Abstract:
An exemplary embodiment of the invention is a method and apparatus for delaying the start of an array built-in self-test (ABIST) until after the ABIST memory arrays have been started. The length of the delay is determined by the value in a programmable delay located on the integrated circuit. The initiation of the ABIST test is delayed by the time specified in the programmable delay.

Description:
FIELD OF THE INVENTION 
     The invention relates to testing integrated circuits, and, more particularly, to a method and apparatus for programmably delaying the start of an array built-in self-test (ABIST) until the ABIST memory arrays have been started and the power supply voltage on the integrated circuit has stabilized. 
     BACKGROUND OF THE INVENTION 
     Array built-in self-test (ABIST) is used to test the memory arrays that are contained in high-end processors. ABIST allows the memory arrays to be tested at and above system clock speeds using a locally generated pattern set that verifies memory array functionality. 
     Conceptually, the ABIST approach is based on the realization that much of a circuit tester&#39;s electronics is semi-conductor based, just like the products it is testing, and that many of the challenges and limitations in testing lie in the interface to the Device Under Test (DUT). The ABIST approach can be described as an attempt to move many of the already semiconductor-based test equipment functions into the products under test and eliminate the complex interfacing. One of the major advantages ABIST has over other means of testing memory arrays is that the operation of the test is self-contained. All of the circuitry required to execute the test at-speed is contained within the integrated circuit. Very limited external controls are needed, so ABIST can be run at all levels of packaging (wafer, TCA, module and system) without requiring expensive external test equipment. 
     ABIST utilizes a boundary-scan design-for-test (DFT) technique. The DFT technique consists of placing a scannable memory element, or boundary-scan chain, adjacent to each integrated circuit I/O so that signals at the integrated circuit boundaries can be controlled and observed using scan operations and without direct contact with the integrated circuit. All internal storage elements are modified such that in test mode they form individual stages of a shift register for scanning in test data stimuli and scanning out test responses. Execution of finite-state-machine ABIST involves initializing the integrated circuit for ABIST, usually through the scannable memory element, and applying a sufficient number of system clocks, either externally or through a self-generated clock, for the finite-state machine to reach its final state. 
     In contrast, execution of programmable ABIST involves scanning the ABIST program to be applied into a custom microcode array, and each instruction is decoded, executed, and applied to the array by the ABIST microprocessor. During the programmable ABIST test, a controller based on a programmable-state machine is used to algorithmically generate a variety of memory test sequences. These test patterns are applied to the embedded memory array at cycle speed. Programmable ABIST, in contrast to finite-state machine ABIST allows for the application of a testing scheme that is flexible enough to help diagnose potential problems, stress memory array performance, and provide production-level testing ability. 
     Over time the memory arrays have become larger and faster and, therefore, consume more power than preceding generations of integrated circuits. When clocks are first applied to the memory arrays during the ABIST test sequence, there is a sudden large current draw from the integrated circuit power supply. Because the integrated circuit power supplies cannot respond with additional current for many microseconds, various levels of capacitors are used to supply the transient currents until the power supplies can respond. During this time, the integrated circuit power supply voltage will start to droop as the capacitors lose charge until the power supply can respond with the required current. The power supply voltage will continue to “ring” until a steady state DC current condition is achieved. 
     CMOS circuit speed is directly affected by power supply voltage. Circuits will run faster with higher voltage and slower with lower voltage. During the initial cycles of the ABIST test, the power supply will be fluctuating and the minimum cycle time for the ABIST test will vary with the power supply voltage. The performance of the memory arrays cannot be accurately measured during this time. 
     BRIEF SUMMARY OF THE INVENTION 
     An exemplary embodiment of the invention is a method and apparatus for delaying the start of an array built-in self-test (ABIST) until after the ABIST memory arrays have been started. The length of the delay is determined by the value in a programmable delay located on the integrated circuit. The initiation of the ABIST test is delayed by the time specified in the programmable delay. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings wherein like elements are numbered alike in the several FIGURES: 
     FIG. 1 depicts the overall flow of the programmable ABIST start delay; 
     FIG. 2 depicts a hardware implementation of the programmable ABIST start delay; and 
     FIG. 3 depicts an alternate hardware embodiment of the programmable ABIST start delay. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 depicts the overall flow of the programmable ABIST start delay. A START_ABIST_TEST signal is received in step  2 . In step  4 , the programmable delay is set to a programmed limit that equals the length of the delay required between the start of the ABIST memory arrays and the start of the ABIST test. This value can be based on how long it will take for the power supply voltage on the integrated circuit to stabilize after the ABIST memory arrays have been started. Next, in step  6 , the clocks for the ABIST memory arrays are started. Then, the programmable delay is checked to see if the programmable delay has reached a predetermined limit. For example, the programmable delay may count down to zero or count up to a stored value. If the programmable delay has not reached the predetermined limit, it continues running at step  10  and then step  8  is repeated. Once the programmable delay reaches the programmed limit, the ABIST clocks are started as shown in step  12 . Last, in step  14  the ABIST test begins. 
     FIG. 2 depicts a hardware implementation of the programmable ABIST start delay. A START_ABIST_TEST signal  102  is applied to initiate ABIST testing. This signal immediately instructs the clock controller  110  to generate a memory array clock signal  112 . The clock controller  110  starts the memory array clocks so that the memory arrays  116  start burning power and drawing current. This START_ABIST_TEST signal  102  also starts a programmable delay  104  that has been programmed with the desired wait time (up to milliseconds) before starting ABIST testing. The programmable delay may be implemented by a counter, the value of which is compared to a programmed limit. Once the programmable delay  104  finishes (e.g., counts down to zero), the programmable delay  104  generates a start ABIST clocks signal  106 . In response to the start ABIST clocks signal  106 , the clock controller  110  provides an ABIST clock signal  114  to the ABIST circuitry  118  to start the ABIST tests. It should also be noted that this implementation is totally contained within the integrated circuit logic and requires no additional external test controls. The programmable delay  104  can be centrally located on the integrated circuit (i.e., accompanying the clock controller)and does not have to be replicated or instantiated with each of the many ABIST clock controllers potentially contained on the integrated circuit. This eliminates the need for the relatively small ABIST controllers each having to contain its own relatively large delay. 
     FIG. 3 shows an alternate implementation where both the memory array and ABIST clocks are started at the same time through an ABIST and memory array clock signal  122 , but start of the ABIST test is delayed. A START_ABIST_TEST signal  102  is applied to initiate ABIST testing. This signal is received by the clock controller  110  which generates the ABIST and memory array clock signal  122 . The memory array  116  and the ABIST circuitry  118  receive the ABIST and memory array clock signal  122 . In response to the clocks starting, the memory arrays  116  start burning power and drawing current. The start of the ABIST test is dependent on receiving an ABIST gate signal  124  from the clock controller  110  and therefore the ABIST does not start in response to the clocks starting. The START_ABIST_TEST signal  102  also starts the programmable delay  104  that has been programmed with the desired wait time (up to milliseconds). Once the programmable delay  104  reaches the programmed limit (e.g., counts down to 0), the programable delay  104  provides a start ABIST gate signal  120  to the clock controller  110 . The clock controller  110  generates the ABIST gate signal  124  which is provided to ABIST circuitry  118  and the ABIST starts. 
     A benefit of delaying the start of the ABIST test until after the ABIST arrays have been initialized is that the test results will be more accurate. As discussed above the performance of the memory arrays cannot be accurately measured when the power supply is fluctuating. Using a programmable delay allows the counter to be set depending on the particular test environment. 
     While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.