areas throughout the chip to determine whether chip timing for a particular path is close to failing, and, if so, takes corrective action before the path fails timing. The invention relates to identifying when and where to apply corrective measures to maintain a chip operation. According to embodiments, the invention can determine which paths are getting close to or approaching failure and correct those areas of the chip. By way of example, the invention can utilize a circuit designed to detect when margin is being lost in a critical path and to enable corrective action to be taken.
According to the invention, designers can concentrate on the area in which the vast majority of the chips will operate, rather than on the other areas in which only a minority of the chip will operate. As a result, a smaller chip die can be realized, as well as lower power consumption due to lower voltage requirements, a faster time to market, and better yields.
FIG. 1 illustrates circuit 10, e.g., a monitoring latch, that can be formed or arranged on the integrated circuit chip to monitor whether a particular path is getting close to failing. Circuit 10 includes a latch pair composed of data flip flops L1 11 and L2 12 and a latch pair composed of delayed data flip flops L1′ 13 and L2′ 14. Moreover, latches L1 11 and L1′ 13 are arranged to receive a data signal from line data-in 15 and clocking signals from L1 clock 16. A delay element 18, e.g., an adjustable delay element, is arranged to output a delayed version of the data signal to latch L1′ 13. A comparator element 19 is arranged to compare the outputs of latches L1 11 and L1′ 13 and the output of comparator element 19 is coupled to latch L2′ 14, while the output of latch L1 11 is coupled to latch L2 12. Latches L2 12 and L2′ 14 are coupled to receive clocking signals from L2 clock 17 and to output a latched data signal 20 and a miscompare signal 21, respectively.
According to the invention, paths on the integrated circuit chip are tested to determine whether any are approaching failing timing. In embodiments, circuit 10 operates to compare the data captured by latch L1 11 to the delayed version of the data captured by latch L1 11, i.e., in latch L1′ 13. Further, FIGS. 2 and 3 illustrate timing diagrams for the clock, data-in, delayed data-in, and miscompare signals in a passing case and failing case, respectively.
As depicted in the timing diagrams of FIG. 2, the data in signal input to latch L1 11 is delayed a time D by delay element 18 to form the delayed data in signal input to latch L1′ 13. As the data in signal and the delayed data in signal each occur within the same clock pulse, the miscompare signal is output low, indicating the path passes. More specifically, a clock pulse is defined by edges of a clock signal. A clock edge is a transition in a clock signal from either a low voltage to a high voltage or a high voltage to a low voltage. For example, in FIGS. 2 and 3 a first clock pulse is defined by leading edge 25 and trailing edge 30. A second clock pulse is defined by leading edge 35 and trailing edge 40, wherein a leading edge of one clock pulse can be a trailing edge of another clock pulse. In FIG. 2, the output of latch Ll 11 and the output of the latch L1′ 13 occur within a same clock pulse because the output of the latch L1 11 and the output of the latch L1′ 13 occur between the leading edge 25 and the trailing edge 30 of the first clock pulse. In other words, a path passes or is within the acceptable operating parameters when the signal in arrives at least D, i.e., the delay time, before the leading transition of the first clock pulse.
Like the timing diagrams of FIG. 2, the timing diagrams of FIG. 3 show the data in signal input to latch L1 11 is delayed a time D by delay element 18 to form the delayed data in signal input to latch L1′ 13. However, in contrast to the passing case example in FIG. 2, the data in signal and the delayed data in signal occur in different clock pulses, so the miscompare signal is output high, which can be indicative of a path getting close to failing timing. More specifically, the output of the latch L1 11 and the output of the latch L1′ 13 occur within different clock pulses when the output of the latch Ll 11 occurs before the leading edge 25 of the first clock pulse and the output of the latch L1′ 13 occurs after the trailing edge 40 of the second clock pulse. Thus, if the signal in arrives later than D, i.e., the delay time, from the transition to the next clock pulse, the outputs of L1 11 and L1′13 will not be equal, so the path will be identified as getting close to failing or operating outside acceptable operating parameters.
Thus, the invention allows critical paths to be measured in real time and to monitor individual paths rather than merely sections of the chip. This offers finer granularity and on the fly prediction of failing paths which allows corrective action to be taken before the path results in data being corrupted in the chip.
In addition to the detection of a miscompare, the invention also relates to corrective action that can be taken before the chip fails. By way of example, if the delay element 18 is a variable delay element, then a determination can be made at test as to how much margin the paths have. This information can be utilized in selecting one of the corrective actions.
According to an embodiment of the invention, if, while the chip is running, the monitor detects a path getting close to failing, a corrective action can be taken before the timing fails and causes a logic error. After a period of time after the corrective action, the path may be checked again and, if the timing is no longer close to failing, the corrective measure can be removed. Such an instance may occur, e.g., when a temperature rising in a certain region of the chip causes the monitor to detect a path in the region as getting close to failing. The corrective action could be, e.g., to raise the voltage to the region. Then, after a period of time, the temperature in the region may have decreased so that the monitor no longer detects a close to failing condition for the path. Thus, the invention ensures the chip maintains its frequency requirement while also running at a lowest possible power.
While the invention has been discussed with regard to an exemplary path and monitoring device, it is understood a number of monitoring units can be arranged throughout the integrated circuit chip to monitor potentially troublesome paths. By way of example, FIG. 4 illustrates a chip-level view of an integrated circuit chip 20 composed of a Central Monitoring Unit (CMU) 21 to control the monitoring of paths A-E on the chip. Paths A-E can be critical paths that have been selected for monitoring. CMU 21 is responsible for, e.g., calibrating the chip, monitoring each path to be sure the paths are being exercised and to determine whether any of the paths are getting close to failure, taking corrective actions, which can be applied on local or global levels, and setting the chip up after reset. By way of example, the corrective actions can include, but are certainly not limited to, adjusting back bias, adjusting pipeline depth, turning hybrid current mode logic (CML) circuits from single ended to dual ended, and increasing voltage.
Further, it is noted when a number of paths without a lot of margin are arranged to go through a central “pinch point,” the pinch point can be monitored for approaching failure. Circuits 10 for each monitored path can be coupled to CMU 21. In this regard, CMU 21 can further change the margin and/or set the delay time in circuits 10.
In an exemplary implementation, when the integrated circuit chip according to the invention powers up, it will need to make sure that all the paths are functional. The reason is, if the data-in signal path does not get to L1 11 in time to be captured, the delayed version of the data-in signal will also not capture the correct logic level. As a result, the comparison of the L1 11 and L1′13 outputs will indicate the path is functioning correctly, when, in fact, it may not. To ensure all the paths are functional when the hardware comes back, an at-speed built-in self test (BIST) could be run to ensure all of the paths on the chip are functional. If the tests fail, then corrective actions can be taken.
An exemplary flow diagram 500 for testing the chip at power up is illustrated in FIG. 5. Flow diagram 500 can perform basic checks to ensure the chip is operational before a calibration step. The chip will set up or load all its default settings at step 501 before running a chip-level at-speed BIST at step 502. If the test passes, then the chip can move on to calibrating the individual paths at step 507. An exemplary calibration procedure is described below with reference to FIG. 6. If at step 503 a determination is made the tests fails, the chip will check to see if all possible corrective actions have been taken at step 504. If all possible corrective actions the chip can make have been made, a determination is made at step 505 that the chip is non-functional and must be replaced. If the determination at step 503 is that all the corrective actions have not yet been taken, the chip will apply a corrective action at step 506 and retest the chip at step 502. This corrective action loop can continue until all corrective actions have been used without success or the test passes at step 503. Since the CMU is monitoring the whole chip, the corrective actions could start locally and move to a chip-wide level. For example, a first corrective action could be to change the local back bias on the circuit close to failing. Moreover, the CMU would also be able to take more chip-wide actions such as raising the voltage to multiple paths when it is determined they are getting close to the point of failure.
In addition to simply monitoring critical paths and correcting them, the present invention provides for exercising the critical paths so, when the critical paths occur, the delays have not degraded to points where the paths miss by amounts greater than or equal to the delays. If this degradation were to happen, the data signal comparison in the latches L1 11 and L1′ 13 would not detect the failure. Accordingly, it is advantageous to periodically check the critical paths, which can be done, by way of example, in the following manners:

    
    
        1) Training cycle. At predetermined intervals, a pattern can be run to exercise the critical path. This method would be a performance hit in some sections of the chip, since the chip would not be performing useful work during these training cycles. These training cycles can be very useful in sections of the chip that are not often used, but when needed must work, e.g., floating point units. The section of the chip containing the path would have to be removed from functional operation before the training cycle is run and returned to functional operation after the training cycle has been performed.
        2) Monitor and train. The paths can be monitored to see if the critical paths were covered. If a predetermined period of time passes without critical paths being exercised, then the chip can run a training sequence for that section of logic.
        3) Design. When a chip is designed to exercise the paths often, the path does not need to be checked, e.g., a cache miss line. If the path is not used very often, then the path is not important to the performance of the chip. Thus, breaking the path into multiple cycles shouldn't be problematic.
    
    


The present invention also provides a process for picking paths for monitoring. In synthesis, the miscompare latch can be used as the target latch. After initial placement and timing, the miscompare latch can be switched for normal latch pairs, e.g., L1/L2 latch, for all latches having a large amount of margin. The monitor latch's compare lines can be connected to a central location where a controller would look for miscompares and take corrective action. By using this method, either a periodic training period can be utilized or an assumption can be made you are covering all the critical paths you will detect when the chip is starting to lose some of its timing margin.
Further, a predetermined number of paths can also be selected, e.g., before synthesis or after initial placement, and it may be preferable to pick the paths before synthesis, which may provide more flexibility in picking which path will be monitored to insure that the critical path is executed. This can be advantageous in that the controller can be designed and/or built to handle a smaller number of paths. This method also allows the advantage of ensuring the monitoring of paths from different sections of the chip and/or on different clock domains.
Once the critical path detectors have been placed, they can be used at test to determine the amount of margin the chip has and then the voltage, back bias, frequency, etc. can be adjusted to give the fastest clock and the lowest voltage. Moreover, while the chip is operational, the critical path detectors can be used to determine if sections of the chip are getting close to not meeting the timing requirements and corrective action can be taken before data becomes corrupted. To accomplish this, the invention can further utilize a calibration step and a monitor and correction step.
A flow diagram for a calibration process 600 is shown in FIG. 6. This process can be utilized to set up the amount of margin for each of the paths. At step 601, all delays may be set to the Max value and the variable (X) that keeps track of which path the central monitor is working on is set to 0. An at-speed BIST is run at step 602, and a determination is made at step 603 whether all paths pass. When all paths pass, control can move to a monitoring section at step 607. An exemplary monitoring procedure is described below with reference to FIG. 7. When all paths do not pass, each failing path can be checked to see if the delay is at the minimum at step 604. If the delay is at the minimum value, a corrective action can be taken at step 605 in a region where the failing path is situated. If the delay is not at the minimum, then the delay will be reduced by 1 step on each failing path at step 606, and the path will be tested again at step 602. The corrective actions can be a series of actions, e.g., the first time the voltage can be increased and the next time back bias applied, etc.
An exemplary process 700 for the monitoring section referred to above in step 607 is depicted in FIG. 7. In the monitoring section, the paths can be monitored to determine whether the paths are near failing and whether corrective action is necessary. The corrective actions can be, e.g., local to begin with and more global if multiple paths start to fail. When a determination is made at step 607 of the calibration process that all paths passed, the monitoring section determines at step 701 whether it is time to check the paths. In this regard, the determination can always be yes, which can allow for constant monitoring of the paths. When not constantly monitored, and it is not time to check the paths, the process returns to step 701. Thus, in contrast to the constant monitored paths, the process at step 701 can keep track of conditions such as temperature or activity and only start checking the paths in particular areas if the temperature or activity increase. By way of example, if it is assumed no data is flowing through a particular area of the chip, so long as the temperature is not increasing in this area, it may not be necessary to check the paths in this area of the chip. At step 702, a determination is made whether all paths pass. When all paths pass, the process returns to step 701. If a path is seen to be failing then a determination is made whether all corrective actions have been taken at step 703. If all corrective actions have not been made, a corrective action is taken in a region where the failing path is located at step 704 and then the path would be rechecked at step 702 to insure that all paths now pass. The corrective actions can be a series of actions, e.g., the first time the voltage could be increased and the next time back bias applied, etc. When all corrective actions have been used, a determination is made at step 705 that the chip is non-functional.
The circuit as described above is part of the design for an integrated circuit chip. The chip design is created in a graphical computer programming language, and stored in a computer storage medium (such as a disk, tape, physical hard drive, or virtual hard drive such as in a storage access network). If the designer does not fabricate chips or the photolithographic masks used to fabricate chips, the designer transmits the resulting design by physical means (e.g., by providing a copy of the storage medium storing the design) or electronically (e.g., through the Internet) to such entities, directly or indirectly. The stored design is then converted into the appropriate format (e.g., GDSII) for the fabrication of photolithographic masks, which typically include multiple copies of the chip design in question that are to be formed on a wafer. The photolithographic masks are utilized to define areas of the wafer (and/or the layers thereon) to be etched or otherwise processed. Moreover, the process as described above is used in the fabrication of integrated circuit chips.
The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor.
While the invention has been described in terms of the identified embodiments, those skilled in the art will recognize that the invention can be practiced with modifications within the spirit and scope of the appended claims.

## Claims

1. A method for correcting timing failures in an integrated circuit comprising:
placing a first latch and a second latch near a critical path, wherein the first latch has an input comprising a data value on the critical path;
generating a delayed data value from the data value;
latching the delayed data value in the second latch;
comparing an output of the first latch with an output of the second latch to determine whether the critical path comprises a timing failure condition; and
using a controller to execute a predetermined corrective measure for the critical path,
wherein:
the determination of whether the critical path comprises the timing failure condition comprises determining whether the output of the first latch and the output of the second latch occur within a same clock pulse or different clock pulses;
the output of the first latch and the output of the second latch occur within a same clock pulse when the output of the first latch and the output of the second latch occur between a leading edge and a trailing edge of the same clock pulse; and
the output of the first latch and the output of the second latch occur within different clock pulses when the output of the first latch occurs before a leading edge of a first clock pulse and the output of the second latch occurs after a trailing edge of a second clock pulse.

2. The method according to claim 1, wherein the predetermined corrective measure comprises at least one of global and local corrections.

3. The method according to claim 1, wherein, if the critical path still comprises the timing failure condition after executing the predetermined corrective measure for the critical path, then the chip is determined to be non-functional.

4. The method according to claim 1, wherein, once the critical path does not comprise the timing failure condition, the method further comprises performing a calibration process.

5. The method according to claim 1, further comprising adjusting a delay before generating the delayed data value from the data value.

6. The method of claim 1, wherein:
if the output of the first latch and the output of the second latch occur within the same clock pulse then this is indicative that the critical path does not have the timing failure condition, and
if the output of the first latch and the output of the second latch occur within different clock pulses then this is indicative that the critical path does have the timing failure condition.

7. The method of claim 6, wherein the delayed data value is delayed from the data value by a predetermined time D, and the critical path does not have the timing failure condition when the output of the first latch occurs at least D before the leading edge of the clock pulse and the critical path does have a timing failure condition when the output of the first latch occurs later than D before the leading edge of the clock pulse.

8. A device for monitoring an integrated circuit comprising:
a first latch arranged in a region of a critical path and structured to receive a data signal;
a second latch arranged in a region of the critical path;
a delay element structured and arranged to couple a delayed version of the data signal to the second latch; and
a comparator device structured and arranged to compare outputs of the first latch and the second latch,
wherein:
a miscompare from the comparator device occurs when the output of the first latch and the output of the second latch occur within different clock pulses and the miscompare is indicative of an approaching timing failing condition; and
the output of the first latch and the output of the second latch occur within different clock pulses when the output of the first latch occurs before a leading edge of a first clock pulse and the output of the second latch occurs after a trailing edge of a second clock pulse.

9. The device according to claim 8, further comprising a device for taking corrective action to avoid the approaching timing failing condition.

10. The device according to claim 9, wherein the corrective action comprises one of global and local actions.

11. The device according to claim 10, wherein the corrective actions comprises at least one of voltage adjustment, back biasing adjustment, and frequency adjustment.

12. The device according to claim 8, further comprising a central monitoring unit coupleable to a plurality of critical paths.

13. The device according to claim 12, wherein the plurality of the critical paths include first and second latches and a comparator device to compare data signals from the first latches to delayed versions of the data signals from the second latches.

14. The device according to claim 13, wherein the plurality of critical paths include delay elements and the central monitoring unit is structured and arranged to adjust a delay for each of the delay elements.

15. The device according to claim 12, wherein the central monitoring unit is structured and arranged to continuously monitor the plurality of critical paths.

16. The device according to claim 12, wherein the central monitoring unit is structured and arranged to remonitor the plurality of critical paths after a period of time.

17. A method for monitoring an integrated circuit comprising:
applying a data signal to a first latch;
applying a delayed version of the data signal to a second latch; and
using a controller to compare outputs of the first latch and the second latch, wherein:
a miscompare from the comparing of outputs occurs when the output of the first latch and the output of the second latch each occur within different clock pulses and the miscompare is indicative of an approaching timing failing condition in a path in a region of the first latch and the second latch; and
the output of the first latch and the output of the second latch occur within different clock pulses when the output of the first latch occurs before a leading edge of a first clock pulse and the output of the second latch occurs after a trailing edge of a second clock pulse.

18. The method according to claim 17, wherein, when a miscompare is detected, the method further comprises taking corrective action to avoid the approaching timing failing condition.

19. The method according to claim 18, wherein the corrective action comprises one of global and local actions.

20. The method according to claim 17, wherein, when a miscompare is not found, the path passes.

21. The method according to claim 17, wherein a central monitoring unit is structured and arranged to monitor the path and to adjust a delay utilized in forming the delayed version of the data signal.

22. The method according to claim 17, wherein, when the path is not used often, the method includes exercising the path.