Abstract:
A method for testing integrated circuits provided on a carrier. The circuits include a series input (22) and a series output (24) for test and result patterns. A mode control register (30) receives a mode control signal train via the serial input. Under the control of said mode control signal train the serial input and output can be shortcircuited to each other, or further registers (32, 34, 36) can be selectively filled and emptied. In this manner, both the interior of the integrated circuit and respective interconnection functions can easily be tested by a universal protocol. Integrated circuits and the carrier only require minor extension/adaptations.

Description:
This is a continuation of application Ser. No. 07/090,489, filed Aug. 28, 1987, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a method of testing integrated circuits provided on a carrier. In the method, a test pattern is presented serially to an integrated circuit set to an input state, by means of a first connection of the integrated circuits. The test pattern is stored temporarily. The integrated circuits are set to an execution state to form a result pattern from the test pattern. The result pattern present in any of the integrated circuits set to an output state is then serially removed by means of a second connection of the integrated circuits to provide, by means of an evaluation on the information contents thereof, a characterization of a correct/incorrect operation of the integrated circuits and their interconnection function, respectively. An example of such carriers comprises printed circuit boards, but the invention is not restricted to the interconnection technology. With integrated circuits becoming more and more complex, the need for a reliable test method increases because rejection of a product in an earlier phase of production is usually much less expensive than rejection in a later production phase. An integrated circuit can be tested exhaustively before being mounted on such a carrier so that the possibility of an undetected defect in such an integrated circuit is negligibly small. Nevertheless, testing of the carrier with mounted circuits in a structure test proves to be useful because an integrated circuit may be damaged during mounting and because an interconnection function may be defective. 
     The interconnection function between two (or more) integrated circuits is to be understood to mean the operational behaviour and hence implicitly the correct/incorrect structure of the following elements or a part thereof, for example: 
     a. the conductor pattern provided on the carrier: test for interruption and/or short circuit; 
     b. the connection between said conductors and the connection pins of the integrated circuit modules: 
     c. the connection between said connection pins and the bonding flaps provided on the substrate of the integrated circuit, for example by means of a bonding wire; 
     d. optionally present buffer elements between the bonding flap and the input/output for the relevant bit of test/result pattern; 
     e. optional further elements provided between the integrated circuits thus connected, at least as far as their digital functioning is concerned. They may be passive elements, for example a terminating resistor which couples an interconnection junction to earth. Alternatively, they may be an integrated circuit which in itself cannot be tested, for example a module constructed in conventional TTL logic, for example a latch circuit. 
     Example of a known technique: 
     It is known to test a combination of integrated circuits according to the scan-test principle as described, for example, in U.S. Pat. No. 3,761,695 in which the various integrated circuits are tested successively. The scan-test principle is based on the fact that in the input and output conditions a number of bistable elements present in the integrated circuit are connected as shift registers so that the test and result patterns, respectively, can be inputted and outputted serially to form said shift registers. In the execution condition said bistable elements are used in a manner as if the circuit were in normal use. The principle stated in said patent specification may be expanded to the &#34;serpentine&#34; concept. The serpentine concept involves that the integrated circuits are connected into a chain. The test patterns and result patterns can move on said chain in that an output line for result patterns of a circuit of the chain also operates as an input line for test patterns of a subsequent circuit of the chain. As a result of the cascade connection of the integrated circuits, the test/result patterns must often traverse several integrated circuits before they arrive at a given location. When several test/result patterns are used simultaneously they must further be spaced at the correct distance along the serpentine connection thus formed to be inputted correctly and evaluated correctly. As a result of this, testing takes a long time and requires continuous supervision by a testing device so that the latter cannot direct its processing capacity alternately to the presentation of a test pattern and the evaluation of a previously received result pattern. 
     A further disadvantage of the serpentine concept described is that in the case of dysfunction of one of the circuits it is in many cases not possible to test the other integrated circuits if test and/or result patterns are multilated by the serial transport. Furthermore, all the integrated circuits thus present must be mutually operated synchronously and they must all have the relevant test facility. All this considerably reduces the applicability of this concept. 
     The above problem also applies when the interconnection function between two or more integrated circuits is tested. In this test, test patterns are applied to one or more integrated circuits, communicated via the interconnection function, and result patterns are removed from one or more integrated circuits (the same or other ones). The test pattern is applied to an output register, while the result pattern is removed from an input register of commonly another integrated circuit. For an internal test on the integrated circuit the test pattern is commonly applied to an input register, while the result pattern is commonly removed from an output register of the same integrated circuit. Selected objects of the invention. 
     It is an object of the invention to provide a method in which a defective circuit arrangement or defective interconnection function need not usually impede the testing of other integrated circuits or interconnection functions on the same carrier because it can be arranged that all the integrated circuits, and interconnection functions except the integrated circuit or interconnection function under test, are usually substantially short-circuited. It is an object, furthermore to provide a method in which control of the respective integrated circuits can be realized at two levels, as a result of which control of individual test modes easy to realize, and in which furthermore, by a standardization of the connections, an environment can be created in which an arbitrary circuit or interconnection function can be made testable without additional connections for information or control being necessary. 
     The connection technique can be simple so that inter alia little area of the carrier need be occupied: arbitrary addressability of the integrated circuits can be realized, the methodology can be applied to the above test procedures; and per integrated circuit function variations and additions may be possible without it being necessary for the interface to be changed for testing. 
     SUMMARY OF THE INVENTION 
     According to the invention a method as set forth in the background of the invention is characterized in that, prior to activating the said input state, the integrated circuits are set to an initiator state. In the initiator state, a mode control signal train is applied to a third connection and is stored in at least one of the integrated circuits in a mode control register. The method according to the invention is further characterized in that a first content of the mode register then provides, as a first alternative, a first control signal to short circuit the third connection of the relevant integrated circuit directly to a fourth connection to directly couple information received on the third connection into a further connection of another integrated circuit which further connection corresponds to the third connection and is connected to the said fourth connection. The method according to the invention is further characterized in that a second content of the mode register, as a second alternative, provides a second control signal to throughconnect the third connection, while bypassing the said mode register, to an input register to apply thereto local test pattern signals, and to couple the fourth connection, while bypassing the said mode control register, to an output register to derive local result pattern signals therefrom so that the said third and fourth connections then operate as the said first and second connections, respectively. As a result of the easily adjustable mode control, many different modes can be realized, if necessary. As a result of the short circuit, tests, of other integrated circuits and interconnection functions present for other integrated circuits, also with considerable defects, can usually be carried out. 
     In the second mode the test pattern may be used both for a test of the interior of an integrated circuit and for a test of an interconnection function. 
     The invention also relates both to a carrier provided with a number of integrated circuits which can easily be tested by means of the method and to an integrated circuit which is suitable for being provided on such a carrier after which the integrated circuit and/or the interconnection functions can be tested. Further advantageous realizations are claimed in the sub-claims. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     The invention will be described in greater detail, by way of example, with reference to the accompanying diagrammatic drawings in which 
     FIG. 1 is a block diagram of an integrated circuit embodying the principle of the invention, 
     FIG. 2 shows a detail of the mutual relationship in a part of FIG. 1. 
     FIG. 3 shows an example of a carrier having several integrated circuits. 
     FIG. 4 shows an alternate embodiment of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Description of an integrated circuit. 
     FIG. 1 is a block diagram of an integrated circuit embodying the principle of the invention. The envelope is denoted by the block 20. The circuit comprises four registers 30, 32, 34, 36. The shift register 30 is fed via switch 42 by input 22 to receive a mode control signal train; in this simple embodiment it accommodates four bits. The storage thereof is synchronized by clock pulses at terminal CL and is done under the control of a first control signal at terminal TST which indicates that a serial pattern is received and a second control signal at terminal C/D which sets the switch 42 in the top position and thus signals that the mode control signal train can be received. In the non-test position the registers 32, 34, 36 are transparent and not noticeable to the outside world. 
     The actual user function of the circuit is fulfilled by the bistable elements of register 34 (in this example also four) and the block 38 which in this case comprises combinational logic not further specified (and optionally further elements). The bistable elements of register 34 operate by their bidirectional coupling with block 38 as internal flipflops of said block. In the input/output states register 34 is operated in the usual scan-test manner to communicate a test/result pattern from to the outside world. In another realization it operates to store information with which internal flipflops can be preset to a relevant value. In that case the actual function of the integrated circuit is fulfilled entirely by block 38. The flipflops of register 34 and the flipflops which are set thereby may be situated at any position within the block 38. For simplicity the user function is not further specified. It is also possible that the internal scan test is not implemented, but that the output of register 32 is directly connected to the input of register 36. 
     Register 32 in this example comprises six stages which can be filled in parallel via input 26. It can furthermore be filled serially via input 22 and the switches 42 and 44 provided these are in the appropriate positions. The register 32 has a serial output to register 34. The register 32 has a parallel output to the block (combinational) logic 38. For testing the interconnection function a result pattern is received in parallel and is removed serially. For testing block 38 a test pattern is received serially and is removed in parallel. It is not strictly necessary for these two provisions to be implemented. 
     Register 36 in this example comprises six stages which can be filled in parallel from the block (combinational) logic 38. It may furthermore be filled serially from register 34. The register 36 can provide its information serially to output 24 provided switches 46 and 48 are in the correct positions. The register 36 has a parallel output via the outputs 28. For testing the interconnection function a test pattern is received serially and is removed in parallel. For testing block 38 a result pattern is received in parallel and is removed serially. It is not strictly necessary for the two provisions to be also implemented. 
     The shift register 30 has a serial output to output 24 provided switch 46 is in the appropriate position. In certain cases this latter connection is superfluous and register 30 has no serial output. Furthermore, a logic short circuit 40 which is active when the four switches 42, 44, 46, 48 are all in the appropriate positions is present between serial input 23 and serial output 24. Optionally, said logic short circuit comprises a clocked buffer 401 as a result of which time delay effects can be standardized and there is no definition problem when a (large) number of these short circuits are connected in series. Finally the integrated circuit comprises a decoder 50 which is connected to (a part of) shift register 30. Decoder 50 can decode certain bits or bit combinations in shift register 30 to then form a given internal control signal for the integrated circuit 20. In another embodiment the contents of shift register 30 have already been decoded entirely and decoder 50 is superfluous. The register 34 in this case present between the registers 32 and 36 may alternatively be situated in another position, for example between input 22 and register 32 or between register 36 and output 24. As shown, it may be single so that it can directly be filled serially. It may also be plural, for example in the form of a register bank or stack so that various test/result patterns can successively be stored therein and be retained collectively. It is also possible for the test patterns to have a fixed length, for example of (in this case) four bits, while internally a much longer test pattern is necessary. Multiple series-parallel conversion then takes place. The same applies to the result patterns. Addressing the register bank/stack can be done again by a suitable control signal from register 30/decoder 50. The operational modes. 
     The FIG. 1 circuit can operate in the following control modes: 
     Under the control of a determined signal at terminal C/D and the test control signal at terminal TST switch 42 is in the top position and register 30 is filled serially with a mode control train. In this example switch 46 is then also in the top position and the mode control train can also be transmitted to other integrated circuits which are connected to connection 24. It is advantageous for all mode control registers 30 of the respective integrated circuits provided on a carrier to have the same length. Upon filling, synchronization takes place by clock pulses at connection CL. 
     When the said signal is not present at terminal C/D, switches 42 and 46 are in the bottom position. This means that when no mode control signals are applied the operation of the circuit can be controlled by the contents of mode control register 30. These control modes may relate to the following: 
     Recognition of an address. The test mode to be described hereinafter is active only if a predetermined pattern is stored in certain bit positions of mode register 30. In all other cases the operational mode is active, in which only the intended user functions are carried out. This recognition takes place in decoder 50. 
     Setting switches 44, 48. The short circuit 40 is active if these are at the &#34;inner&#34; position so that a subsequent signal train applied to input 22 is directly removed at output 24. This means that a test pattern or result pattern can be coupled into another integrated circuit substantially without delay. When the switches 44, 48 are in the &#34;outer&#34; position and a &#34;test&#34; control signal is received at terminal TST a signal train at connection 22 is to be treated as a test pattern and may be used to fill the registers 32 and/or 34 and/or 36 serially with a suitable test pattern. The function of the respective test bits will be explained hereinafter. When after said supply the &#34;test&#34; control signal at terminal TST disappears, the test pattern is processed in the (combinational) logic circuit 38 as a result of which a result pattern appears in the registers 34 and/or 36, alternatively, the test pattern is transported via an interconnection function, after which the result of a test of the interconnection function appears in register 32. The result pattern of a test of an interconnection function will commonly appear in an integrated circuit other than that of which the test pattern for said test has been applied. If then the test control signal at terminal TST appears again (after one or a previously determined larger number of clock pulses at terminal CL) the test pattern can be removed for evaluation via connection 24. The input state and the output state are both synchronized by clock pulses at terminal CL. The execution state may optionally also be synchronized by clock pulses at terminal CL. This will often not occur, for example when hand-shake protocols are implemented between various integrated circuits (these could otherwise not be tested) and in general when the use of the serial shift clock would not be optimum, for example as a result of too low a frequency. In that case either an external clock or an internal clock will be provided. 
     Further modes which can be controlled selectively may be one or more of the following. Register 34 is short circuited by means of a couple of switches in the same manner as switches 44/46 realize a short circuit through line 40. The same may occur for the registers 32 and/or 36. It is also possible to directly short circuit the input 22 to the serial input of register 36. It is also possible to directly short circuit the serial output of register 32 to output 24. Furthermore a given internal control signal for the interior of block 38 may be realized. 
     FIG. 2 shows another example of the organizational relationship between the registers 32, 34 and 36 and a block 380 which in this case also comprises a number of flipflops in addition to combinational logic (not shown). In the first place a selectively controllable shortcircuit for register 34 is realized by means of switches 60, 62 and line 64. The flipflops of register 32 contain a series of six information bits which can be received from outside the circuit (via connection 26). The flipflops of register 36 after a test of block 380 contain representations of six information bits to be applied to further elements. Alternatively they contain, before a test of an interconnection function, a test pattern to be applied to said interconnection function, which test pattern then can be received, for example, in another integrated circuit. In addition to the above it is possible that a number of the register stages of registers 32, 36 are connected bidirectionally to internal components of block 38/380. 
     FIG. 2 shows that the interior of block 380 also comprises a number of flipflops. They are indicated by blocks. The logic depth measured in bits is three bits for all the input stages of register 32 up to the output stages of register 36. The combinational logic may give rise to all kinds of transverse connections. In fact the functional circuit is divided into six parallel organized independently loadable scan test chains, for which purpose register 32 forms a series-to-parallel converter and register 36 forms a parallel-to-series converter. Conversely, for testing an interconnection function between integrated circuits on one carrier, register 36 may form a series/parallel converter and register 32 may form a parallel/series converter. 
     It is shown in FIG. 1 that register 34 is connected bit-wise bidirectionally to block 38. In FIG. 2 each individual bit position is connected unidirectionally to a respective previously determined internal flipflop within block 380. As a result of this, the contents of the bit positions 52, 54 in this example can be set to predetermined values. Furthermore, the information contents of bit positions 56, 58 can be derived in a corresponding selective manner without further interactions with later flipflops of the scan test chain. The advantage of such an operation is that in certain cases it will suffice to fill register 36 only once, even when the depth of the individual scan test chains within block 380 is (much) larger than 1 bit position. 
     The following modes are possible: 
     a. short circuit by means of connection 40; 
     b. the scan test is carried out with a test pattern of six bits wide and three bits deep, the dimension of the result pattern being the same. In practice such patterns will usually have considerably larger dimensions; 
     c. the register 32 is filled with a test pattern of six bits wide and one bit deep; a result pattern of the same dimension is formed in register 36, register 34 being not activated; 
     d. the same as c, but in this case register 34 is also activated; 
     e. only register 34 is activated, registers 32, 36 being short circuited (the advantage of this is that a pattern can then rapidly be applied via input 22 and be removed via output 24). All kinds of modes can be implemented, some of which are advantageous in particular for testing the integrated circuit itself, and some of which are advantageous in particular for testing the interconnection functions. Description of a carrier. 
     FIG. 3 shows a carrier having three integrated circuits to which the invention can be applied. Integrated circuit 70 looks to the exterior like the FIG. 1 circuit: three control connections TST, CL, C/D, six pins at 260 on the input side, six pins at 280 on the output side. Furthermore there is an input 220 and an output 240 for a mode control signal train. The integrated circuits 72, 74 are shown in a corresponding manner but individually they may have quite different functions. In the Figure it is shown only that they each have only four pins on the input side and the same number of pins on the output side. The connections 220, 222, 224 correspond to each other, as do the connections 240, 242 and 244. Connections 240 and 222 are interconnected. So when in the circuit 70 the short circuit around the mode control register is activated, a mode control signal train can be directly applied to the integrated circuit 72. The same also applies when said signal train is a test pattern or, if applicable, a result pattern. The connections 242 and 224 are also interconnected; the three integrated circuits thus are arranged in a daisy chain, but as a result of the short-circuitability of the various components the signal traverse nevertheless is rapid. (Optionally the delay is one clock pulse if a clocked buffer 401 is present per integrated circuit, which is shown in FIG. 4). 
     Connection 280 comprises six pins, whereas the connections 262, 264 each comprise only four pins. The interconnection patterns are determined in the first place by the functions which the carrier provided with integrated circuits has to fulfill. If no mode control signal train is present, nor a serial test pattern which has to be presented to a corresponding serial input (22 in FIG. 1), the output 240 will temporarily not be used. Therefore it is possible in such a case to involve said pin in the functional connection pattern of the carrier provided with integrated circuits. The relevant information signal may then be derived directly from terminal 220.