Abstract:
The invention relates to a method for protecting data during a self-test of a microcontrollers, in which all of the circuit elements within the microcontroller can be tested, where the course of the self-test cannot be altered via the external pins, and no intermediate results are passed to the outside via the pins. The invention also relates to an configuration in the form of an integrated circuit which can be used to implement the method, and to correspondingly equipped microcontrollers.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of copending International Application No. PCT/DE00/03271, filed Sep. 20, 2000, which designated the United States. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a method and a configuration for protecting data during a self-test of a microcontroller. The configuration is in the form of an integrated circuit. 
     For the manufacture of integrated circuits (ICs), the production test is an aspect that plays a crucial part in the overall determination of the quality. Normally, the production test is performed by stimulating the integrated circuits on a test system using a multiplicity of test patterns on the input pins of the ICs, measuring the test responses on the output pins of the ICs, and comparing them with the nominal values stored in the test system. 
     This presents a problem for a particular class of ICs, the “security ICs”, as are used for data encryption or for the hardware-based protection of transfer protocols, for example. In the case of these ICs, configuration and operation are subject to certain security requirements that—couched in simple terms—require that the internal circuit elements of the IC must not be able to be accessed by unauthorized parties via the pins of the IC. For this reason, the test method outlined cannot be applied in this instance, since knowledge and storage of input data and of the associated nominal output data in a test system or a PC, for example, would allow the configuration and operation of the IC to be inferred. Nevertheless, a suitable production test needs to be conducted for this class of ICs too. 
     To solve this problem, self-tests (BIST, Built-In Self-Test) have been developed. A self-test normally requires only a start signal to be specified; after a prescribed time, the test response conditioned in the IC can then be read out. Since this test response contains the compression of a large number of individual test results to form a single data record, it is generally not possible for the configuration and operation of the IC to be inferred from this test response. 
     To implement a self-test, two different approaches can be distinguished for microcontrollers: A self-test containing the execution of a test program stored in the microcontroller itself; and a self-test such as described in IEEE standard 1149.1 “Standard Test Access Port and Boundary-Scan Architecture” under the term “RUNBIST” as an optional function. 
     The fundamental advantage of a self-test using a test program stored in the microcontroller itself is that the test program can check the authorization to perform the self-test in a variety of ways before the actual test is performed. This can be done, inter alia, by the external input of a password that is compared with a nominal value stored in the microcontroller, the self-test being started only if the result of the check is positive. 
     However, the approach using a test program stored in the microcontroller itself generally has substantial drawbacks. One drawback is that software cannot stimulate or observe all of the circuit elements within the microcontroller. The result of this is that the quality of the production test is not sufficiently high. Also, the additionally integrated memory required for the test instructions increases the chip area. Another drawback is that the execution of a test program—as compared with a normal test using a test machine—is generally at least ten times slower. 
     The approach via the self-test using the boundary scan described in IEEE standard 1149.1, for example, generally solves the problem of the accessibility of internal circuit nodes, since in this case all of the inputs and outputs of the IC are involved in the generation of test patterns and test responses. However, this self-test satisfies only minor security requirements, because the generation of test patterns takes place internally but the test responses are emitted serially by the boundary scan, and the flow control of the self-test by the standardized TAP controller can be altered from the outside at any time. Thus, a “hacker” could misuse this self-test in order to access secrets in the chip. To do so, he would repeat the self-test as often as desired, but would constantly and systematically vary the duration of the self-test by the number of clock cycles which keeps the system in the “Run-Test/Idle” state and would infer the chip content from the results which are then read out in each case. The control signals needed for this are freely accessible externally in line with the IEEE standard, and there is no kind of protection against such “spying”. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a method and configuration for protecting data during a self-test of a microcontroller that overcomes the hereinaforementioned disadvantages of the heretofore-known devices of this general type and that allow a full and fast self-test that is protected against hacking attempts. 
     With the foregoing and other objects in view, there is provided, in accordance with the invention, a method for protecting data while self-testing a microcontroller and testing circuit elements within the microcontroller, a course of the self-test not being alterable via an external pin, and no intermediate results being passed to the outside. The method includes the step of providing the microcontroller with a boundary scan having registers. The next step is storing a built-in self-test (BIST) in the microcontroller. The next step is generating test patterns within the microcontroller with the BIST by using the registers of the boundary scan. 
     With the objects of the invention in view, there is also provided an integrated circuit configuration for protecting data during a self-test for a microcontroller including devices for starting the self-test and for performing the self-test internally. The self-test covers all fundamental elements of the microcontroller and outputs a corresponding test response. The configuration includes devices for storing a self-test program. In addition, the configuration includes devices for isolating external input and output devices from the devices for performing the self-test throughout execution of the self-test. 
     Accordingly, the microcontroller has a boundary scan, preferably in line with IEEE standard 1149.1. However, only the obligatory test instructions listed in the IEEE standard are implemented, as are provided for the production test on a printed circuit board (and not on the IC), in line with the intention of the boundary scan. 
     The BIST now uses the registers of the boundary scan in order to generate pseudo-random test patterns within the chip and to compress test responses. Since the boundary scan register encompasses the entire logic of the chip as far as the input and output cells, it is optimally suited to reaching a maximum number of internal circuit nodes using the self-test, which is in turn a prerequisite for the greatest possible fault coverage and hence quality of the production test. 
     In accordance with the invention, this BIST is not started directly from the outside (e.g. by the instruction “RUNBIST”, as indicated in the IEEE standard), however, but rather with the microcontroller. Beforehand, the microcontroller checks the authorization to perform the self-test. During the self-test, the microcontroller monitors correct execution of the self-test, which includes not allowing the self-test to be interrupted. This also includes not accepting any test instructions during the BIST using the boundary scan (in line with the IEEE standard), that is to say the TAP controller indicated in the IEEE standard is thus decoupled from the boundary scan cells by using logic. At the end of the test, the compressed test response can either be output by the microcontroller or can be compared with a nominal response stored in a read only memory in the microcontroller. 
     Thus, in principle, the invention combines the two known approaches and the associated logic sequence while preserving the advantages. First, the test program integrated in the microcontroller checks the authorization to execute the self-test. If this condition is satisfied, the test program starts the self-test by decoupling the TAP controller&#39;s control signals from the boundary scan register by using logic, putting the test logic into a defined initial state and starting the self-test. At the end of the test, the test program reads the test response stored in the boundary scan register into an internal memory and then erases the test response in the boundary scan register. The TAP controller is then enabled again. The test response stored in the microcontroller can now be compared with a nominal response stored in the test program or can be output to the outside, depending on implementation. 
     This procedure ensures that only authorized parties can execute the self-test, that the course of the self-test cannot be altered via the external pins, and that no intermediate results are passed to the outside via the pins. Despite this, a fast full self-test covering all of the elements of the microprocessor is made possible. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a method and a configuration for protecting data during a self-test of a microcontroller, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a microcontroller according to the invention; 
     FIG. 2 is a flowchart for describing a preferred embodiment of the invention; 
     FIG. 3 is a circuit diagram of an input boundary scan cell for producing test patterns; 
     FIG. 4 is a circuit diagram for an output boundary scan cell for holding test responses; and 
     FIG. 5 is a block diagram showing basic flow of signals for the embodiment shown in FIGS. 2 to  4 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown a block diagram of a microcontroller  10  with boundary scan. The microcontroller  10  includes a CPU  12 , RAM  14 , ROM  16 , input/output unit  18 , peripheral units (e.g. timers, coprocessors)  20 , BIST controller  22 , boundary scan TAP controller  24 , and boundary scan cells  26 . The latter communicate with pads (A in FIGS. 3 and 4) and with a circuit core (B in FIGS.  3  and  4 ). 
     FIG. 2 shows an example of the logical course of the self-test. A request to the microcontroller  10  to start the self-test can be made in various ways: by an external signal, by an instruction transferred to the microcontroller  10 , by an internal timer, etc. First, the test program integrated in the microcontroller  10  checks the authorization to execute the self-test. Depending on the form, the check can be conducted by comparing an actual value with a nominal value stored in the microcontroller, by checking a password, by the microcontroller  10  authenticating the external instructing authority (e.g. host computer, test system), or in another suitable manner. 
     If the check is successful, the TAP controller is blocked, the test logic is reset and the test program starts the self-test. In this regard, it is sufficient for the microcontroller  10  to set a corresponding start bit in the BIST controller  22 . Alternatively, an assembler instruction “RUNBIST” is also conceivable which can be executed only if the check is successful, and implicitly starts the BIST controller  22 . If the authorization check is not successful, the test program is exited and a reset is triggered, for example. 
     After the start of the self-test, the microcontroller  10  waits until the hardware of the BIST controller  22  signals the end of the test to the microcontroller  10 . This signaling can be conducted in a variety of ways by requesting a status bit in the BIST controller  22 , by an interrupt triggered by the BIST controller  22 , or by an integrated timer. Next, the BIST signature is ascertained by reading out the boundary scan register to the internal RAM  14 . After that, the boundary scan register is reset and the TAP controller  24  is then enabled again. Resetting the boundary scan register makes it impossible for the signature (or parts thereof) to be read out via the TAP controller  24 . 
     The signature can be checked either in the microcontroller  10  itself or externally. 
     FIGS. 3 and 4 show exemplary implementations of boundary scan register cells  26  used for generating test patterns (FIG. 3) and for evaluating the test responses (FIG.  4 ). 
     FIG. 3 shows a 3-bit linear feedback shift register (LFSR) as part of a circuit for an inventive BS cell. Switching elements XOR  32  and a multiplexer  34 , additionally inserted in line with the invention, are shown in bold. The signal LFSR is used during the self-test to separate the boundary scan shift register and to generate test patterns using the LFSR. In this case, the signals SHIFTDR and MODE are set to the ‘1’ level. Each clock cycle of the signal CLOCKDR produces a new test pattern. 
     FIG. 4 shows a 3-bit multiple input signature register (MISR)  40  as an example of a circuit for holding the test responses. The switching elements XOR  42  and multiplexer, additionally inserted in line with the invention, are shown in bold. The signal LFSR is used during the self-test to separate the boundary scan shift register. The MISR  40  sums the test responses from the circuit core. In this case, the signals SHIFTDR and MODE are set to the ‘1’ level. Each clock cycle of the signal CLOCKDR sums another test response. The signal UPDATEDR is blocked by the BIST controller so that no results are passed to the external pins during the self-test. 
     FIG. 5 illustrates the flow of signals between the BIST controller  22  and the TAP controller  24  in terms of the control signals derived from the two. The standard-conforming TAP controller  24  is controlled by the external signals TCK (Test Clock), TMS (Test Mode Select), TDI (Test Data Input) and TDO (Test Data Output). The internal control signals (MODE_T, UPDATE_T, SHIFT_T, CLK_T) generated by the TAP controller  24  are blocked by signals from the BIST controller  22  (SHIFT_B, BIST) while the self-test is running. 
     During the self-test, the BIST controller  22  sets the signals BIST, LFSR, MISR and SHIFT_B. In this case, the BIST controller itself is clocked by SYS_CLK, the system clock of the microcontroller  10 . 
     A signal MODE is held at the ‘1’ level by an OR function, as a result of which the multiplexers  34 ,  44  actuated by the signal MODE in FIGS. 3 and 4 are set such that no external data are passed to the circuit core (B) via the input pins (A) and would thus be able to manipulate the course of the self-test (FIG.  3 ), and no (intermediate) test responses can be passed to the output pins (A). 
     A signal UPDATEDR is held at the ‘0’ level by the AND function with a negated input so that (intermediate) test responses can be passed to the output pins (A) during the self-test. 
     A signal SHIFTDR is set to the ‘1’ level for the LFSRs  30  in order to form the shift function; for the MISRs  40 , the level of this signal is of no significance. 
     A signal CLOCKDR, used to clock LFSR  30  and MISR  40 , is changed over to SYS-CLK, the system clock of the microcontroller  10 . 
     A signal SDO is held at the ‘0’ level by the AND function with a negated input so that no (intermediate) test responses can be passed to the external output TDO of the TAP controller  24  from the boundary scan shift register.