Abstract:
A semiconductor device, having an input terminal configured to receive a multi-valued input signal as input, the multi-valued input signal including a plurality of values, a multi-valued logic circuit that operates with a multi-valued function and output binary signals to an output section in response to the input signal that has been input to the input terminal, the output section having a number of nodes being one less than a number of the plurality of values of the multi-valued input signal, and an impedance control circuit that is connected to the input terminal and the output section, and changes a combined resistance value in response to the binary signals of the plurality of nodes to change a current which flows in the input terminal.

Description:
[0001]    The present application is a Continuation application of U.S. patent application Ser. No. 12/659,286 filed on Dec. 16, 2009, which is based on Japanese Patent Application No. 2009-95331, filed on Feb. 27, 2009, the entire contents of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a semiconductor device, and more particularly to a semiconductor device having a test circuit of a multivalued logic circuit. 
         [0004]    2. Background of the Invention 
         [0005]    In the semiconductor device field, with the large-scaled system and the increasing number of logic gates to be mounted, the number of input/output terminals is also remarkably increased. However, the number of input/output terminals that can be mounted on the semiconductor device is limited by a package. For that reason, there is a case in which a larger package must be employed when the number of input/output terminals exceeds a mountable limit. Under the circumstances, a reduction in the number of input/output is executed by using the multivalued logic circuit. However, there is a need to provide the semiconductor device with a test-use output terminal, which leads to a problem on a reduction in the number of terminals. Further, the number of multivalued logics that can be dealt with in the individual multivalued logic circuits is increased, whereby a technique enabling the multivalued logic circuit to be efficiently tested is increasingly required. 
         [0006]    As a test of the semiconductor device including the multivalued logic circuit, there is a threshold voltage test of the multivalued input terminal. JP-A-03-209181 has proposed a method for reducing the number of terminals to be used in a threshold voltage test. 
         [0007]    First, a description will be given of the configuration of a semiconductor device discloses in JP-A-03-209181 with reference to  FIG. 9 . As shown in  FIG. 9 , an n-valued input terminal  1  is connected with (n−1) comparators C 1  to Cn−1. The comparators C 1  to Cn−1 are connected to an internal logic  3  and a threshold voltage test circuit  6 , respectively. Output terminals  5  are connected to the internal logic  3 , respectively, and an output terminal  7  is connected to the internal logic  3  or the threshold voltage test circuit  6  through an output mode selector switch  4 . The respective comparators C 1  to Cn−1 are set with threshold voltages different gradually such as V 1 &lt;V 2 &lt; . . . &lt;Vn−1, respectively. 
         [0008]    The operation of testing the semiconductor device shown in  FIG. 9  will be described. First, in order to observe an output signal of the threshold voltage test circuit  6  by using the output terminal  7 , the output mode selector switch  4  changes over to connect the threshold voltage test circuit  6  and the output terminal  7 . 
         [0009]    In that state, an input voltage of the n-valued input terminal  1  is sequentially raised from 0 volt, an output signal of the comparator C 1  whose threshold voltage is V 1  changes to “H” level from “L” level. When the input voltage is raised as it is, an output signal of the comparator C 2  whose threshold voltage is V 2  changes to “H” level from “L” level. This operation is sequentially repeated with the results that an output signal of the comparator Cn−1 whose threshold voltage is Vn−1 finally changes to “H” level from “L” level. 
         [0010]    In this situation, as shown in  FIG. 10 , a test output signal of the threshold voltage test circuit  6  is also inverted every time the output signals of the comparators C 1  to Cn−1 are inverted. That is, an output signal observed at the output terminal is inverted when the input voltage of the n-valued input terminal  1  first reaches V 1  volt, and again inverted when the input voltage then reaches V 2  volt. This operation is repeated, and when the input voltage of the n-valued input terminal  1  reaches Vn−1 volt, (n−1)-th inversion is conducted. 
         [0011]    Accordingly, when the voltage of the n-valued input terminal  1  when the test output signal is inverted is measured, and compared with the threshold voltages of the comparators C 1  to Cn−1, it is possible to determine whether the comparators C 1  to Cn−1 normally operate, or not. 
         [0012]    The above description is applied to a case of typical n-valued input. A specific example of four-valued input will be described.  FIG. 11  is a circuit configuration diagram showing the configuration of the threshold voltage test circuit  6  in the case of four-valued input. As shown in  FIG. 11 , the threshold voltage test circuit  6  includes two inversion logic blocks and three NAND logic blocks. The threshold voltage test circuit  6  is connected to the n-valued input terminal  1  through the comparators C 1  to C 3 , and also to the output terminal  7 . With the above configuration, the threshold voltage test circuit  6  sequentially generates inversion logic according to the output signals of the comparators C 1  to C 3 , as shown in a characteristic diagram of  FIG. 12 . 
       SUMMARY 
       [0013]    In the above-described semiconductor device, the test output signal from the threshold voltage test circuit  6  is observed by using the output terminal  7 . Hence, the output mode selector switch  4  naturally required in the normal operation must be provided. That is, even in the normal operation, the output signal from the internal logic  3  is transmitted to the output terminal  7  via the output mode selector switch  4 . Because this output signal, as compared with other output signals to be transmitted to the output terminal  5 , passes through the output mode selector switch  4 , there arises such a problem that a transmission delay occurs. As a result, the output signals from the output terminal  5  and the output terminal  7 , which should be naturally synchronized with each other, are not synchronous with each other. 
         [0014]    According to an aspect of the present invention, a semiconductor device includes: an input terminal to which an input signal is input; plural comparators that operate at different threshold voltages in response to the input signal that has been input to the input terminal, respectively; and an impedance control circuit that is connected to the input terminal and the outputs of the plural comparators, respectively, and changes a combined resistance value in response to output signals of the plural comparators, thereby changing a current which flows in the input terminal. 
         [0015]    According to the present invention, since the operation of the comparators can be tested by only measuring a value of a current flowing in the input terminal to which the input signal is input, there is no need to newly provide an output terminal for observing the test output signal. Also, since there is no need to provide the output mode selector switch for outputting the test output signal by using the output terminal of the multivalued logic circuit, an increase in the transmission delay of the output signal of the multivalued logic circuit can be prevented. 
         [0016]    According to the present invention, there can be provided the semiconductor device having the test circuit of the multivalued logic circuit without newly provision of the output terminal for the test signal, and with no increase in the transmission delay in the output signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a circuit configuration diagram showing a semiconductor device according to a first embodiment of the present invention; 
           [0018]      FIG. 2  is a circuit configuration diagram showing an impedance control circuit according to the first embodiment of the present invention; 
           [0019]      FIG. 3  is a circuit configuration diagram showing a semiconductor device according to a first example; 
           [0020]      FIG. 4  is a circuit configuration diagram showing an impedance control circuit according to the first example; 
           [0021]      FIG. 5  is a true value table showing comparator outputs according to the first example; 
           [0022]      FIG. 6  is a characteristic diagram showing the semiconductor device according to the first example; 
           [0023]      FIG. 7  is a circuit configuration diagram showing a semiconductor device according to a second embodiment of the present invention; 
           [0024]      FIG. 8  is a circuit configuration diagram showing an impedance control circuit according to the second embodiment of the present invention; 
           [0025]      FIG. 9  is a circuit configuration diagram showing a semiconductor device; 
           [0026]      FIG. 10  is a characteristic diagram showing a threshold voltage test circuit; 
           [0027]      FIG. 11  is a circuit configuration diagram showing a semiconductor device in the case of four-valued input; and 
           [0028]      FIG. 12  is a circuit configuration diagram showing a threshold voltage test circuit in the case of four-valued input. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment of the Invention 
       [0029]    First, a description will be given of a circuit configuration of a semiconductor device according to a first embodiment of the present invention with reference to  FIG. 1 . As shown in  FIG. 1 , the semiconductor device includes an n-valued input terminal  1  (n is a natural number of 3 or more), (n−1) comparators C 1  to Cn−1, an internal logic  3 , output terminals  5 , an impedance control circuit  8 , an input node Y 0 , and (n−1) comparator output nodes Y 1  to Yn−1. 
         [0030]    The n-valued input terminal  1  receives an n-valued signal from the external of the semiconductor device. 
         [0031]    The comparators C 1  to Cn−1 are set with different threshold voltages V 1  to Vn−1 which satisfy a relationship of V 1 &lt;V 2 &lt; . . . &lt;Vn−1, respectively. Each of the comparators C 1  to Cn−1 outputs a signal “0” when an input voltage of the n-valued signal is lower than a threshold voltage. On the other hand, each comparator outputs a signal “1” when the input voltage of the n-valued signal is equal to or higher than the threshold voltage. 
         [0032]    The internal logic  3  operates upon receiving the output signals of the comparators C 1  to Cn−1, and outputs a signal to the output terminals  5 . 
         [0033]    The configuration of the impedance control circuit  8  will be described with reference to  FIG. 2 . As shown in  FIG. 2 , (n−1) impedance switch circuits S 1  to Sn−1 are connected in parallel between the input node Y 0  and a ground potential being a reference potential. The impedance switch circuits S 1  to Sn−1 are made up of n-channel transistors M 1  to Mn−1, and resistors R 1  to Rn−1, respectively. Gates of the n-channel transistors M 1  to Mn−1 are connected with the comparator output nodes Y 1  to Yn−1, respectively. Sources of the n-channel transistors M 1  to Mn−1 are connected to the ground potential being the reference potential. Drains of the n-channel transistors M 1  to Mn−1 are connected to the input node Y 0  through pull-down resistors R 1  to Rn−1, respectively. 
         [0034]    Subsequently, the operation of the semiconductor device shown in  FIG. 1  will be described. When the input voltage which is applied to the n-valued input terminal  1  sequentially increases from 0 volt, the output signal of the comparator C 1  first reaches “1”. When the input voltage sequentially increases, the output signal of the comparator C 2  then reaches “1”. Likewise, when the input voltage sequentially increases, the output signal of the comparator Cn−1 finally reaches “1”. 
         [0035]    The operation of the impedance control circuit  8  shown in  FIG. 2  will be described. When the input voltage which is applied to the n-valued input terminal  1  sequentially increases from 0 volt, the output signal “1” of the comparator C 1  is supplied to the n-channel transistor M 1  through the comparator output node Y 1 . Hence, the n-channel transistor M 1  turns on. Since a source of the n-channel transistor M 1  is connected with the ground potential, and a drain thereof is connected with the resistor R 1 , a current I 1  produced by a potential difference between the input voltage of the n-valued input terminal  1  and the ground potential flows into the impedance switch circuit S 1 . 
         [0036]    When the input voltage sequentially increases, the output signal “1” of the comparator C 2  is supplied to the n-channel transistor M 2  through the comparator output node Y 2 . Hence, the n-channel transistor M 2  turns on, and a current I 2  flows into the impedance switch circuit S 2 . Likewise, when the input voltage sequentially increases, the output signal “1” of the comparator Cn−1 is finally supplied to the n-channel transistor Mn−1 through the comparator output node Yn−1. Hence, the n-channel transistor Mn−1 turns on, and a current In−1 flows into the impedance switch circuit Sn−1. That is, with an increase in the input voltage, the impedance switch circuits S 1  to Sn−1 are sequentially rendered conductive, a combined resistance value of the entire impedance control circuit  8  decreases, and a current I (I=I 1 +I 2 + . . . +In−1) flowing in the n-valued input terminal  1  increases. 
         [0037]    In this situation, when the resistance values of the resistors R 1  to Rn−1 are set in advance, a relationship between the input voltage which is applied to the n-valued input terminal  1  and the current I can be known. Accordingly, through observation of the current I, it is possible to determine whether the comparators C 1  to Cn−1 normally operate at the set threshold voltages, or not. 
         [0038]    That is, the operation of the comparators C 1  to Cn−1 can be tested by using the originally existing n-valued input terminal  1 . Hence, there is no need to provide the output mode selector switch  4  shown in  FIG. 9 . 
         [0039]    Hence, the delay of the output signal from the internal logic  3  can be prevented from increasing. 
         [0040]    Further, since an increase in the delay of the output signal may not be taken into consideration, there is no need to provide the internal logic  3  with a delay compensation unit for the output signal, and a timing design of the internal logic gets facilitated. 
         [0041]    A control terminal and a wiring for controlling the above-described switch circuit are not required. Hence, it is possible to reduce the number of terminals in the semiconductor device, and to substantially enlarge an internal region of the semiconductor device. 
         [0042]    Also, according to the configuration of  FIG. 9 , the threshold voltage must be determined by counting the number of toggles of the output signal. In conducting a test, there is required a test time taking, into consideration, a delay time of the threshold voltage test circuit  6 , a delay time at the output mode selector switch  4 , and a delay time of the output circuit. That is, a voltage to be applied to the n-valued input terminal  1  must be sequentially changed from 0V to a supply voltage while taking time. However, according to the above-described configuration, since a relationship between the input voltage and the current I flowing in the n-valued input terminal  1  has been known, it is only necessary to measure the current I, and the test time can be reduced. 
         [0043]    Also, in the typical semiconductor device, there is a need to set a combination of the input terminal of the multivalued input circuit to be measured with the output terminals from which the test results are output, and to create a test program for switching between a test mode and a normal operation mode. However, according to this configuration, since only the input terminal of the n-valued input circuit is used, there is no need to switch between the test mode and the normal operation mode, thereby enabling a test to be conducted by a simpler test program. Accordingly, a period of time for creating the test program can be reduced. 
       First Example 
       [0044]    The configuration of a semiconductor device according to a first example will be described with reference to  FIG. 3 .  FIG. 3  shows a circuit configuration of the semiconductor device shown in  FIG. 1  in the case of inputting a four-valued input signal. Accordingly, in  FIG. 3 , there are disposed three comparators C 1  to C 3 , three comparator output nodes Y 1  to Y 3 , and an impedance control circuit  8   a  accepting four-valued input. Other configurations are identical with those in  FIG. 1 , and therefore duplicate description will be omitted. The respective threshold voltages are 0.8 V in the comparator C 1 , 1.6 V in the comparator C 2 , and 2.5 V in the comparator C 3 . 
         [0045]    The configuration of the impedance control circuit  8   a  will be described with reference to  FIG. 4 . Because the impedance control circuit  8   a  accepts a four-valued input signal, the impedance control circuit  8   a  has three impedance switch circuits S 1  to S 3 . Other configurations are identical with those in  FIG. 2 , and therefore their description will be omitted. Resistance values of the impedance switch circuits S 1  to S 3  are 30 kΩ, respectively. 
         [0046]    The operation of the semiconductor device according to the first example will be described with reference to  FIGS. 3 and 5 . When, for example, 0 V is applied to the n-valued input terminal  1  as the input voltage, the output signals of the comparators C 1 , C 2 , and C 3  are “0”, “0”, and “0”, respectively. 
         [0047]    When, for example, 1.2 V is applied to the n-valued input terminal  1  as the input voltage, the output signals of the comparators C 1 , C 2 , and C 3  are “1”, “0”, and “0”, respectively. 
         [0048]    When, for example, 2.2 V is applied to the n-valued input terminal  1  as the input voltage, the output signals of the comparators C 1 , C 2 , and C 3  are “1”, “1”, and “0”, respectively. 
         [0049]    When, for example, 3.3 V is applied to the n-valued input terminal  1  as the input voltage, the output signals of the comparators C 1 , C 2 , and C 3  are “1”, “1”, and “1”, respectively. 
         [0050]      FIG. 6  shows a relationship between the input voltage applied to the n-valued input terminal  1  and the current I flowing in the n-valued input terminal  1 . When the input voltage ranges from 0 V to 0.8 V, because all of the n-channel transistors M 1  to M 3  shown in  FIG. 4  get in an off state, the combined resistance value of the impedance control circuit  8   a  becomes infinite, and no current I flows. 
         [0051]    When the input voltage ranges from 0.8 V to 1.6 V, only the n-channel transistor M 1  gets in an on state, and the n-channel transistors M 2  and M 3  get in the off state. Therefore, the combined resistance value of the impedance control circuit  8   a  becomes 30 KΩ. Accordingly, the current I corresponding to the combined resistance value 30 KΩ of the impedance control circuit  8   a  flows in the n-valued input terminal  1 . 
         [0052]    When the input voltage ranges from 1.6 V to 2.5 V, the n-channel transistors M 1  and M 2  get in the on state, and the n-channel transistor M 3  get in the off state. Therefore, the combined resistance value of the impedance control circuit  8   a  becomes 15 KΩ. Accordingly, the current I corresponding to the combined resistance value 15 KΩ of the impedance control circuit  8   a  flows in the n-valued input terminal  1 . 
         [0053]    When the input voltage is equal to or higher than 2.5 V, all of the n-channel transistors M 1  to M 3  get in the on state. Therefore, the combined resistance value of the impedance control circuit  8   a  becomes 10 KΩ. Accordingly, the current I corresponding to the combined resistance value 10 KΩ of the impedance control circuit  8   a  flows in the n-valued input terminal  1 . 
         [0054]    Hence, as shown in  FIG. 6 , a change in the input voltage applied to the n-valued input terminal  1  and the current I are measured, thereby making it possible to determine whether the comparators C 1  to C 3  normally operate at the threshold voltages set for the respective comparators, or not. 
       Second Embodiment of the Invention 
       [0055]    A second embodiment of the present invention changes the configuration of the impedance control circuit in the first embodiment of the invention. 
         [0056]    First, the configuration of a semiconductor device according to the second embodiment of the present invention will be described with reference to  FIG. 7 . In  FIG. 7 , the impedance control circuit  8  in  FIG. 1  is substituted with an impedance control circuit  9 . Also, the comparators C 1  to Cn−1 are set with different threshold voltages V 1  to Vn−1 which satisfy a relationship of V 1 &gt;V 2 &gt; . . . &gt;Vn−1, respectively. Other configurations are identical with those in  FIG. 1 , and therefore their description will be omitted. 
         [0057]    The circuit configuration of the impedance control circuit  9  will be described with reference to  FIG. 8 . As shown in  FIG. 8 , (n−1) impedance switch circuits SU 1  to SUn−1 are arranged between the input node Y 0  and a ground potential being a reference potential. The impedance switch circuits SU 1  to SUn−1 are made up of p-channel transistors MU 1  to MUn−1, and resistors RU 1  to RUn−1, respectively. Gates of the p-channel transistors MU 1  to MUn−1 are connected with the comparator output nodes Y 1  to Yn−1, respectively. Sources of the p-channel transistors MU 1  to MUn−1 are connected to a power potential being the reference potential. Drains of the p-channel transistors MU 1  to MUn−1 are connected to the input node Y 0  through pull-up resistors RU 1  to RUn−1, respectively. 
         [0058]    The operation of the semiconductor device shown in  FIG. 7  will be described. When the input voltage which is applied to the n-valued input terminal  1  sequentially decreases from the supply voltage, the output signal of the comparator C 1  first reaches “0”. When the input voltage sequentially decreases, the output signal of the comparator C 2  then reaches “0”. Likewise, when the input voltage sequentially decreases, the output signal of the comparator Cn−1 finally reaches “0”. 
         [0059]    The operation of the impedance control circuit  9  shown in  FIG. 8  will be described. When the input voltage which is applied to the n-valued input terminal  1  sequentially decreases from the supply voltage, the output signal “0” of the comparator C 1  is supplied to the p-channel transistor MU 1  through the comparator output node Y 1 . Hence, the p-channel transistor MU 1  turns on. Since a source of the p-channel transistor MU 1  is connected with the power potential, and a drain thereof is connected with the resistor RU 1 , a current IU 1  produced by a potential difference between the input voltage of the n-valued input terminal  1  and the power potential flows into the impedance switch circuit SU 1 . 
         [0060]    When the input voltage sequentially decreases, the output signal “0” of the comparator C 2  is supplied to the p-channel transistor MU 2  through the comparator output node Y 2 . Hence, the p-channel transistor MU 2  turns on, and a current IU 2  flows into the impedance switch circuit SU 2 . Likewise, when the input voltage sequentially decreases, the output signal “0” of the comparator Cn−1 is finally supplied to the p-channel transistor MUn−1 through the comparator output node Yn−1. Hence, the p-channel transistor MUn−1 turns on, and a current IUn−1 flows into the impedance switch circuit SUn−1. That is, with a decrease in the input voltage, the impedance switch circuits SU 1  to SUn−1 are sequentially rendered conductive, a combined resistance value of the entire impedance control circuit  9  decreases, and a current IU (IU=IU 1 +IU 2 + . . . +IUn−1) flowing in the n-valued input terminal  1  increases. 
         [0061]    In this situation, when the resistance values of the resistors RU 1  to RUn−1 are set in advance, a relationship between the input voltage which is applied to the n-valued input terminal  1  and the current IU can be known. That is, according to this configuration, the observation of the current IU makes it possible to determine whether the comparators C 1  to Cn−1 normally operate at the set threshold voltages, or not. Accordingly, the same functions as those of the semiconductor device shown in  FIG. 1  can be realized. 
         [0062]    The present invention is not limited to or by the above embodiments, but can be appropriately changed without departing from the subject matter of the invention. For example, the impedance control circuit or the impedance switch circuits may be of other configurations if the same functions can be realized. 
         [0063]    In the above-described embodiments, the internal logic and the impedance control circuit are separated from each other, but the impedance control circuit may be incorporated into the internal logic. Further, the impedance switch circuits may be dispersed into a logic circuit configuring the internal logic.