Isolation/Removal of Faults During LBIST Testing

This invention is related to computer system chips and in particular to an improved isolation/removal of faults during logic built in self test (LBIST) testing via clock system controls.

LBIST (Logic Built In Self Test) was (and still is) used extensively within IBM on chips to test for both DC and AC faults in the logic and some portions of the arrays. Published materials about LBIST include IBM's TDB v38 n11 of 11-95, dealing with a method an apparatus for handling multiple clock domains within a single logic built-in self test structure. This is but an example of testing during the debug of IBM's system chips. LBIST testing has been used to diagnose and characterize chip problems and uncovered a number of unique failure mechanisms. Two examples of these failure mechanisms are:

1) Coupled noise causing critical signals to change their delay as well as causing latches to change states when certain test patterns were executed.

2) A wide range in process parameters of the chips that were manufactured (for example very strong P-fets with very weak N-fets) stressed some of the dynamic circuits in the arrays, causing intermittent failures.

If these failure mechanisms result in "hard" repeatable faults, LBIST testing could still be used to test the remaining logic since there are known mathematical formulas for determining a new "valid signature" when a "hard" fault exists.

Unfortunately these failure mechanisms resulted in faults that were intermittent and very sensitive to environmental conditions such as power supply and temperature. Hence, LBIST could not be used to test the remaining logic on the chip because we could not determine what the "valid signature" should be. This one intermittent fault could be masking a number of other faults, that may be real design problems, yet LBIST would not be able to test for these faults. This would be desirable.

Our invention provides a way of testing a chip in parts by testing areas where control can be maintained for those sections of the chip that receive system clocks. In the case where intermittent faults are found to exist on the chip, the sections of the chip that produce intermittent faults can be isolated and removed from generating intermittent LBIST signatures. In addition, we can minimize the power supply noise to an acceptable level by maintaining control over those sections of the chip that receive system clocks.

The improvements which we have made achieve a way to use LBIST as a standard industry manufacturing test technique for these problem areas and this approach expands the capabilities of LBIST such that it can be used to diagnose and characterize chip problems by the industry.

These and other improvements are set forth in the following detailed description. For a better understanding of the invention with advantages and features, refer to the description with reference to the following drawing.

FIG. 1 shows schematically an overview of the preferred embodiment of a chip.

(Note: For convenience of illustration, the FIGURE may be separated in parts and as a convention we place the top of the FIGURE as the first sheet, with subsequent sheets proceeding down and across when viewing the FIGURE, in the event that multiple sheets are used.)

Before considering our preferred embodiments in detail, it may be worthwhile to note, by way of example, that in accordance with our invention, if we start LBIST testing the entire chip (i.e. all logic and arrays are getting system clocks) and find an intermittent fault in the L1 cache, we could set the GPTR 10 latches shown in FIG. 1 having bits 1, 2, 3 . . . N such that the L1 cache will no longer receive system clocks during LBIST testing. Hence the logic causing the intermittent failure will no longer receive system clocks and hence will no longer cause intermittent LBIST signatures. LBIST testing can proceed on looking for the next failure, if one existed, or proving that the remaining logic contains no faults.

Turning now to our invention in greater detail, it will be seen from FIG. 1 showing a chip with a basic clock distribution and control system that the chip is divided into a number (N) of functional units with each unit receiving system clocks from its own clock control macro. Each clock control macro receives an oscillator signal and a bit from the GPTR (General Purpose Test Register). All the functional units, the L1 cache array 11, the L1 directory array 12, the Instruction Unit 13, the Execution Unit 14, listed in FIG. 1 and any others on the chip contain latches that are connected into one scan chain (Scan In to Scan Out). FIG. 1 which illustrates our preferred embodiment in which the chip is divided up into a number (N) of functional units (e.g. 11, 12, 13, 14) with each unit's system clock being controlled independently from its own unique clock control macro 15, 16, 17, 18. The clock macro uses the oscillator input 19 to generate system clocks to the functional unit that it is connected to via the main clock distribution unit 20. The GPTR bit is used by the clock control macro to stop or gate-off the system clocks when the GPTR bit is set to a binary `1` value. If and when a functional unit on the chip experiences an intermittent fault, we can simply set the GPTR bit that stops the system clocks to that functional unit. The LBIST test can then be run with the clocks to that functional unit shut off to test the remaining logic on the chip that receives clocks.

As we just described we isolate and remove the logic that was causing an intermittent fail so LBIST could continue to be used to search for the next failure if any existed by using on-chip clock control and distribution logic to individually control the clocks to each unique array and to a number of different logic sections on the chip. A set of scan only GPTR (General Purpose Test Registers) is used to determine what arrays and sections of logic will receive system clocks during LBIST testing.

A sufficient number of GPTR's will be used to divide a chip into many individually controllable sections. Under this design multiple faults can be isolated and removed from the generation of the LBIST signature.

This solution has another advantage that we believe is important for future high performance machines. Since LBIST uses pseudo random patterns to test the chip, the switching activity of the logic under test is on average 50%, which is much higher than the switching activity when the chip is functioning under "normal" operating conditions. As chip speeds and density increase, the amount of instantaneous power supply noise and its influence of the operation of the circuits is becoming a real concern under these very stressful test conditions. By maintaining control of the system clocks to various sections of the chips, one can minimize the power supply noise to an acceptable level.

While we have described our preferred embodiments of our invention, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first disclosed.