Abstract:
In previously known methods for testing internal signals of an integrated circuit, additional output pins were required which, in general, were linked to additional measuring pads within the integrated circuit.  
     In the new method, the circuit functions are tested by using the output pins at which the output signal is present during normal operation of the integrated circuit. By means of a simple, external connection, with which a defined voltage value is set at the signal output, the integrated circuit is switched by means of an integrated control unit into a test mode in which it applies selected signals, which are to be tested, at the signal output. There is no need for additional internal measuring pads or additional output pins.

Description:
BACKGROUND  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a method for testing an integrated circuit in accordance with the preamble of patent claim  1  and a circuit arrangement for performing this method.  
           [0003]    2. Description of the Related Technology  
           [0004]    In the case of integrated circuits (ICs), electrical measurements are made after the manufacturing process in order to test the circuits. The high degree of complexity makes it necessary to measure not only the output signals of the entire circuit but also the signals of individual internal functional groups of a circuit. In the development phase, additional internal contact surfaces, so-called measuring pads which are assigned to the individual circuit blocks, are integrated for this purpose. Function checks can be performed at these measuring pads provided that the integrated circuits are not yet enclosed in a casing. In the case of the finished integrated circuits, some of the measuring pads are linked to external bond pads for the purpose of checking the correct function, and also in order to measure the signals to be tested at additional output pins even in the assembled state. Both the additional measuring pads within the integrated circuits as well as the additional pins on the finished IC require an additional area which, in the case of miniaturization, increases as a percentage of the whole.  
           [0005]    Examples for the previous method are provided by known integrated circuits (IC), such as the U2548 and U2521 from the ATMEL Germany GmbH company, for example. In these cases, a part of the measuring pads present within the integrated circuit was linked to additional measuring pads in order to test signals of individual circuit functions.  
           [0006]    Another method working according to the previous state-of-the-art is known from the publication EP 0535776 B1. A test mode is activated at an additional input pin by an available signal and by means of an internal logic in order to thus apply selected signals, which are to be tested, of individual circuit units of the integrated circuit at the additional input pins of the integrated circuit.  
           [0007]    The disadvantage of the known methods according to the state of the art is that the areas needed for the function checks occupy a substantial part of the total area of the circuit, particularly in the case of small, but highly integrated circuits, in which the area is divided between the measuring pads within the circuit and the additional pins which are needed for the external measurement of the signals. Because of the large part of the total chip area, this gives rise to a substantial proportion of the total cost of a circuit. This has a negative effect on profitability, particularly in the case of small circuits which are manufactured in large numbers.  
         SUMMARY OF THE INVENTION  
         [0008]    The object of the present invention is to provide a method by which the internal signals of an integrated circuit are made available for external function checks with the use of already existing pins. A further object of the invention is to specify a circuit arrangement for implementing the method which can be easily and economically manufactured.  
           [0009]    The first-named object of the invention is solved by the features described in patent claim 1. The circuit arrangement is described by the features of patent claims 10 and 11. Favorable embodiments are the objects of the subclaims.  
           [0010]    Accordingly, the essence of the invention lies in switching the signals generated by a circuit unit within an integrated circuit, which are not measurable at the outputs in the normal operating mode, as test signals to the existing signal outputs for function checks. To do this, a defined potential value is applied to at least one signal output of the integrated circuit while the supply voltage is available, and the integrated circuit is thus switched into a test mode. A test signal generated by a circuit unit of the integrated circuit is thus applied at the signal output. In order to be able to achieve a reliable switchover into the test mode, it is necessary for the potential value applied at the signal output to be different to the value of the output voltage available at the signal output in normal operation of the integrated circuit. It is particularly advantageous if the potential value is applied or set at the signal output by means of a passive component, for example by a resistor.  
           [0011]    In comparison to the previous state of the art, it is advantageous that internal measuring pads do not have to be provided for the individual circuit elements of the circuit unit during the circuit design, and chip area is thus saved. Moreover, no additional pins or thus bond pads are needed for switching the integrated circuit into a test mode or for measuring the test signals. Particularly in the case of integrated circuits with signal outputs which are designed as “open collectors” with “pull up” resistors, a potential value, with which the integrated circuit can be switched into a test mode, can be set in a very inexpensive manner by means of a resistor. Moreover, it is irrelevant to the new method whether or not an input signal is available at a signal input of the integrated circuit. In particular, the output signals of circuit elements of the integrated circuit which is to be tested, which has an oscillator stage for example, can be made available as test signals at the signal output. Moreover, it is also possible to measure the test signal if there is an input signal available at the signal input of the integrated circuit, in order to check, in particular, the signal processing circuit elements of the circuit unit.  
           [0012]    In a development of the method, a plurality of potential values can also be applied at the signal output. By assigning each of the potential values to a different test signal, different test signals can be measured one after the other at a single signal output. In the case of integrated circuits which have only a single signal output, this enables different test signals to be analyzed one after the other at a single signal output with low circuit complexity.  
           [0013]    In a development of the method, it is advantageous to test the values of the potential values applied at the signal output for conformity with a reference value by means of a control unit during a defined time window before switching the integrated circuit into the test mode, and to switch the integrated circuit into the test mode in a second time window provided that the potential value available at the signal output corresponds to the defined reference value. In so doing, it is advantageous if the potential value set at the signal output remains constant over time and the test signal represents an alternating voltage. The direct voltage components can thus be easily separated with an external measurement arrangement. In comparison to the previous state of the art, an integrated circuit can be switched into a test mode by means of a single resistor in a particularly simple and inexpensive manner with the method according-to the invention.  
           [0014]    In the case of a plurality of signal outputs, it is possible in a development of the method for the control unit to test the potential value set at the signal output and apply the test signal at another signal output. The advantages of this are that there is no superimposition with the set direct voltage value and that a direct voltage offset of the test signal can be measured.  
           [0015]    In another development of the method, the control unit performs a Boolean logic operation with the potential value set at the signal output and a signal of a circuit component of the circuit unit. An AND logic operation with the negative signal value enables, for example, the integrated circuit to be prevented from being switched into a test mode by a potential available at the signal output. For this purpose, it is advantageous if the logic operation is performed with a control voltage from an output stage of the integrated circuit. This embodiment of the method according to the invention enables the test signal in the test mode to be prevented from being superimposed at the signal output on the output signals available in normal operation, that is if input signals are available for example, in a particularly reliable manner.  
           [0016]    Investigations by the applicant have shown that it is advantageous if different test signals are selected as a function of the voltage values set at the signal output, in that, for example, specific parts of the circuit are activated or deactivated by the control unit when there are defined potential values within the circuit unit. In this manner, defined signal shapes can be generated in the case of the test signals with or without an available input signal.  
           [0017]    In a further development of the method according to the invention, the integrated circuit is switched into the test mode if the potential value available at the signal output of the integrated circuit lies within an interval of a window discriminator. In so doing, it is advantageous if the signal heights of the test signals, that is their amplitudes and their direct voltage offsets, are set by means of signal amplifiers so that the direct voltage offset of the respective test signal corresponds to the potential value set at the signal output, and the maximum amplitude of the test signal lies within the interval defined by the respective window discriminator. Crosstalk between adjacent windows of the discriminators can be prevented in this way provided that a plurality of window discriminators are used. Furthermore, the direct voltage value at the signal output is little changed.  
           [0018]    The present, new circuit arrangement can be used in an advantageous manner for implementing the method according to the invention. An advantage of the integration of a control unit and at least one switching element according to the second-mentioned object of the present invention is that all internal measuring pads may be eliminated while incurring little additional circuit complexity. Another advantage is that a single resistor, which is connected externally to the signal output, is sufficient to switch the integrated circuit into a test mode if the supply voltage is available. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0019]    The method according to the invention is described in the following by means of an embodiment in conjunction with the drawings. They show:  
         [0020]    [0020]FIG. 1 A first circuit arrangement for implementing the method according to the invention, and  
         [0021]    [0021]FIG. 2 A second circuit arrangement for implementing the method according to the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    The object of the integrated circuit IC shown in FIG. 1 is to apply the output signals from a circuit unit as test signals to a signal output of the integrated circuit provided that, at the signal output, an externally applied potential value, which can be set by means of an external resistor for example, corresponds to a defined reference value. The integrated circuit IC has an input pin IN and an output pin OUT for this purpose. The output pin OUT is externally linked to a reference potential RV via a node  100  which is connected to a resistor W 1  by means of a switch T 1  or to a resistor W 2  by means of a switch T 2 . Furthermore, the integrated circuit has another pin at which a supply voltage VS is available and a pin which is linked to the reference potential RV.  
         [0023]    There are two functional units within the integrated circuit IC. The first functional unit contains the circuit functions needed for normal operation of the integrated circuit, which are represented, with the exception of a load element RL connected as a “pull up” between the voltage VDD and a node  50 , by a circuit unit SCH, the second functional unit comprises the test mode detection which consists of a control unit ST and a first and a second voltage-controlled switching element E 1  and E 2 . The switching unit SCH has a first input which is linked to the signal input IN of the integrated circuit IC, and a second input at which a signal MS is available, and a first output line which is linked to the control unit ST, a second output line which is linked to the switching element E 1 , at which a signal SW 1  which is to be tested is available, and a third output line which is linked to the switching element E 2 , at which a signal SW 2  which is to be tested is available. The outputs of the two switching elements E 1  and E 2  are linked to the node  50 . Furthermore, the node  50  is linked to the signal output OUT of the integrated circuit IC and by a wire  5  to the control unit ST. The control unit ST has a first output, at which the signal MS is available, which is linked to a control input of the switching element E 1  and to the second input of the circuit unit SCH, and a second output which is linked to a control input of the switching element E 2 .  
         [0024]    The principle of operation of the circuit is explained in the following. In this connection, a differentiation can be made between two operating modes of the integrated circuit.  
         [0025]    In the first operating mode, the resistor W 1  is separated by the switch T 1  from the signal output OUT. As there is thus no potential available at the signal output OUT of the integrated circuit IC that corresponds to the value predefined by the control unit ST, the integrated circuit is not switched into the test mode. Provided that an input signal ES is available at the input pin IN, a derived signal OS is applied to the control unit ST at the first output of the circuit unit SCH. The control unit ST passes the signal on the line  5  unchanged on to the node  50 , and thus applies the signal OS as an output signal to the output OUT of the integrated circuit IC.  
         [0026]    In the second operating mode, a defined potential is set at the output OUT by means of the switch TI connecting the resistor W 1  to the load element RL, as a result of which the integrated circuit IC is switched into a test mode. A circuit arrangement for potential detection is illustrated in FIG. 2. By the switchover into the test mode, a predefined circuit block within the circuit unit SCH, which outputs the signal SW 1  to the switching element E 1 , is selected from the control unit ST by means of the signal MS. Furthermore, the switching element E 1  is closed by the signal MS, and the signal SW 1  is applied at the signal output OUT as a test signal. The test signal SW 1  is an alternating voltage signal in order to minimize the effect on the direct voltage potential at the node  50 . If the resistor WI is separated from the signal output OUT by means of the switch T 1 , the potential at the node  50  increases to the voltage VDD, and the control unit ST switches the integrated circuit IC back into the normal operating state, that is the signal SW 1  is separated from the node  50  by means of the switching element E 1 , and the selection of the predefined circuit block is cancelled by the signal MS.  
         [0027]    The object of the integrated circuit IC shown in FIG. 2 is to supply an alternating voltage signal in normal operation. Whereas the external wiring of the integrated circuit IC is identical with the embodiment illustrated in FIG. 1, an advantageous implementation of a control unit ST is presented in a further development of the embodiment in FIG. 1. In the embodiment illustrated, the switchover of the integrated circuit into the test mode is a function of the result of a logical operation of the set potential value and the control signal of an output stage of the integrated circuit IC. The input IN is linked to a first input of a switching element SCH 1  within the integrated circuit IC. Furthermore, the switching element SCH 1  has a second input at which a signal MS is available, and a first output at which a signal OS is available which is linked to a node  10 , and a second output, at which a first signal S 1  which is to be tested is available, which is linked to a non-inverting input of a first amplifier LE 1 , and a third output, at which a second signal S 2 , which is to be tested is available, which is linked to a non-inverting input of a second amplifier LE 2 . Furthermore, the input of a signal output stage AS, for example an impedance amplifier, is still linked to the node  10 , and at any one time a first negative input  20  and  30  of an AND logic gate L 1  and L 2 . The output of the signal output stage AS is linked to the node  50 , to which are linked, apart from the signal output OUT, a load resistor RL downstream of the voltage VDD and in each case the output of a voltage-controlled switching element E 1  and a voltage-controlled switching element E 2 . Furthermore, a first non-inverting input of a comparator  11  and a first non-inverting input of a comparator  12  are linked to the node  50 .- A lower threshold voltage Vi is available at the inverting input of the comparator  11 , which together with the second upper threshold voltage V 2  available at the inverting input of the comparator  12  forms a window discriminator. The output of the comparator  11  is linked to an affirmative input of an AND logic gate L 1 , the output of the second comparator  12  is linked to a second negative input of an AND logic gate L 1 . The output of the logic gate L 1 , at which a signal SE 1  is available, is linked to both the control input of a switching element E 1  and to the second input of the circuit unit SCH 1 . Furthermore, the node  50  is linked to a first non-inverting input of a comparator  13  and to a first non-inverting input of a comparator  14 . A lower threshold voltage V 3  is available at the inverting input of the comparator  13 , which together with the upper threshold voltage V 4  available at the inverting input of the comparator  14  forms a second window discriminator. The output of the comparator  13  is linked to an affirmative input of an AND logic gate L 2 , the output of the comparator  14  is linked to the second negative input of an AND logic gate L 2 . The outlet of the logic gate L 2 , at which a signal SE 2  is available, is linked to the control input of a load element E 2 . Furthermore, a reference voltage P 1  is available at the inverting input of the amplifier LE 1 . The output of the amplifier LE 1 , at which the signal SW 1  is available, is linked to the node  50  by means of the voltage-controlled switching element E 1 . Furthermore, a reference voltage P 2  is available at the inverting input of the regulated amplifier LE 2 . The output of the amplifier LE 2 , at which the signal SW 2  is available, is linked to the node  50  by means of the voltage-controlled switching element E 2 .  
         [0028]    The principle of operation of the integrated circuit IC depending upon the external wiring is described in the following. There are two different operating modes. In the first operating mode, which represents the normal operating mode, an input signal ES is available at the signal input IN, from which the circuit unit SCH 1  derives the input signal OS for the output amplifier AS. As a signal is available at the node  10  and thus at both of the first inputs of the logic gates L 1  and L 2 , the result of the two AND logic operations is “false”. The potential of the node  50  is thus not taken into consideration, that is even a potential value set by the resistor W 1  would not switch the integrated circuit into the test mode. As a result, both the voltage-controlled switching elements E 1  and E 2  remain open. The amplified signal OS is available at the signal output OUT, whereby this represents the output signal of the integrated circuit IC in normal operation.  
         [0029]    In the second operating mode, there is no signal OS available at the node  10 . The integrated circuit IC is thus switched into a test mode by an external wiring, provided that the potential value set at the node  50  lies within the interval of one of the two window discriminators. In the illustrated embodiment, a potential, which lies within the voltage interval given by the first window discriminator, is set at the node  50  by means of the switch T 1  connecting the resistor W 1  to the load element RL. As the signals only have the correct polarity at the three inputs of the logic gate L 1 , only the result of the AND logic operation of the logic gate L 1  is “true” and the output signal SE 1  is switched to “high”. At the same time as the switching element E 1  closes, a predefined circuit component in the circuit unit SCH 1  is selected by the signal SE 1 , and a signal shape is generated which is applied in the shape of the signal S 1  to the node  50  as signal SW 1  by the amplifier LE 1 . The signal SW 1  to be tested is thus available at the signal output OUT. If the switch T 1  is opened, the potential at the node  50  increases to above the upper limiting value of the first window discriminator, and the output signal SE 1  of the logic gate L 1  is switched to “low” because the result of its logic operation is now “false”. The selection of the predefined circuit component is terminated and the switching element E 1  separates the signal SW 1  from the node  50 . The node  50  is raised to the value of the voltage VDD by the load element RL (pull up) so long as no further signals are fed to the node  50 . If the resistor W 2  is linked to the signal output OUT by means of the switch T 2 , a potential appears at the node  50  which lies within the interval of the second window discriminator. Thus only the result of the logic gate L 2  is “true”, and the test signal SW 2  is applied to the signal output OUT by means of the switching element E 2 . If the resistor W 2  is separated, the logic gate L 2  separates the test signal SW 2  from the signal output OUT by means of the switching element E 2 , and the potential of the node  50  is raised to the value of the voltage VDD.  
         [0030]    So that the two test signals SW 1  and SW 2  have minimal effect on the prevailing direct voltage value at the node  50 , or a coupling with the prevailing other window discriminator does not occur as a result of signals SW 1  and SW 2  having an excessive alternating voltage amplitude, the signals S 1  and S 2  are defined by the amplifiers LE 1  and LE 2  as regards their maximum amplitude as well as the direct voltage offset. In order to achieve the greatest possible amplitude swing, it is advantageous to select the direct voltage offset of the signals SW 1  and SW 2  so that this lies in the middle of the interval defined by the respective window discriminator. If the respective switching elements E 1  and E 2  are open, the test signals are only available at the respective inputs of the switching elements E 1  and E 2 .  
         [0031]    In contrast to the logic gate L 1 , the output of the logic gate L 2  is exclusively linked to the control input of the switching element E 2 . Therefore, a selection, that is an activation or deactivation of circuit functions within the circuit unit SCH 1 , cannot be made with the signal SE 2 .  
         [0032]    Finally, it should be noted that with the new method the number of test signals is limited only by the amplitude values of the test signals and the distances required between the individual voltage values available at the signal output.