Patent Abstract:
A semiconductor device is provided having an internal circuit in which input data is gated and supplied to the internal circuit according to an internal control signal generated within the semiconductor device. The semiconductor device has N number (N being two or greater integers) of data input terminals for inputting input data, and a test mode input terminal for inputting a test mode signal. An OR device is provided for obtaining a logical sum of the internal control signal and the test mode signal. The semiconductor device also has N number of gate circuits that are supplied with the input data applied to the N data input terminals, respectively. When an output of the OR device is active, those of the N gate circuits responsive to the output of the OR device pass the input data applied to the data input terminals. The internal circuit is supplied with outputs of the N gate circuits.

Full Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention generally relates to semiconductor devices equipped with a test mode and a method for operating the same. More particularly, the present invention relates to a semiconductor device with a built-in measurement circuit that supports examinations of the semiconductor device when the semiconductor device is examined by a test apparatus such as an IC tester in a test mode. 
     2. Description of Related Art 
     FIG. 3 shows a conventional semiconductor device. In the semiconductor device shown in FIG. 3, buffer circuits  101 ,  102 ,  103 , . . . , each having two inverters, are inserted as input circuits between input terminals  1 ,  2 ,  3 ,. . . and an internal circuit  30 . 
     As shown in FIG. 3, when input data is applied to the input terminals  1 ,  2 ,  3 , . . . from a test apparatus such as an IC tester, the input data is supplied to the internal circuit  30  through the buffer circuits  101 ,  102 ,  103 , . . . , respectively. Therefore, when this type of semiconductor device is examined by using the test apparatus, logic levels to be inputted in the internal circuit need to be measured. For this purpose, it is proposed to provide a measurement circuit that operates in a test mode within the semiconductor device, to thereby measure logic levels that are inputted in the internal circuit. In the semiconductor device shown in FIG. 3, for example, AND circuits  21 ,  22 ,  23 , . . . , each including a NAND gate and an inverter, are provided as measurement circuits. 
     The AND circuits are connected to a series of the data input terminals in a chain like manner. More particularly, for example, input data from the second data input terminal  2  is supplied through the buffer circuit  102  to one of two inputs of the second AND circuit  22 . Also, an output from the AND circuit  21  that is connected to the second AND circuit  22  in an immediately proceeding stage is supplied to the other input of the AND circuit  22 . Furthermore, an output of the AND circuit  22  is supplied to one input of the AND circuit  23 , and input data from the third data input terminal  3  is inputted to the other input of the AND circuit  23 . In this manner, the multiple AND circuits are connected in a chain-like manner. 
     A test mode signal TEST is supplied through a test mode signal input terminal  60  to one input of the AND circuit  21  in the first stage. The test mode signal TEST is at high level in a test mode. Also, an output of one of the AND circuits in the last stage is supplied to one input of a selection circuit  70 . An output of the internal circuit  30  is supplied to the other input of the selection circuit  70 . The selection circuit  70  is controlled by the test mode signal TEST. The selection circuit  70  selects the output of the internal circuit  30  in a normal operation mode, and selects the output of the AND circuit in the last stage in a test mode. An output of the selection circuit  70  is read out through an output terminal  80  by an external device. 
     It is noted that, in the normal operation mode, the test mode signal TEST is at low level. Therefore, outputs from the AND circuits  21 ,  22 ,  23 , . . . are at low level without regard to the level of the input data. On the other hand, the test mode signal TEST is at high level in the test mode. Therefore, when input data on input systems other than an input system that is subject to measurement are fixed at high level, and the logic level of input data (for example, input data applied to the data input terminal  1 ) in the input system that is subject to measurement is changed, the logic level inputted in the input system of the internal circuit  30  is accordingly changed. The change is transferred through the AND circuits  21 ,  22 ,  23 , . . . that are connected in a chain-like manner, and outputted through the selection circuit  70  and then through the output terminal  80 . In this manner, the logic level of an input within the internal circuit  30  can be measured without regard to differences in the specification of the input circuits of the semiconductor device. 
     SUMMARY OF THE INVENTION 
     When the buffer circuits are used as input circuits in a manner shown in FIG. 3, a problem occurs when a power supply to a separate system that supplies input data is tuned off. In other words, in such an instance, the data input terminals of the semiconductor device are placed in a high-impedance state, an input to the buffer circuits may have a potential close to an intermediate potential between a power supply potential V DD  and a power supply voltage V SS , i.e., a value of (V DD +V SS )/2. Alternatively, an input to the buffer circuits may have a potential close to a value of V DD /2 when a power supply voltage V SS  is at a grounding potential. As a result, a drain current may constantly flow through the inverters that form the buffer circuits. 
     In order to prevent wasteful current from flowing even in the instance described above, some techniques are proposed. For example, an AND circuit  11  shown in FIG. 4 or an OR circuit  91  shown in FIG. 5 is used to form an input circuit instead of the buffer circuit  101  used in the semiconductor device shown in FIG.  3 . 
     Referring to FIG. 4, the AND circuit  11  includes a NAND gate and an inverter. One of input terminals of the NAND gate is connected to the data input terminal  1 . The other input terminal of the NAND gate is supplied with a control signal C that is internally generated in the semiconductor device. Even when the data input terminal  1  is placed in a high-impedance state, an output of the NAND gate of the AND circuit  11  is always at high level if the control signal C is maintained at low level. Therefore, wasteful current does not flow. 
     Referring to FIG. 5, the OR circuit  91  includes a NOR gate and an inverter. One of input terminals of the NOR gate is connected to the data input terminal  1 . The other input terminal of the NOR gate is supplied with a control signal C bar that is internally generated in the semiconductor device. Even when the data input terminal  1  is placed in a high-impedance state, an output of the NOR gate of the OR circuit  91  is always at low level if the control signal C bar is maintained at high level. Therefore, wasteful current does not flow. 
     However, when the semiconductor device having an input circuit that is formed with the AND circuit  11  shown in FIG. 4 is tested, the output of the AND circuit  11  is fixed at low level and does not change even when the logic level on the data input terminal  1  is changed, unless the control signal C is changed to high level. Also, when the semiconductor device having an input circuit that is formed with the OR circuit  91  shown in FIG. 5 is tested, the output of the OR circuit  91  is fixed at high level and does not change even when the logic level on the data input terminal  1  is changed, unless the control signal C bar is changed to low level. 
     Accordingly, when the AND circuit  11  shown in FIG. 4 or the OR circuit  91  shown in FIG. 5 is inserted in an input system of the semiconductor device having a measurement circuit that uses the AND circuits  21 ,  22 ,  23 , . . . shown in FIG. 3, the logic level of an input in the input circuit cannot be measured unless the internal control signal is changed. 
     In view of the above, it would be desired to provide a semiconductor device having an input circuit and a method for operating the same, in which the logic level of an input on the input circuit can be measured by a test apparatus such as an IC tester even when a gate circuit that uses an internally generated control signal is used in a first stage of the input circuit. 
     A semiconductor device in accordance with one exemplary embodiment of the present invention has an internal circuit in which input data is gated and supplied to the internal circuit according to an internal control signal generated within the semiconductor device. The semiconductor device has N number (N being two or greater integers) of data input terminals for inputting input data, and a test mode input terminal for inputting a test mode signal. An OR device is provided for obtaining a logical sum of the internal control signal and the test mode signal. The semiconductor device also has N number of gate circuits that are supplied with the input data applied to the N data input terminals, respectively. When an output of the OR device is active, those of the N gate circuits responsive to the output of the OR device pass the input data applied to the data input terminals. The internal circuit is supplied with outputs of the N gate circuits. The semiconductor device has a first stage AND device and second through Nth stage AND devices. The first stage AND device has a first input that is supplied with an output of a first one of the N gate circuits and a second input that is supplied with the test mode signal. The second through Nth stage AND devices respectively have first input terminals that are supplied with outputs of second through Nth ones of the N gate circuits, respectively,  25  and second input terminals that are supplied with outputs of the first through (N−1)th stage AND devices, respectively. 
     The semiconductor device may further include a selection circuit that selects an output of the internal circuit in a normal operation mode and selects an output of the Nth stage AND device in a test mode, and an output terminal that is supplied with an output of the selection circuit. 
     In the semiconductor device, the internal control signal and an output of the OR device may be active at high level. Alternatively, the internal control signal and an output of the OR device may be active at low level. 
     By a semiconductor device having the structure described above in accordance with the embodiment of the present invention, even when a gate circuit that uses an internally generated control signal is used in one of the input circuits in the first stage thereof, the operation of the gate circuit can be controlled by using a test mode signal. Therefore, the logic level of an input in the input circuits can be measured by using a test apparatus such as an IC tester. 
     Other features and advantages of the invention will be apparent from the following detailed description, taken in conjunction with the accompanying drawings that illustrate, by way of example, various features of embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically shows a structure of a semicoductor device in accordance with a first exemplary embodiment of the present invention. 
     FIG. 2 schematically shows a structure of a semiconductor device in accordance with a second exemplary embodiment of the present invention. 
     FIG. 3 schematically shows a structure of a semiconductor device including buffer circuits in an input system. 
     FIG. 4 shows an example of a gate circuit that is inserted in the input system of the semiconductor device. 
     FIG. 5 shows another example of a gate circuit that is inserted in the input system of the semiconductor device. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferred exemplary embodiments of the present invention will be described below with reference to the accompanying drawings. 
     FIG. 1 shows a structure of a semiconductor device in accordance with a first exemplary embodiment of the present invention. Referring to FIG. 1, the semiconductor device has a plurality of data input terminals  1 ,  2 ,  3 , . . . , N. It is noted that FIG. 1 shows only three data input terminals, and fourth through Nth data input terminals are omitted to simplify the illustration. Input data are inputted to the respective data input terminals  1 ,  2 ,  3 , . . . , N from an external test apparatus. Also, the semiconductor device also has a test mode signal input terminal  60 . A test mode signal TEST that is at high level in a test mode is supplied to the test mode signal input terminal  60  from the external test apparatus. 
     The semiconductor device of the present embodiment includes AND circuits  11 ,  12 ,  13 , . . . N as gate circuits that gate the input data. The input data applied to the data input terminals  1 ,  2 ,  3 , . . . , N are supplied to an internal circuit  30  of the semiconductor device through the AND circuits  11 ,  12 ,  13 , . . . N that provide logical multiplication of the input data and an internal control signal C. It is noted that FIG. 1 shows only three AND circuits  11 ,  12 ,  13  as gate circuits, and fourth through Nth AND circuits are omitted to simplify the illustration. Each of the AND circuits  11 ,  12 ,  13 , . . . . N include a NAND gate and an inverter. Input data is supplied to one of two inputs of each of the NAND gates from the corresponding one of the data input terminals, and an output from an OR circuit  50  is inputted in the other input of each of the NAND gates. The OR circuit  50  includes a NOR gate and an inverter. The internal control signal C that is internally generated in the internal circuit  30  is inputted in one of two inputs of the NOR gate of the OR circuit  50  and the test mode signal TEST is inputted to the other input of the NOR gate of the OR circuit  50 . The OR circuit  50  provides a logical sum of the internal control signal C and the test mode signal TEST. 
     Even when the internal control signal C is normally at low level in the test mode, the test mode signal TEST is at high level. As a result, the output of the OR circuit  50  is at high level. Therefore, when the logic levels of the data input terminals  1 ,  2 ,  3 , . . . , N are changed, outputs of the AND circuits  11 ,  12 ,  13 , . . . . N are accordingly changed. 
     Furthermore, the semiconductor device includes AND circuits  21 ,  22 ,  23 , . . . , N as measurement circuits within the semiconductor device. 
     Each of the AND circuits  21 ,  22 ,  23 , . . . . N includes a NAND gate and an inverter. It is noted that FIG. 1 shows only three AND circuits  21 ,  22 ,  23  as measurement circuits within the semiconductor, and fourth through Nth AND circuits are omitted to simplify the illustration. 
     A first one ( 21 ) of the AND circuits as measurement circuits has a first input that is supplied with an output of a first one ( 11 ) of the AND circuits as gate circuits and a second input that is supplied with the test mode signal. Second one ( 22 ) through Nth AND circuits as measurement circuits have first inputs that are supplied with outputs of the second ( 12 ) through Nth ones of the AND circuits as gate circuits, respectively, and second inputs that are supplied with outputs of immediately preceding ones of the AND circuits as measurement circuits (i.e., the AND circuit  21  through (N−1)th AND circuit), respectively. 
     In one embodiment, for example, input data from the second data input terminal  2  is supplied through the AND circuit  12  (i.e., second gate circuit) to one of two inputs of the second stage AND circuit  22 . Also, an output from the AND circuit  21  in an immediately proceeding stage is supplied to the other input of the AND circuit  22 . Furthermore, an output of the AND circuit  22  is supplied to one of two inputs of the AND circuit  23  in the next stage, and input data from the third data input terminal  3  is inputted to the other input of the AND circuit  23 . In this manner, the multiple AND circuits are connected to one another in a chain-like manner. 
     The test mode signal TEST is supplied to one of two inputs of the AND circuit  21  in the first stage. Also, an output of the AND circuit in the measurement circuits in the last stage (i.e., the Nth stage AND circuit) is supplied to one of two inputs of a selection circuit  70 . An output of the internal circuit  30  is inputted to the other input of the selection circuit  70 . The selection circuit  70  is controlled by the test mode signal TEST. The selection circuit  70  selects the output of the internal circuit  30  in a normal operation mode, and selects the output of the AND circuit in the last stage in a test mode. An output of the selection circuit  70  is read out through an output terminal  80  by an external device. 
     It is noted that, in the normal operation mode, the test mode signal TEST is at low level. Therefore, outputs from the AND circuits  21 ,  22 ,  23 , . . . , N are at low level without regard to the level of the input data. On the other hand, the test mode signal TEST at high level is provided in the test mode. Therefore, when input data on the input systems other than the input system that is subject to measurement is fixed at high level, and the logic level of input data on the input system that is subject to measurement is changed, the change is transferred through the AND circuits  21 ,  22 ,  23 , . . . , N that are connected in a chain-like manner and through the selection circuit  70 , and outputted from the output terminal  80 . In this manner, inputted logic levels on the AND circuits  11 ,  12 ,  13 , . . . , N can be measured without regard to variations in the specification of the input circuits of the semiconductor device. 
     Next, a second exemplary embodiment of the present invention is described below with reference to FIG.  2 . The second embodiment is different from the first embodiment in that OR circuits are used instead of the AND gates as gate circuits. 
     The semiconductor device of the second embodiment includes OR circuits  91 ,  92 ,  93 , . . . N as gate circuits that gate the input data. The input data applied to the data input terminals  1 ,  2 ,  3 , . . . , N are supplied to an internal circuit  30  of the semiconductor device through the OR circuits  91 ,  92 ,  93 , . . . N that provide logical sums of the input data and an internal control signal C bar. It is noted that FIG. 2 shows only three OR circuits  91 ,  92 ,  93  as gate circuits, and fourth through Nth OR circuits are omitted to simplify the illustration. Each of the OR circuits  91 ,  92 ,  93 , . . . N include a NOR gate and an inverter. Input data is supplied to one of two inputs of each of the NOR gates of the respective OR circuits  91 ,  92 ,  93 , . . . N from the corresponding one of the data input terminals, and an output from a NOR gate  51  is inputted in the other input of each of the NOR gates of the respective OR circuits  91 ,  92 ,  93 , . . . N. The internal control signal C bar that is internally generated in the internal circuit  30  is supplied through an inverter  52  to one of two inputs of the NOR gate  51 , and the test mode signal TEST is supplied to the other input of the NOR gate  51 . The inverter  52  inverts the internal control signal C bar to form an internal control signal C. The NOR gate  51  provides a logical sum of the internal control signal C and the test mode signal TEST, inverts its result and outputs the same. 
     When the internal control signal C bar is normally at high level in a test mode, the test mode signal TEST is at high level, and therefore an output of the NOR gate  51  is at low level. As a result, when logic levels on the data input terminals  1 ,  2 ,  3 , . . . , N are changed, outputs of the gate circuits  91 ,  92 ,  93 , . . . , N are accordingly changed. 
     The semiconductor device of the second embodiment has AND circuits  21 ,  22 ,  23 , . . . , N, each including a NAND gate and an inverter, a selection circuit  70 , and an output terminal  80 , in a similar manner as the first embodiment. Also, the semiconductor device of the second embodiment measures the logic levels of inputs to the internal circuit  30  in a similar manner conducted in the first embodiment. 
     It is noted that, in the exemplary embodiments described above, one type of internal control signal is used. However, the present invention is also applicable to other cases in which a plurality of internal control signals are used. In such cases, OR circuits in FIG. 1 or NOR gates  51  and inverters  52  in FIG. 2 may be provided in the same number of the internal control signals, respectively. 
     In accordance with the present invention, even when a gate circuit that uses an internal control signal is used in one of the input circuits in the first stage thereof, the operation of the gate circuit can be controlled by using a test mode signal. Therefore, the logic level of an input in the input circuits can be measured by using a test apparatus such as an IC tester. 
     While the description above refers to particular embodiments of the present invention, it will be understood that many modifications may be made without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention. 
     The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Technology Classification (CPC): 6