Method and device for testing multiple power supply connections of an integrated circuit on a printed-circuit board

Abstract: A test organization is described for testing the correct operation of multiple power supply connections for the same rated supply voltage of an integrated circuit. The integrated circuit is provided with a test comparison circuit which is capable of measuring a voltage difference, if any, by way of respective test leads. Should one of the power supply connections no longer be correctly connected, the voltage difference may become too high; this is signaled by way of an error signal.

Description: BACKGROUND OF THE INVENTION The invention relates to a method of testing for the same rated supply voltage on multiple power supply connections which are connected to a functional part of an integrated circuit mounted on a printed circuit board. Various large scale integrated circuits are constructed to have multiple power supply connections for the same rated supply voltage. The number of power supply connections thus exceeds the number of supply voltages. For example, the Motorola MC 68010 dual-in-line package microprocessor comprises four power supply connections, that is to say two for VCC and two for ground potential GND. The two types of power supply connections are present in triple form in the square chip carrier version. The integrated digital circuit 80386 by INTEL CORPORATION has a 20-fold power supply connection VCC and even an 21-fold VSS connection. This multiplicity enables a higher clock frequency, because the power supply connections then represent a lower current resistance or lower inductance. On the integrated circuit itself there are two possibilities for power supply connections for the same rated supply voltage: they may be interconnected, for example by means of a conductor track of aluminium or polysilicon. Alternatively, they are not interconnected. When the number is larger than two, both possibilities may be realized in a single circuit. The above circuits are mentioned merely by way of example. The described problem and its solution can in principle also be relevant for circuits for processing analog signals; in any case, the integration technology (bipolar, MOS, etc.) is irrelevant to a high degree. SUMMARY OF THE INVENTION An integrated circuit which is mounted on and connected to a printed circuit board can operate correctly at the highest permissible clock frequency and the lowest/highest permissible temperature only if all power supply connections are suitably connected to conductor tracks on the printed circuit board. In standard operating conditions the circuit will operate correctly in many cases, notably when the power supply connections on the substrate of the integrated circuit are interconnected. From US-A- 3,783,372 a test apparatus is known for detecting open circuits. This publication does not mention the problems encountered in testing integrated circuits with multiple power supply connections for the same rated supply voltage. It concerns a general purpose, self contained test apparatus, with two probe lead which must be attached to a circuit under test. The probe leads are connected to a high gain amplifier, coupled to readout means, which distinguishes a zero voltage drop along a short circuit from a non-zero voltage drop across an open circuit. The use of attached probe leads makes its use in automated testing complex. It is inter alia an object of the invention to perform, without intervening with the function of the integrated circuit, a simple test in the core of the circuit in order to test the functional presence of all power supply connections, without a so-called functional test being required, testing being performed as regards complete absence of the power supply connection as well as regards many categories of faults in the power supply connections, for example soldering defects, interrupted leads and others. To achieve this, in accordance with one of the aspects of the invention a test comparison circuit, included in said integrated circuit in addition to the functional part, is connected between respective power supply connections for the same rated power supply voltage by means of respective test leads, and a standard voltage difference is detected with said test comparison circuit in a test state in the case of correct functioning of said power supply connections and an error signal is generated using said test comparison circuit in the case of a critical deviation from said voltage difference across the test leads. The test state is realized in that the integrated circuit has a comparatively high current consumption; this means that no standby state is assumed. The invention also relates to a device for performing the method, and notably to a printed circuit board provided with an integrated circuit comprising test means for performing said test. The invention also relates to an integrated circuit comprising test means for performing said test. Further attractive aspects are disclosed in dependent Claims. BRIEF DESCRIPTION OF THE FIGURES The invention will be described in detail hereinafter with reference to some Figures. Therein: Fig. 1 diagrammatically shows a printed cirucit board with an integrated circuit mounted thereon; Fig. 2 shows an electrical equivalent diagram of integrated circuit and printed circuit board; Fig. 3 shows an electrical equivalent diagram thereof when the invention is used. DESCRIPTION OF THE ENVIRONMENT OF THE INVENTION Fig. 1 diagrammatically shows a printed circuit board with an integrated circuit mounted thereon. Reference numeral 20 partly symbolically denotes the package, for example a plastics DIL package. This package comprises connection pins 26. The pins are mounted, for example by soldering, on printed circuit board 28. The actual integrated circuit is symbolized by element 32. It is assumed that the two connection pins shown are intended for the same rated supply voltage. This may be an arbitrary voltage per se (a positive voltage, a negative voltage or ground potential), the value of the supply voltage also being arbitrary. On the printed circuit board the connection pins are interconnected by way of a copper connection in the form of a track 30. Various technologies are feasible in this respect. The resistance of such a connection is in the range of from 5 to 25 mOhms. The pins 26 are both connected to a track 24 by way of bond wires 22. This track can be realized in aluminium using one type of technology or another. The width/length ratio of such a track is in the order of magnitude W/L≈1/50 = 0.02. The sheet resistance of aluminium generally amounts to approximately 60 mOhms per square. This means a resistance in the order of magnitude of 3 Ohms for the track. In other cases this track may consist completely or partly of polysilicon. This results in even higher track resistance. In given cases the track will be interrupted or be absent, deliberately or not; this results in a resistance between the bond wires 22 as realized by the integrated circuit separately which may be infinitely high in theory; in practice the resistance is limited by leakage resistances on the substrate and possibly by input and output resistances of functional parts of the integrated circuit. The physical realization of the integrated circuit and the mounting on the panel 28 can be realized in various ways. Surface mounted (SMD) components, mounting by way of so-called piggy- back facilities, and other versions are also feasible. The multiple power supply connections may be provided for 1, 2, 3 or more rated supply voltages; in addition to electrically conductive connections there may also be provided, for example optical and also capacitive or inductive connections for signals. Fig. 2 shows an electrical equivalent diagram of integrated circuit and printed circuit board. On the printed circuit board 28 there is provided an integrated circuit 20 by way of two pins 34, 36 for the substrate voltage V SS and two pins 38, 40 for the voltage V DD , being 5V in this case. The equivalent resistances on the printed circuit board are represented by the reference Rp, and those on the integrated circuit itself by the reference Ral. The functional part of the integrated circuit is diagrammatically represented by a "core" 42 which is powered by said two voltages via connections which have been omitted for the sake of simplicity. The realization of this power supply on the integrated circuit and further connections of the integrated circuit to the environment have been omitted. There is also shown a power supply source (44). This source may be provided on the printed circuit board or note, the nature of the power supply is irrelevant (mains supply, battery, solar cell, inductive power supply or others). Fig. 3 shows an electrical equivalent diagram of a part of Fig. 2 when the invention is used. The connections 34, 36 are shown, and also the equivalent resistances Rp, Ral, the connection to the power supply source 44, and the connections to the functional core of the function 46, 48. There is also shown a comparison circuit 50. When both power supply connections function correctly, the pins A, B will generally carry substantially the same current, for example 50 mA. In that case the difference between these currents amounts to only a fraction of the current itself, for example 10%. In such cases the voltage drop across the resistance Ral (3 Ohms) amounts to no more than a fraction of 150 mV, being the voltage drop which would occur if said 50 mA current were to flow entirely through the 3-Ohm resistor. In this case the voltage drop probably amounts to some 15 mV. Should one of the two power supply pins not function correctly, this voltage drop becomes +150 mV or -150 mV, depending on which power supply pin is defective. It is assumed that the internal resistance of the function part 42 in fig. 2 is comparatively high, so that the overall current remains the same. When the resistor Ral is made of polysilicon, the voltage difference may be higher in the case of an incorrectly operating power supply connection. This also holds when the connection is actually non-existent (Rp infinitely high). In that case the voltage difference detected by the comparison cirucit may be higher, for example as high as at most V dd . The comparison device 50 may be constructed, for example in the same technology as the remainder of the integrated circuit, as a sense amplifier as customarily connected to a data output of a memory (for example, a RAM). Preferably, such a comparison device must be capable of detecting positive as well as negative difference voltages and it preferably has a threshold voltage. This threshold voltage is proportioned as a function of the intended current decrease per power supply connection; for example, it amounts to 75 mV in the above case. For other values of the normal current the threshold voltage is proportionally higher or lower. The comparison circuit 50 comprises an output 52 for applying an error signal to a processing circuit which is not shown for the sake of simplicity. In a simple version this error signal consists of one bit indicating "correct" versus "incorrect". It may be multivalent in order to indicate which of the power supply connections does not function correctly. In the case of two power supply connections it may concern a series of two bits, for example encoded as: 01-first connection faulty; 10-second connection faulty; 00-both connections correct. In that case the comparison device 50 may comprise, for example two parallel outputs for simultaneously producing these signals. The possibility that both power supply connections are faulty is ignored. The processing circuit may form part of the integrated circuit itself or may be provided separately. An attractive solution consists in that, after discrimination to 1 bit or to 2 bits as described above, the output signal of the comparison circuit 50 is stored directly in a flip-flop, as will be described hereinafter. Fig. 4 shows an electrical equivalent diagram of a more elaborate application of the invention. In this case the integrated circuit comprises three power supply connections 56, 58, 60 for a first power supply voltage and three power supply connections 62, 64, 66 for a second power supply voltage. The power supply source 68 is again shown in the form of a battery. Between the power supply connections the resistances Rp and Ral are again shown. In practice the resistances, for example Rp, will not always have the same value. The equivalent diagram may also be a more complex network. It is essential that the equivalent value of this network between two power supply connections is sufficiently high. The circuit also comprises three switching transistors 70, 72, 74, with respective series resistances 76, 78, 80; there are as many branches as there are pairs of power supply connections. In another set-up, the three branches need not be arranged in parallel; more branches may be involved for as long as they always allow for a single power supply connection to be reached directly from the functional part of the integrated circuit, whereas the other power supply connections for the same rated supply voltage are reached only via a resistance Ral. In the present case the functional part has been omitted for the sake of simplicity. It may be connected in an arbitrary manner between the power supply connections 56 ... 66; for each power supply connection the switching transistors realize a selectively activatable near-shortcircuit which is limited only by the series resistance. Assume that the transistor 70 is turned on and the other two transistors 72, 74 are turned off. In that case in principle six voltage differences (three for each supply voltage) are to be compared with a standard. In order to facilitate the selection therebetween, there is provided a switching matrix 82 which comprises at its switch-points a number of transistors which can be turned on. For a simple set-up there are provided only those transistors which are denoted by lozenges. The resultant configuration is capable of making a decision between correct functioning and incorrect functioning. When all transistors which can be turned on (also the transistors which are denoted by a delta) are present, all faulty power supply connections can be detected and localized. Moreover, in that case for each drive configuration the comparison circuit 84 need supply only one bit value, because there also exists a commutation facility. For the sake of simplicity, the control of the switches 70 ... 72 and the switchcing matrix 82 has been omitted in the Figure. The control of the various switches and the storage of the results can be realized by way of flip-flops. These flip-flops may be included in a shift register chain suitable for performing a so-called boundary scan as described in Netherlands Patent Application 8502476, corresponding United States Patent Application Serial No. 902,910 (PHN 11.484). The interconnection function described therein also offers the necessary hardware components for performing tests on other, internal parts of the circuit. Instead of test patterns, in accordance with the present Patent Application control patterns are serially applied, the result patterns (1 = incorrect, 0 = correct) are subsequently serially output to a test evaluator. An additional advantage of the above method consists in that it enables direct diagnoses of errors which would otherwise be impossible to detect or only with great difficulty. The boundary test scan can test is suitable for testing all IC pins. The additional logic circuitry can be provided at the edge of the integrated circuit or underneath the power supply tracks.

Claims: A method of testing for the same rated supply voltage on multiple power supply connections (34, 36, 38, 40, 56, 58, 60, 62, 64, 66) which are connected to a functional part (42) of an integrated circuit (20) mounted on a printed circuit board (28), characterized in that a test comparison circuit (R al , 50, 84), included in said integrated circuit in addition to the functional part, is connected between respective power supply connections (34, 36, 38, 40, 56, 58, 60, 62, 64, 66) for the same rated power supply voltage by means of respective test leads, and a standard voltage difference is detected with said test comparison circuit (R al , 50, 84) in a test state in the case of correct functioning of said power supply connections (34, 36, 38, 40, 56, 58, 60, 62, 64, 66), and an error signal is generated using said test comparison circuit (R al , 50, 84) in the case of a critical deviation from said voltage difference across the test leads. A method as claimed in Claim 1, characterized in that when at least three power supply connections (56, 58, 60, 62, 64, 66) are provided for the same rated supply voltage, a selection mechanism (82) on the integrated circuit (20) is connected between the power supply connections and a discriminator circuit (84) part of the comparison circuit (R al , 84), whereby only two of said power supply connections are selectively connected to a respective input of the discriminator circuit (84). An integrated circuit (20) containing a functional part (42), with at least two separate power supply connections (34, 36, 38, 40, 56, 58, 60, 62, 64, 66) for the same rated supply voltage, characterized in that said separate power supply connections (34, 36, 38, 40, 56, 58, 60, 62, 64, 66) are connected via respective test leads to respective connections of a test comparison circuit (R al , 50, 84) comprised in said integrated circuit (20) in addition to the functional part, the test comparison circuit (R al , 50, 84) being arranged for producing in a test state, when a deviation from a standard difference voltage across said connections (34, 36, 38, 40, 56, 58, 60, 62, 64, 66) exceeds a critical value, an error signal on a signal output (52, 86) of the test comparison circuit (R al , 50, 84). A printed circuit board (28), comprising an integrated circuit (20) as claimed in Claim 3. An integrated circuit as claimed in Claim 3, characterized in that the test comparison circuit comprises a discriminator circuit (50, 84) for detecting said critical deviation and an output circuit which is fed thereby in order to form a standard binary representation of the error signal. An integrated circuit as claimed in Claim 3, characterized in that at least three power supply connections (56, 58, 60, 62, 64, 66) are provided for the same rated supply voltage and a selection mechanism (82) is connected between the power supply connections (56, 58, 60, 62, 64, 66) and a discriminator circuit (84) part of the test comparison circuit (R al , 84) for connecting only two of said power supply connections (56, 58, 60, 62, 64, 66) selectively to a respective connection of the discriminator circuit (84). An integrated circuit as claimed in Claim 3, characterized in that multiple power supply connections (56, 58, 60, 62, 64, 66) are provided for at least two different supply voltages, a selection mechanism (82) being provided between the supply connections (56, 58, 60, 62, 64, 66) and a discriminator circuit (84) part of the test comparison circuit (R al , 84) for connecting only power supply connections (56, 58, 60, 62, 64, 66) associated with the same supply voltage selectively to each time a respective connection of the discriminator circuit (84). An integrated circuit as claimed in Claim 6 or 7, characterized in that said selection mechanism (82) comprises a commutation mechanism for inverting the selection of two power supply connections for connections of the test comparison circuit. An integrated circuit as claimed in any one of the Claims 3 or 5 to 8 characterized in that there is provided a data flip-flop which is fed by the output (52, 86) of the test comparison circuit (50, 84) in order to output a result bit for external evaluation by including the flip-flop in a serial shift register. An integrated circuit as claimed in any one of the Claims 3 or 5 to 9, characterized in that there is provided a second data flip-flop for the storage of a control bit for selecting said test leads and for receiving, by including the second data flip-flop in a serial shift register, a control bit from an external control device. An integrated circuit as claimed in Claim 10, characterized in that there is provided a third data flip-flop for storing a second control bit for the selective activation of any power supply connection and for receiving, by including the third data flip-flop in a serial shift register organization, a further control bit from the external control device.

Inventors: VAN DE LAGEMAAT DIRK, VAN DE LAGEMAAT, DIRK
Applicants: PHILIPS NV, N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1989
Main classification: G01R31
IPC codes: G01R31, G06F11