Abstract:
An integrated memory circuit has a memory cell array and a test circuit. The test circuit generates an assessment datum, the assessment datum is dependent on a result of a comparison between a datum read from the memory cell array and a datum previously written to the memory cell array. A coding unit is coupled to the test circuit in order to code a plurality of assessment signals to form a coded test signal, a voltage signal is assigned to the plurality of test data as coded test datum.

Description:
BACKGROUND OF THE INVENTION  
       Field of the Invention  
         [0001]    The invention relates to an integrated memory circuit having a test circuit that generates assessment data and transmits the latter to a tester unit. The invention furthermore relates to a method for testing the integrated memory circuit with the test circuit.  
           [0002]    Integrated memory circuits are generally tested by a test system under various specification-conforming conditions. The testing is usually carried out by writing test data to a memory cell array of the memory circuit and subsequently reading out the data that have been written in. A comparison of the data read out with the data previously written in leads to assessment data which indicate whether the results of the comparison have yielded identity between the written-in and read-out data or a difference between written-in and read-out data. The comparison is usually carried out in a test circuit situated in the integrated memory circuit.  
           [0003]    The assessment data thus obtained serve for determining defective memory areas, which are replaced by redundant memory areas in subsequent repair steps in order to repair the integrated memory circuit.  
           [0004]    In order to calculate the repair solution, during testing the assessment data have to be transmitted from the integrated memory circuit to a tester unit, in which the optimum repair solution is calculated. The optimum repair solution indicates how the defective memory areas are to be replaced by redundant memory areas since it is possible to replace a defective memory area by redundant word lines or by redundant bit lines.  
           [0005]    Usually, a plurality of integrated memory circuits are tested simultaneously in a test system, the parallelism being prescribed by the number of test lines between the tester unit and the number of integrated memory circuits. The time for the testing an integrated memory circuit by the tester unit and the parallelism prescribe the throughput of the test system.  
           [0006]    What is a significant determining factor for the time duration for testing an integrated memory circuit is the time duration that is required for transmitting the assessment data from the memory circuit to the tester unit. In order to minimize this time, the assessment data are already compressed in a redundancy-conforming manner in the memory circuit, so that only information about errors in memory areas that are subsequently replaced by a redundant memory area is transmitted. In this case, the compression is based on the fact that bit-fine knowledge of the assessment data is not required, for example, for the repair of memory cells with memory cells from a so-called redundancy area. Consequently, it is possible to combine specific error areas, as a result of which information compression can be achieved. With this type of compression the assessment data are compressed by assessment logic and combined to form an individual assessment datum for each replaceable memory area and forwarded to the tester unit.  
           [0007]    However, even the transmission of all the assessment data compressed in a redundancy-conforming manner gives rise to considerable volumes of data to be transmitted, so that even the transmission of assessment data that have already been compressed requires a considerable time.  
         SUMMARY OF THE INVENTION  
         [0008]    It is accordingly an object of the invention to provide a test circuit of an integrated memory circuit for coding assessment data and a method for testing the memory circuit that overcome the above-mentioned disadvantages of the prior art devices and methods of this general type, which minimizes the test time for testing an integrated memory circuit.  
           [0009]    With the foregoing and other objects in view there is provided, in accordance with the invention, an integrated memory circuit. The memory circuit has a memory cell array, and a test circuit connected to the memory cell array. The test circuit generates an assessment datum, the assessment datum is dependent on a result of a comparison between a datum read from the memory cell array and a datum previously written to the memory cell array. A coding unit is coupled to the test circuit for coding a plurality of assessment data to form a coded test signal. The coded test signal is a voltage signal representing the plurality of assessment data and functioning as a coded test datum.  
           [0010]    A first aspect of the present invention provides an integrated memory circuit having a memory cell array and a test circuit. The test circuit generates an assessment signal, the assessment signal is dependent on a result of a comparison between a datum read from the memory cell array and a datum previously written to the memory cell array. According to the invention, a coding unit is coupled to the test circuit in order to code a plurality of assessment signals to form a coded test signal. In this case, a voltage signal is assigned to a plurality of assessment signals as a coded test signal, the voltage level of the voltage signal unambiguously describing the plurality of assessment signals.  
           [0011]    The coding unit according to the invention thus has the effect of carrying out a coding of the assessment signals, a voltage signal being assigned to the assessment signals. The voltage signal is an analog signal that can assume different voltage levels. It thus contains a higher information density than digitized signals. Consequently, during the testing of the integrated memory circuit, instead of transmitting a plurality of digital assessment signals in parallel or serially via test lines to a tester unit, it is possible to transmit a single analog signal via a test line to the tester unit, which signal contains the information of the plurality of assessment signals.  
           [0012]    It may preferably be provided that the coding unit has a digital-to-analog converter circuit, so that the plurality of assessment signals are converted into a voltage level, each voltage level being assigned to a specific pattern of a plurality of assessment signals.  
           [0013]    In order to read the coded test signal from the integrated memory circuit, an external terminal is preferably provided, via which the integrated circuit can be connected to a tester unit.  
           [0014]    A further aspect of the present invention provides a tester unit in order to receive coded test signals. The tester unit has a decoding circuit, a coded test signal containing a voltage signal that can assume a plurality of signal levels. The decoding circuit is configured in such a way as to assign a respective number of assessment data to the voltage levels of the received voltage signal. What is essential is that each voltage level is respectively assigned a series of assessment data, so that, after the reception of the test data in the tester unit, the assessment data can be assigned to the defective memory areas. In this way, it is possible to provide a tester unit which receives compressed test data and decodes the latter in order to make them available e.g. to an evaluation unit which, in the tester unit, determines a redundancy solution for the optimum replacement of defective memory areas.  
           [0015]    The decoding circuit preferably contains an A/D converter circuit in order to convert the voltage levels of the test signal into a digital value containing bits that represent the original assessment data.  
           [0016]    A further aspect of the present invention provides a test system having an integrated memory circuit according to the invention, the integrated memory circuit being connected to a tester unit, so that the coded test signal can be transmitted to the tester unit. A test system with which integrated memory circuits can be tested more rapidly is made available in this way.  
           [0017]    A further aspect of the present invention provides a method for testing an integrated memory circuit. A datum read from the memory cell array and a datum previously written to the memory cell array are compared with one another during the testing, an assessment signal which is dependent on the result of the comparison being generated. According to the invention, a plurality of assessment signals are coded into a coded test signal, the coded test signal being transmitted to a tester unit, the transmitted coded test signal being decoded to form a plurality of assessment signals.  
           [0018]    The method according to the invention has the advantage that the assessment data can be transmitted in compressed fashion to the tester unit, as a result of which it is possible, on the one hand, to save test lines, in order thus to increase the parallelism of the test system, and, on the other hand, to increase the transmission speed for transmitting the assessment data, so that more assessment data can be communicated to the tester unit within a specific time.  
           [0019]    Other features which are considered as characteristic for the invention are set forth in the appended claims.  
           [0020]    Although the invention is illustrated and described herein as embodied in a test circuit of an integrated memory circuit for coding assessment data and a method for testing the memory circuit, 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.  
           [0021]    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  
       [0022]    [0022]FIG. 1 is a block diagram of a test system according to the invention;  
         [0023]    [0023]FIG. 2 is a circuit diagram of a coding unit for an integrated memory circuit according to the invention;  
         [0024]    [0024]FIG. 3 is a table representing the coding of assessment data into different voltage levels; and  
         [0025]    [0025]FIG. 4 is a block diagram of a decoder circuit for a tester unit according to the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]    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 test system according to the invention. The test system has an integrated memory circuit  1 , in particular a DRAM memory circuit having a memory cell array  2  and a test circuit  3 . The integrated memory circuit  1  is connected to a tester unit  5  via a signal bus  4 . Via the signal bus  4 , address data and control data can be transmitted from the tester unit  5  to the integrated memory circuit  1  and assessment data can be transmitted from the integrated memory circuit  1  to the tester unit  5 .  
         [0027]    The test circuit  3  has a comparator circuit  6 , which serves for comparing the data written to the memory cell array  2  with the data read from the memory cell array  2  and for generating assessment data as the result, the assessment data being dependent on whether the written-in and read-out data are identical to or different from one another. Thus, by way of example, an assessment datum has the value “0” if the written-in and read-out data are identical and the value “1” if the written-in and read-out data are different.  
         [0028]    Usually, the data are compared with one another bit by bit or memory area by memory area, thereby generating assessment data that indicate bit by bit whether the written-in and read-out data are identical to or different from one another. The assessment data are forwarded to a coding unit  7 , which performs a compression of the assessment data.  
         [0029]    After the assessment data has been compressed, the coded test data thus determined are communicated via the signal bus  4  to the tester unit  5 , where the coded test data are decoded in a decoding unit  8  and fed to an evaluation circuit  9 , which determines a redundancy solution from the assessment data.  
         [0030]    [0030]FIG. 2 illustrates a circuit diagram of a coding unit for the integrated memory circuit according to the invention. The coding unit illustrated essentially represents a two-bit digital-to-analog converter that converts two bits of data into an analog voltage value. For the sake of clarity, the illustration of the coding unit is restricted to a two-bit digital-to-analog converter. However, it is conceivable for an arbitrary number of bits to be coded in this way. This is limited only by the resolution accuracy of the decoding unit  8  in the tester unit  5  and by the susceptibility to interference of the signal lines of the signal bus  4 .  
         [0031]    The coding unit  7  is essentially constructed in two parts for each possible bit combination of the assessment data to be coded. A first part essentially contains an AND gate  10   a - 10   d , to whose inputs the assessment data to be coded are applied either in non-inverted form or in a manner inverted by an inverter. The output of the respective AND gate leads to a control input, a switch  13   a - 13   d , the respective switch  13   a - 13   d  being closed in the event of a high state of the output of the respective AND gate  10   a - 10   d , so that a voltage generated by a voltage divider  11  is applied to an output line  12 . The AND gates  10   a - 10   d  are connected in such a way that only one of the switches  13   a - 13   d  is closed in each case. Each of the switches  13   a - 13   d  switches a voltage potential onto the output line  12  which unambiguously determines the bit combination present, i.e. it differs from the other voltage potentials.  
         [0032]    Thus, the first bit and the second bit are applied in non-inverted form to the inputs of the first AND gate  10   a , the first bit and the second bit are applied in inverted and non-inverted form, respectively, to the second AND gate  10   b , the first bit and the second bit are applied in non-inverted and inverted form, respectively, to the third AND gate  10   c , and the first bit and the second bit are applied in inverted form to the fourth AND gate  10   d.  In this way, the four states which can be represented by the first bit and the second bit can be coded into four voltage levels. This is illustrated in the table according to FIG. 3.  
         [0033]    Such a coding unit can be extended arbitrarily and thus be used for coding two, three, four or more assessment data.  
         [0034]    The voltage divider  11  is essentially constructed by a series of resistors  14   a ,  14   b ,  14   c , between which various predefined voltage levels can be tapped off. The series of resistors  14   a ,  14   b ,  14   c  is disposed between a supply voltage potential VDD and a ground potential GND. The supply voltage potential VDD represents the first voltage level V 1  and the ground potential GND represents the fourth voltage level V 4 . The second voltage level V 2  can be tapped off the node between the first resistor  14   a  and the second resistor  14   b , and the third voltage level V 3  can be tapped off at the node between the second resistor  14   b  and the third resistor  14   c . The first voltage level V 1  is connected to a terminal of the first switch  13   a , the second voltage level V 2  is connected to a terminal of the second switch  13   b , the third voltage level V 3  is connected to a terminal of the third switch  13   c , and the fourth voltage level V 4  is connected to a terminal of the fourth switch  13   d.    
         [0035]    Such a voltage divider circuit can likewise be extended in the coding of more than two bits, so that not only four, but 8, 16 and more voltage levels can be generated.  
         [0036]    [0036]FIG. 4 illustrates a circuit diagram of the decoding unit  8 , which may be provided in the tester unit  5 , for example. The decoding unit  8  is essentially an analog-to-digital converter circuit, with a resolution that corresponds at least to the number of assessment data compressed in the test signal. It is provided with a second voltage divider circuit  20  having four resistors  21   a ,  21   b ,  21   c ,  21   d  connected in series. The first and fourth resistors  21   a ,  21   d  have respectively half the resistance of respectively the second and third resistors  21   b ,  21   c . The node between the first resistor  21   a  and the second resistor  21   b  is connected to a non-inverting input of a first comparator. The node between the second resistor  21   b  and the third resistor  21   c  is connected to the non-inverting input of a second comparator circuit  21   b , and the node between the third resistor  21   c  and the fourth resistor  21   d  is connected to the non-inverting input of a third comparator circuit  22   c . The coded test signal is applied to the inverting inputs of the first, second and third comparator circuits  22   a ,  22   b ,  22   c . The comparator circuits  22   a ,  22   b ,  22   c  are in each case connected by an output to a converter circuit  23  having two outputs. Signal bits coded in the coded test signal are present at the two outputs during the decoding.  
         [0037]    The converter circuit  23  decodes the three signals present at the outputs of the comparator circuits  22   a ,  22   b ,  22   c , thereby recovering the first and second bits of the original assessment data. Depending on the voltage level of the coded test signal, none, one or all of the outputs of the comparator circuits  22   a ,  22   b ,  22   c  have a high state. If the voltage level is 0 V or the voltage V 1 , then all the outputs of the comparator circuit  22  are at a high level, while in the event of a voltage level of the coded test signal with the magnitude of the supply voltage or the voltage V 4 , then all three outputs of the comparator circuits are in a low state.  
         [0038]    In this way, it is possible to provide a decoding unit  8  which is able to process a coded test signal in which four states which can be represented by two bits are decoded, in which the voltage level ranges of the defined voltage levels of the coded test signal are respectively assigned to a state defined by a two-bit combination by the first bit and the second bit.  
         [0039]    The invention relates to compressing an arbitrary number of cell signals to form a single item of error information and in transferring it from the integrated memory circuit to an external tester unit.  
         [0040]    The invention furthermore contains the tester unit having the decoding circuit that is able to decode the compressed error information, i.e. to assign a series of assessment data to the voltage levels of the coded test signal. The compression enables a high information transmission rate per unit time.  
         [0041]    The coded test signals that have been generated with the aid of the coding unit illustrated in FIG. 2 are preferably transmitted to the tester unit  5  within a clock cycle. In this way, the transmission of the assessment data can be accelerated by a factor of 4 since information from four-bit assessment data is transmitted with the aid of a coded test signal that can be transmitted per clock cycle.  
         [0042]    The decoding unit  8  may either be disposed within the tester unit or be connected upstream of the tester unit, so that there is no need to modify a conventional tester unit. Consequently, the tester lines disposed between the integrated memory circuit and the tester unit are not connected to the tester unit directly, but rather via the decoding unit  8 , at whose outputs the decoded assessment data can then be tapped off. In this way it is possible to avoid intervention in the tester devices of complex construction.