Abstract:
A test circuit is capable of testing functions of a microprocessor without involving any performance penalty or substantial increase in area overhead. The test circuit includes a test control register for providing test instructions to an instruction decoder of the microprocessor, a first multiplexer for selecting either the test instructions from the test control register or instructions from an instruction fetch unit, a linear feedback shift register for providing test operand to an instruction execution unit of the microprocessor wherein the test operand is random data for executing the instruction execution unit multiple times per instruction from the test control register, a second multiplexer for selecting either the test operand from the linear feedback shift register or operand from the main memory, a multi-input feedback shift register for receiving results from the instruction execution unit, and a controller for providing the test instruction to the test control register and the linear feedback shift register and evaluating an output signature of the multi-input feedback shift register.

Description:
FIELD OF THE INVENTION 
     This invention relates to a circuit structure and method for testing microprocessors and microcontrollers, and more particularly, to a circuit structure and method for testing microprocessors and microcontrollers in an IC chip or circuit board with substantially no increase in hardware overhead. 
     BACKGROUND OF THE INVENTION 
     Microprocessor and microcontroller (hereinafter “microprocessor”) testing is considered one of the most complex problem in IC testing. In general, an automatic test equipment (ATE) such as an IC tester is commonly used for testing a microprocessor. An IC tester provides a test pattern to the microprocessor under test and the resultant response of the microprocessor is evaluated by expected value data. Because the recent microprocessors have dramatically improved their performance, such as operating speeds, density, functionality, and pin counts, an IC tester for testing such microprocessors needs to be very large scale, high speed, and accordingly very expensive. For example, such an IC tester has several hundreds or more test pins (test channels), each of which includes a pattern generator, timing generator and a frame processor, resulting in a very large and high cost system. 
     In other approach, various design-for-test (DFT) and built-in self-test (BIST) schemes such as scan, partial scan, logic BIST, scan-based BIST are used to test various logic blocks within a microprocessor. The main problem in these approaches is the requirement of large amount of additional hardware area (extra logic circuits) to implement the test logic. For example, scan implementation in general requires approximately 10% area overhead and scan-based BIST requires approximately 10-15% area overhead on top of the scan implementation. This large area overhead causes larger die, which results into smaller number of dies per wafer, lower yield and higher cost. 
     In addition, these test schemes also cause a 5-10% performance penalty. Typically, such a performance penalty is a signal propagation delay in the microprocessor because of the additional hardware overhead in the microprocessor. For example, in the scan implementation, each flip-flop circuit in the microprocessor is preceded by a selector (multiplexer) to selectively provide the flip-flop either a scan-in signal or a normal signal. Such an additional selector causes a delay time in the overall performance of the flip-flop circuit. Thus, the design-for-test and built-in self-test schemes adversely affect the microprocessor&#39;s performance, such as an operating speed because of the signal propagation delays. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to provide a method and structure for testing microprocessors without having the disadvantages involved in the conventional technologies. 
     It is another object of the present invention to provide a method and structure for testing microprocessors which require substantially no hardware overhead in the IC chip. 
     It is a further object of the present invention to provide a method and structure for testing microprocessors which involve no performance penalty in the microprocessors to be tested. 
     It is a further object of the present invention to provide a method and structure for testing microprocessors which are capable of testing the microprocessors with low cost and high efficiency. 
     In the present invention, three extra registers are added to the periphery of the microprocessor and random test patterns are provided to the microprocessor and the response of the microprocessor is compressed before being compared with the pre-computed signature. 
     The present invention, for testing a microprocessor having an instruction fetch unit, an instruction decoder, a system memory, and an instruction execution unit, is comprised of: 
     a test control register for providing instructions to the instruction decoder of the microprocessor during testing; 
     a first multiplexer for selecting either the test instructions from the test control register or instructions from the instruction fetch unit; 
     a linear feedback shift register for providing test operand to the instruction execution unit of the microprocessor; 
     a second multiplexer for selecting either the test operand from the linear feedback shift register or operand from the system memory; 
     a multi-input feedback shift register for receiving results from the instruction execution unit, and a controller for providing the test instruction to the test control register and the linear feedback shift register and evaluating an output signature of the multi-input feedback shift register. 
     Another aspect of the present invention is a method of testing a microprocessor which is comprised of the following steps of: 
     (a) activating a test mode; 
     (b) initializing a test control register, a linear feedback shift register, and a multi-input feedback shift register; 
     (c) loading the test control register with opcode (operation code) of a test instruction; 
     (d) providing contents of the test control register to the instruction fetch; 
     (e) clocking the linear feedback shift register and multi-input feedback shift register either for a fixed number of cycles or 2 N -1 cycles, where N is a number of stages of the linear feedback shift register and multi-input feedback shift register; 
     (f) taking out contents (signature) of the multi-input feedback shift register; 
     (g) comparing the signature of the multi-input feedback shift register with pre-computed simulation signature to determine if there is a fault; and 
     (h) repeating the foregoing steps (a)-(g) with different instruction until all instructions are exercised. 
     The present invention does not require large area overhead, it requires only three registers, and causes no performance penalty. This method is applicable to both standard product microprocessors as well as embedded microprocessors, embedded cores, 2D/3D graphics accelerators, DSP (digital signal processor), audio/video, and multi-media chips. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram showing a basic structure of a microprocessor. 
     FIG. 2 is a schematic diagram showing an example of structure to be added to the microprocessor for implementing the test method of the present invention. 
     FIGS. 3A and 3B show examples of pseudo-code for simulation testbench to collect fault free values of MISR (multi-input feedback shift register) for each instruction in accordance with the present invention. 
     FIGS. 4A-4C are schematic diagrams showing examples of shift registers TCR (test control register), LFSR (linear feedback shift register) and MISR (multi-input feedback shift register) to be used in the test method and structure of the present invention. 
     FIGS. 5A and 5B are block diagrams showing further examples of structure of LFSR (linear feedback shift register) and MISR (multi-input feedback shift register) of FIGS. 4B and 4C. 
     FIG. 6 is a schematic block diagram showing an example of structure for implementing the present invention through a boundary scan TAP (test access port) controller. 
     FIG. 7 is a schematic block diagram showing an example of structure for implementing the present invention for testing a microprocessor having a complex execution unit. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The general structure of a microprocessor and microcontroller (hereinafter collectively “microprocessor”) is shown in FIG. 1. A typical microprocessor  10  includes a program counter and instruction fetch unit  12 , an instruction decode logic  14 , an instruction execution unit  16 , and a system memory (not shown). The instruction fetch unit  12  obtains the opcode of the next instruction based upon the address in the program counter. This opcode is decoded by the instruction decode logic  14 , which generates function select and control signals for the instruction execution unit  16 . The instruction execution unit  16  also receives the operand (data enters or results from the operation) as shown in FIG.  1 . Based upon these control signals, one of the logic blocks within the instruction execution unit  16  computes its function. The operand or data for this computation are obtained from the system memory. 
     In the present invention, the inventors have modified this general structure by three registers and two multiplexers. The basic idea of the invention is to use extra registers to scan-in one instruction, execute it multiple times with pseudo random data and take out the resultant signature. One of the extra registers is Test Control Register (TCR) to provide the opcode of microprocessor&#39;s instructions during the test mode. Two additional registers, one Linear Feedback Shift Register (LFSR) and another Multi-Input Feedback Shift Register (MISR) are used to generate random data and to compress the test response respectively. The data from LFSR is used as operand for the instruction provided by the TCR. The computed result is stored in MISR. The idea behind using the random data and compression thereof is to substantially reduce the test time and obtain a high fault coverage. 
     This principle is illustrated in FIG.  2 . In FIG. 2, it is shown instruction and data flows during a test mode and a normal mode. The additional registers TCR  22 , LFSR  24 , and MISR  26  as well as additional multiplexers  32  and  34  are also shown. The additional registers TCR  22 , LFSR  24 , and MISR  26  are used in the test mode. The multiplexers  32  and  34  are used to switch the paths for these additional registers between the test mode and the normal mode. TCR  22  is to provide the opcode of microprocessor&#39;s instructions during the test mode. LFSR  24  and MISR  26  are used to generate random test data and to compress the test response, respectively. 
     LFSR  24  generates the test operand for the instruction provided by TCR  22 . The computed result from the execution unit  16  is stored in MISR  26 . A multiplexer  32  and a multiplexer  34  are provided to TCR  22  and LFSR  24 , respectively, as shown in FIG. 2 to switch between the test mode and the ordinary mode. FIG. 2 also shows a controller  28  which may be an IC tester, a boundary scan TAP (test access port) controller, or an on-chip test controller. The controller  28  is to control an overall test process by transmitting a mode select signal to the multiplexers  32  and  34  as well as test instructions to the microprocessor  10  under test through TCR  22  and LFSR  24 , and compares the resultant response of the microprocessor  10  with the pre-computed expected value. 
     In the test mode, by a mode select signal to the multiplexers  32  and  34 , TCR  22  and LFSR  24  are electrically connected to the microprocessor  10 . Test instruction is provided to the TCR  22  through the controller  28  (IC tester or other control means) as noted above. The test instruction is then given to the instruction decode logic  14  through the multiplexer  32  and to the instruction execution unit  16 . Based on the test instruction, LFSR  24  generates test patterns which are applied to the execution unit  16  through the multiplexer  34 . In this example, the test pattern generated by the LFSR  24  is a random pattern as noted above. The resultant output of the execution unit  16  is compressed by MISR  26  to produce a signature which is evaluated by the IC tester or other control means. 
     The sequence of testing operation in this scheme is summarized as follows: 
     (1) Activate the test mode. In this mode, contents of TCR  22  are used as instruction rather than that of the instruction fetch unit  12 . 
     (2) Initialize TCR  22 , LFSR  24  and MISR  26  either by direct control or via test control signals from the controller  28  which is an IC tester, boundary scan controller or on-chip test controller, depending upon implementation. 
     (3) Load TCR  22  with the opcode of an instruction. Based upon implementation, it can be either parallel load or serial load (such as scan-in). 
     (4) Clock LFSR  24  and MISR  26  either for a fixed number of cycles or 2 N -1 cycles (full length) for N-bit LFSR. This step repeatedly executes the instruction in TCR  22  with LFSR data. For example, if 1,000 clocks are used, the instruction in TCR  22  is executed 1,000 times with 1,000 different operand (random data provided by the LFSR  24 ). 
     (5) Take out the content (signature) of MISR  26  to determine pass/fail. 
     (6) Compare the content of MISR  26  with pre-computed simulation signature to determine if there is a fault. An IC tester or other automatic test equipment (ATE) can perform this comparison. 
     (7) Repeat the steps (2) to (6) with different instruction until all instructions are exercised. 
     The above sequence of operation assumes that after the design completion of the microprocessor to be tested, a simulation testbench is developed which exercises all instructions with LFSR data and MISR signatures after each run has been recorded. Thus, the fault free MISR content after the each instruction run is known through the simulation. An example of the pseudo code to develop such simulation testbench for each instruction is given in FIGS. 3A and 3B. 
     The above procedure determines that each instruction is executed correctly and hence, it provides functional fault coverage. If stuck-at fault (fault in a circuit which causes a line to remain permanently either at logic 1 or at logic 0) coverage is also desired, then fault simulation can be performed with various values of m (number of patterns generated by LFSR). For example, fault simulation with m=1,000, or 10,000 or exhaustive length will provide different levels of stuck-at fault coverage. Based upon this, one can select a value of m for a particular application. For fault simulation, any commercial EDA tool, such as Cadence&#39;s VeriFault can be used. 
     FIG. 4 shows examples of structure of the registers TCR, LFSR and MISR. There can be many ways to implement this scheme depending how the overall control is done. Such control can be obtained by an IC tester, implemented through a boundary scan TAP controller or an independent on-chip test controller. The implementation of TCR, LFSR and MISR will also vary slightly based upon the overall controlling mechanism. Thus, one possible description of TCR, LFSR and MISR is given in FIGS. 4A-4C which provide a behavioral level description (in-terms of input/output form) for which many gate-level or transistor level implementations are possible. In the present invention, the basic requirements for these registers are: 
     Test control register (TCR): 
     It should be a parallel-out register. It can be either a serial input or parallel input type; however, the parallel input type will cause a large number of wires to become primary input at the chip-level. The serial input implementation will require only one wire to be primary input. If it is implemented as a serial input type, it will also require a mode select signal to switch it from serial-in to parallel-out and vice-versa as shown in FIG.  4 A. 
     Linear feedback shift register (LFSR): 
     It needs initialize and start/stop signals as shown in FIG.  4 B. The start/stop signal can be the same as a test control signal or a signal derived from the test control signal, as shown in FIG.  2 . Any polynomial can be implemented in LFSR  24 , however, prime polynomial is advantageous to obtain 2 N -1 patterns. The pseudo random pattern generated by LFSR  24  is provided to the execution unit  16 . 
     Multi-Input feedback shift register (MISR): 
     It also needs initialize and start/stop signals as shown in FIG.  4 C. In addition, it also needs a serial-out and a mode-select signal to switch it from a data compression mode to a shift register mode and vice-versa. Instead of serial-out, it can be parallel out, however, that will cause a large number of wires to be primary output at the chip-level. The start signal of MISR  26  can be derived from the start signal of the LFSR  24 . To avoid the unknown data in a pipeline of the execution unit  16 , the start signal for MISR  26  should be delayed by a time equal to (or more) the latency of pipeline. 
     FIGS. 5A and 5B show examples of LFSR and MISR in a flip-flop level. An example of LFSR in FIG. 5A is formed of series connected D flip-flops D 0 -D 2  and exclusive OR gates E 0 -E 2  for establishing feedback connections for the flip-flops. As is well known in the art, this structure is to generate a pseudo random signal. An example of MISR is substantially the same as LFSR except for an additional input at each of the exclusive OR gates E 0 -E 2 . 
     The control of this scheme can be implemented in many ways as indicated by the examples of controller  28  in FIG.  2 . The implementation directly through the logic tester (ATE) is straightforward. In such a case, the logic tester provides the clock and test control signals as well as evaluates the test response (MISR signature) to determine pass/fail. 
     In a case when an IC under test has either an on-chip test controller or boundary scan capability, the test control signals and test response is passed through (or even controlled by) the on-chip test controller or boundary scan TAP controller. For example, RUNBIST instruction in the boundary scan TAP controller can be implemented to generate the test control signals; the opcode in TCR can be scanned-in and test response can be scanned-out through the TAP controller. 
     FIG. 6 illustrates such implementation by the boundary scan TAP controller, which is based on a standard boundary scan architecture defined by IEEE/ANSI standard 1149.1. A boundary scan register (scan chain)  40  is introduced at input-output pins of the microprocessor to be tested. A test data input (TDI) pin is connected to an instruction register  48  as well as to TCR  22 , a device ID register  44 , a bypass register  46 . The TAP controller  28  includes I/O pins of a test clock (TCK), test mode select signal (TSM) and test reset (TRST). Multiplexers  36  and  38  are provided to transmit the signature from MISR  26  to a test data output (TDO) pin. In this arrangement, the test is performed by shifting the test vectors and instructions into the microprocessor  10  through the TDI pin, TCR  22  and LFSR  24 , and scanning out the response of the microprocessor  10  through MISR  26  and the TDO pin. 
     Both FIGS. 2 and 6 show the execution unit as a black box. Depending upon the IC, it can be a simple ALU or a group of complex blocks implementing integer and floating point arithmetic and logic operations. One such example is given in FIG.  7 . 
     In the example of FIG. 7, an execution unit  16  includes multiple blocks of integer and floating point arithmetic units  51 - 56 . However, it should be noticed from FIG. 7 that the implementation of this new test method has not changed and it is still the same as FIGS. 2 and 6. The only difference is that the function select multiplexer  34  of the execution unit  16  has been modified so that it will also function as the operand select multiplexer. In FIGS. 2 and 6, there is no function select multiplexer, hence, an additional multiplexer is added to select LFSR data as operand during testing corresponding to each of the arithmetic functions  51 - 56 . In another situation when a function select multiplexer is implemented inside the execution unit  16 , the implementation of test method will be identical to that shown in FIGS. 2 and 6. 
     The major benefits of the present invention is that the hardware overhead in this scheme is negligible and it provides 100% functional fault coverage and additional stuck-at fault coverage. The hardware overhead needed in this invention is negligible and it causes no performance penalty in the normal operation of the IC. The present invention is applicable to a wide range of ICs such as standard product microprocessors or microcontrollers, embedded microprocessors, embedded cores, 2D/3D graphics accelerators, DSP, audio/video and multi-media chips. 
     This scheme provides following two kinds of fault coverage: 
     (1) Functional fault coverage: As each instruction is executed multiple times with different data, it ensures the functional correctness of each instruction. 
     (2) Stuck-at fault coverage: As different lines are sensitized depending upon instruction in the TCR and LFSR data, comparing MISR signature with simulation signature ensures that various lines do not have stuck-at faults. The fault simulation of the gate level model of the execution unit with LFSR, MISR and TCR can quantify the exact stuck-at fault coverage during manufacturing testing. 
     In terms of area overhead, this scheme requires only three extra registers and at-most two multiplexers. The three registers are, one TCR of length equal to microprocessor&#39;s word-size (width of instruction), one LFSR and one MISR. Both LFSR and MISR can be of any length. For all practical purposes either 16-bits or 32-bits wide LFSR and MISR are good enough. If the width of data bus is different than the width of LFSR, multiple fan-out can be taken from LFSR output to match the data-width. 
     Although only preferred embodiments are specifically illustrated and described herein, it will be appreciated that many modifications and variations of the present invention are possible in light of the above teachings and within the purview of the appended claims without departing the spirit and intended scope of the invention.