Abstract:
A signal generating circuit of a semiconductor device comprises n input test pins for receiving respective coded input signals. At least one of the input signals is coded in more than two possible levels, such as 3 levels or four levels. The device also includes an indicator I/O signal generators, each coupled respectively with an associated input test pin. Each indicator signal generator generates two-level indicator signals in response to the coded input signal received by its associated input test pin. A decoder receives the indicator signals to produce decoded signals, and a mode selecting circuit generates mode selecting signals with the decoded signals responsive to mode setting signals. Each indicator signal generators outputs a regular signal when the input test signal is an ordinary low, a control signal when the input test signal is an ordinary high, and a higher first level signal when the input test signal is a super high. If more than three levels are used, the indicator signal generator further generates corresponding signals for these higher values.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a semiconductor device, and more particularly to a signal generating circuit of the semiconductor device which generates a number of test mode selecting signals for performing a test during an operational mode, and further generates signals at a normal operational mode. 
     Description of Prior Art 
     In general, operations of semiconductor devices can be classified into normal and test modes. Test modes are subdivided into a plurality of test items, for which respective tests are performed to determine whether a product is good or defective. 
     In order to get the semiconductor device ready to test each test item, it is necessary to have test mode selecting signals generated internally within the semiconductor device to set up the test mode. For this purpose, a test mode selecting circuit is constructed with a predetermined number of pins among address or data pins of the semiconductor device, which are used as test mode selecting pins for buffering and decoding signals transmitted to the pins, to thereby generate test mode selecting signals for testing a plurality of test items. 
     The conventional test mode selecting circuit of the semiconductor device can only generate n 2  test mode selecting signals, where, n denotes the number of pins. For example, if the number of test items to test at the test operational mode is  8 ,  3  test mode selecting pins are required. If more test items are required, more test mode selecting pins should be allocated. 
     The same is also true for internally generated signals at the normal operational mode. Their maximum number is  2 , where n denotes the number of pins. Therefore, if internally generated signals are required, more pins are also required for the operation. 
     In other words, in semiconductor devices the number of pins allocated for testing should increase when the number of signals to be internally generated increases, whether at test or normal operational modes. This is a problem, because semiconductor devices have the tendency of increasing integration, which decreases the size of the chip. Any increase in the number of pins limits the efforts to reduce the size of the chip. 
     FIG. 1 is a block diagram for illustrating a conventional test mode selecting circuit of a semiconductor device, which has been already disclosed in U.S. Pat. No. 5,036,272, which is hereby incorporated by reference. 
     There are input pins  10 ,  14 - 1 ,  14 - 2 ,  14 - 3 ,  14 - 4 , buffers  12 ,  16 - 1 ,  16 - 2 ,  16 - 3 ,  16 - 4 , a high voltage sensing circuit  18 , decoders  20 ,  22  and a mode selecting circuit  24  at the block diagram in FIG.  1 . As can be seen, a predetermined number of pins of the semiconductor device used for normal operations at the normal operational mode are used as pins for generating test mode selecting signals at the test operational mode. 
     Since there are only four input pins  14 - 1 ,  14 - 2 ,  14 - 3 ,  14 - 4 , the conventional test mode selecting signal generating circuit shown in FIG. 1 can only generate up to  16  test mode selecting signals. This, in turn, limits the number of test items. 
     As more test items are required, more pins in the circuit are required. Therefore, there still remains a problem in the conventional test mode selecting signal generating circuit, in that the increase in the number of pins to be used at the test operational mode can not be restricted, in spite of reduction in the size of a chip. 
     SUMMARY OF THE INVENTION 
     The present invention recognizes that the prior art was so limited because it only allowed each test input pin to receive a signal of only two possible values (high and low). The present invention provides a device and a method that allows more values for each of the input pins, and therefore permits more test modes without requiring more pins. 
     The present invention therefore provides a signal generating circuit of a semiconductor device, and a method for invoking test modes in the semiconductor device. 
     The signal generating circuit of the device of the invention includes n input test pins for receiving respective coded input signals. At least one of the input signals is coded in more than two possible levels, such as  3  levels or four levels. 
     The device further includes an indicator signal generators, each coupled respectively with an associated input test pin. Each indicator signal generator generates indicator signals in response to the coded input signal received by its associated input test pin. The indicator signals are of only two levels. 
     A decoder receives the indicator signals to produce decoded signals, and a mode selecting circuit generates mode selecting signals with the decoded signals responsive to mode setting signals. 
     The indicator signal generators preferably include a buffer outputting a regular signal, indicative of whether the associated input signal has an ordinary low level, a first higher level voltage detector for outputting a higher first level signal indicative of whether the associated input signal has a first extra high level higher than the ordinary low level, and a scrambling circuit for producing, in response to the regular signal and to the higher first level signal, a control signal indicative of whether the associated input signal has an ordinary high level higher than the ordinary low level and lower than the first extra high level. 
     If more than three levels are used, the indicator signal generators further include a second higher level voltage detector for outputting a higher second level signal indicative of whether the associated input signal has a second extra high level higher than the first extra high level. 
     The method of the invention is for invoking a test mode in a circuit of a semiconductor device. The method includes applying coded input signals to input test terminals of the circuit, where at least one of the input signals has more than two possible levels, then generating two-level indicator signals in response to the coded input signals, and decoding the indicator signals to generate decoded signals. 
     These and other features and advantages of the present invention will be understood from the Detailed Description and the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram for illustrating a mode selecting signal generating circuit of a conventional semiconductor device; 
     FIG. 2 is a block diagram for illustrating a signal generating circuit of a semiconductor device in accordance with an embodiment of the present invention, wherein each input test pin can receive three input values; 
     FIG. 3 is a diagram showing a signal of  3  possible levels being applied to an input test terminal of the device of FIG. 3; 
     FIG. 4 is a circuit diagram for implementing a portion of the block diagram shown in FIG. 2; 
     FIG. 5 is a circuit diagram for illustrating a high voltage detector in accordance with an embodiment of the present invention; 
     FIG. 6 is a circuit diagram for illustrating a high voltage detector in accordance with another embodiment of the present invention; 
     FIG. 7 is a truth table of a circuit shown in FIG. 4; 
     FIG. 8 is a block diagram for illustrating a signal generating circuit of a semiconductor device in accordance with another embodiment of the present invention, wherein each input test pin can receive up to four input values; 
     FIG. 9 is a signal of  4  possible levels being applied to an input test terminal of the device of FIG. 8; 
     FIG. 10 is a circuit diagram for implementing a portion of the block diagram of FIG. 8; 
     FIG. 11 is a truth table of a circuit shown in FIG. 10; and 
     FIG. 12 is a flowchart illustrating a method of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A signal generating circuit of a semiconductor device of the present invention will be described with reference to accompanying drawings. 
     FIG. 2 is a block diagram for illustrating the signal generating circuit of the semiconductor device in accordance with an embodiment of the present invention. The circuit includes input pins  30 ,  34 - 1 , . . . ,  34 -n, buffers  36 - 1 , . . . ,  36 -n, high voltage detectors  38 - 1 ,  38 - 2 , . . . ,  38 -n, scrambling circuits  40 - 1 ,  40 - 2 , . . . ,  40 -n, a high voltage sensing circuit  42 , a decoder  44  and a mode selecting circuit  46 . 
     In FIG. 2, the input pin  30  is used as a pin for transmitting a signal to select test or normal operational mode. Input test pins  34 - 1 ,  34 - 2 , . . . ,  34 -n are also called input pins, and generally receive an input signal. The input pins are used as pins for transmitting selecting signals to select test items that are to be tested at the test operational mode. 
     Buffers  32 ,  36 - 1 ,  36 - 2 , . . . ,  36 -n in general output a regular signal indicative of whether the input signal has one of an ordinary low level or not. In particular, they respectively buffer input signals Ai (i= 1 ,  2 , . . . , n), to respectively generate complementary levels of two signals PAiB, PAi (i= 1 ,  2 , . . . , n). 
     The high voltage detectors  38 - 1 ,  38 - 2 , . . . ,  38 -n in general output a higher first level signal SAI indicative of whether the input signal has a first extra high voltage higher than an ordinary high level. In particular, they generate signals SAi (i= 1 ,  2 , . . . , n). These correspond to additional possible signal values, namely voltages higher than “high”. 
     In addition, the scrambling circuits  40 - 1 ,  40 - 2 , . . . ,  40 -n produce control signals in response to the regular signals and any higher level signals. In particular, they respectively scramble input signals PAiB, PAi, SAi (i= 1 ,  2 , . . . , n) to generate control signals PAIB, PPAi, SAj (i= 2 , . . . , n, j = 1 ,  2 , . . . , n-I). The control signals indicate whether the input signal has an ordinary high level, between the ordinary low and the first extra high. 
     Collectively, the regular signal, control signals, and the higher level signals are called indicator signals. They are used to decode, in two levels, the levels of the multi-level coded signals that are applied to the test pins. 
     The high voltage sensing circuit  42  senses high voltage inputted from the input pin  30 , and generates a signal MRS. Signal MRS is high at the test operational mode, and low at the normal operational mode. 
     The decoder  44  decodes the output signals generated from the scrambling circuits  401 ,  40 - 2 , . . . ,  40 -n, to generate mode selecting signals. The mode selecting circuit  46  responds to the MRS signal being high, to select and latch the mode selecting signals generated from the decoder  44 . The selecting signals generated as such make the internal state of the semiconductor device set up for testing specifically designated test items at the test operational mode. 
     In the signal generating circuit of the semiconductor device shown in FIG. 2, signals of three levels are inputted from the tester to at least one of the test pins, at the test operational mode, and signals of two or three levels are inputted from an external system out of the semiconductor device. It is preferred that signals of three levels are transmitted to all test pins. 
     The various levels are now explained in more detail with reference to FIG.  3 . At least one of the input test pins  34 -i receives an input signal IS 3 . Generally, signal IS 3  has one of  3  preselected levels, namely an ordinary low (OL), an ordinary high (OH), and a first extra high or super high (SH) level. Other names are also applicable to such values. In this case, the signal IS 3  has the SH value. 
     These levels can be selected in any advantageous way. One such way is to select the OL level to be OV, the OH level to be the high level ordinarily used within the circuit, and the SH level to be even higher. That is not necessary, however. The CMOS technology is preferred, because it offers a large range of operating voltages from which to chose the levels. In other words, signals of low, high or high voltage levels can be inputted at the test operational mode, and signals of low or high level, or low, high or high voltage level should be inputted at the normal operational mode. If signals of three levels are inputted to address input pins at the normal operational mode, the number of address input pins can be reduced. This, however, would require other circuits to cooperate also in a non-test environment. 
     FIG. 4 is a circuit diagram for illustrating the structure of an embodiment of the block diagram shown in FIG. 2, wherein input signals of three levels are transmitted to two input pins, to generate  9  mode selecting signals M 1 , M 2 , . . . , M 9 . 
     The scrambling circuit  40 - 1  is constructed with inverters I 1 , I 2 , I 3  and a NAND gate  10  NA 1 , and the scrambling circuit  4 - 2  is constructed with inverters  14 ,  15 ,  16  and a NAND gate NA 2 . 
     Additionally, the decoder  44  is constructed with NAND gates NA 3 , NA 4 , . . . , NA 11  and inverters  17 ,  18 , . . . , I 15 . 
     The mode selecting circuit  46  is constructed with CMOS transmission gates C 1 , C 2 , is . . . , C 9 , an inverter I 16  and latches L 1 , L 2 , . . . , L 9 . 
     The functions of respective parts in the circuit thus constructed and shown in FIG. 4 will be described below. 
     Buffers  36 - 1 ,  36 - 2  respectively buffer the signals inputted from the input pins  34 - 1 ,  34 - 2  to generate complementary output signals (PA 2 B, PA 2 ), (PA 3 B, PA 3 ). The high  20  voltage detectors  38 - 1 ,  38 - 2  respectively detect and buffer the high voltage inputted from the input pins  34 - 1 ,  34 - 2  to generate high level of signals SA 1 , SA 2 . 
     The scrambling circuit  40 - 1  outputs signals PA 2 B, SA 1  as they are, inverts signals PA 2 B, SA 1  through the inverters  11 ,  12  and ANDs the output signals of the inverters  11 ,  12  and the signal PA 2  through the NAND gate NAI and an inverter  13  to generate a signal PPA 2 . Similarly, the scrambling circuit  40 - 2  outputs the signals PA 3 B, SA 2  as they are, and generates a signal PPA 3 . 
     The decoder  44  ANDs the signals PA 2 B, PA 3 B through a NAND gate NA 3  and an inverter  17  to generate a signal d 1 , ANDs the signals PA 2 B, PPA 3  through a NAND gate NA 4  and an inverter  18  to generate a signal d 2 , ANDs the signals PA 2 B, SA 2  through a  30  NAND gate NA 5  and an inverter  19  to generate a signal d 3 , ANDs the signals PPA 2 , PA 3 B through a NAND gate NA 6  and an inverter  110  to generate a signal d 4 , ANDs the signals PPA 2 , PPA 3  through a NAND gate NA 7  and an inverter  111  to generate a signal d 5 , ANDs the signals PPA 2 , SA 2  through a NAND gate NA 8  and an inverter  112  to generate a signal d 6 , ANDs the signals SA 1 , PA 3 B through a NAND gate NA 9  and an inverter  113  to generate a signal d 7 , ANDs the signals SA 1  PPA 3  through a NAND gate NA 1  and an inverter  114  to generate a signal d 8  and ANDs the signals SA 1 , SA 2  through a NAND gate NA 11  and an inverter  15  to generate a signal d 9 . 
     The mode selecting circuit  46  responds to a high level of a mode setting signal MRS to respectively trait output signals d 1 , d 2 , . . . , d 9  outputted from the decoder  44  through CMOS transmission gates C 1 , C 2 , . . . , C 9 . The latches L 1  L 2 , . . . , L 9  respectively latch the output signals of the CMOS transmission gates C 1 , C 2 , . . . , C 9  to output mode selecting signals M 1 , M 2 , . . . , M 9 . 
     FIG. 5 is a circuit diagram of the high voltage detector in accordance with an embodiment of the present invention, comprising PMOS transistors PI, P 2 , NMOS transistors N 1 , N 2  and inverters I 17 . In other words, the high voltage detector shown in FIG. 5 is constructed with differential amplifiers. 
     The operation of the circuit shown in FIG. 5 is now described. 
     First of all, the reference voltage Vref input to the high voltage detector is set to be a level of high voltage, higher than the high level thereof. For example, if low, high and high voltage levels of voltage are respectively set at OV,  3 V and  6 V, the reference voltage Vref inputted from the high voltage detector should be set at a value such as  4 V or  5 V. Under the assumption that all those levels of voltage are set as such, operations of the high voltage detector will be described below. 
     If the voltage of the signal Ai transmitted to an input pin is a low level thereof, the NMOS transistor NI turns off and the NMOS transistor N 2  turns on to thereby turn on the PMOS transistors P 1 , P 2 . Therefore, a voltage of high level is set at the drain of the PMOS transistor P 1 . The inverter I 17  inverts a high level of voltage to generate a low level of output voltage SAi. 
     Also, if the voltage of the signal Ai inputted to an input pin is a high level thereof, the NMOS transistor N 1  turns off and the NMOS transistor N 2  turns on to generate a low level of output voltage SAi. 
     If the voltage of the signal Ai inputted to an input pin is a high voltage level thereon, the NMOS transistor NI turns on and the NMOS transistor N 2  turns off to thereby set a low level of voltage at the drain of the PMOS transistor PI. Then, the inverter  17  inverts a low level of voltage to generate a high level of output voltage SAi. 
     That is, the high voltage detector shown in FIG. 5 generates a high level of output voltage SAi if a-high voltage level of voltage is applied through the input pin Ai, and a low level of output voltage SAi if a high or low level of voltage is applied through the input pin Ai. 
     FIG. 6 is a circuit diagram of a high voltage detector in accordance with another embodiment of the present invention. Generally, the circuit includes a voltage dropping circuit, such as that made of a series of diodes. It also includes a buffer for buffering and outputting the reduced signal from the voltage level dropping circuits. In this embodiment, the diodes are made from NMOS transistors N 3 , N 4 , . . . , N 7 , and the buffer is made from inverters  118 ,  119  and a resistance R. 
     The operation of the high voltage detector shown in FIG. 6 is now described. 
     Under the assumptions that low (OL), high (OH), and high voltage (SH) levels of the signal Ai transmitted to the input pin are respectively set at OV,  3 V and  6 V, and that the turned-on resistance of the NMOS transistors N 3 , N 4 , . . . , N 7  connected in series is much lower than the resistance R, operations of the high voltage detector shown in FIG. 6 at the test operational mode will be described below. First of all, if voltage of the signal Ai transmitted to the input pin is OV or  3 V, a low level of a signal is outputted through the NMOS transistors N 3 , N 4 , . . . , N 7 . The inverters  118 ,  119  buffer a low level of the signal, to generate a low voltage detecting signal SAi. 
     On the other hand, if a high voltage level of the signal Ai is transmitted, the NMOS transistors N 3 , N 4 , . . . , N 7  connected in series act as diodes. In other words, they turn on, to output a high level of the signal through the resistance R. The diodes are such in number so as to convert ash level to an OH level signal. Inverters I 18 , I 19  buffer a high level of the signal, to generate a high level of the high voltage detecting signal SAi. 
     Therefore, it can be concluded that the high voltage detectors shown in FIGS. 5 and 6 perform the same operations. 
     The truth table of the circuit shown in FIG. 4 is shown in the table of FIG.  7 . In that table, I, H and SH respectively symbolize low, high, high voltage levels of voltage. A 2  and A 3  symbolize signals respectively transmitted to input pins  34 - 1 ,  34 - 2 ; PA 3 B and PA 3  output signals outputted from the buffer  36 - 2 ; SAI an output signal outputted from the high voltage detector  38 - 1 ; SA 2  an output signal outputted from the high voltage detector  38 - 2 ; PPA 2  an output signal outputted from the scrambling circuit  40 - 1 , PPA 3  an output signal outputted from the scrambling circuit  40 - 2 ; and d 1 , d 2 , d 3 , d 4 , d 5 , d 6 , d 7 , d 8 , and d 9  output signals outputted from the decoder  44 . 
     By using the table of FIG. 7, the operations to generate the mode selecting signals M 1 , M 2 , M 3 , M 4 , M 5 , M 6 , M 7 , M 8 , M 9  will be described in accordance with the  5  semiconductor device of the present invention below. 
     In the table of FIG. 7, if all the levels of the input signals A 2 , A 3  are L level thereof, the output signals PA 2 B, PA 2 , SAI outputted from the buffer  36 - 1  and the high voltage detector  38 - 1  are respectively set at H, L, L levels thereof, and the output signals PA 3 B, PA 3 , SA 2  are respectively set at H, L, L levels thereof. Furthermore, the output signals (PA 2 B, PPA 2 ), (PA 3 B, PPA 3 , SA 2 ) are respectively set at H, L, L levels thereof. Therefore, the output signals d 1 , d 2 , . . . , d 9  of the decoder  44  are set at H, L, . . . L level thereof. The mode selecting circuit  46  responds to the mode setting signal MRS being high, to turn the output signals outputted from the decoder  44  into mode selecting signals MI, M 2 , . . . , M 9  and output them. 
     In addition, in the table of FIG. 7, if levels of the input signals A 2 , A 3  are respectively set at SH, H, the output signals PA 2 B, PA 2 , SA 1  outputted from the buffer  36 - 1  and a high voltage detector  38 - 1  are respectively L, H, H levels thereof, and the output signals PA 3 B, PA 3 , SA 2  are respectively set at L, H, L. Furthermore, the output signals (PA 2 B, PPA 2 , SA 1 ), (PA 3 B, PPA 3 , SA 2 ) outputted from the scrambling circuits  40 - 1 ,  402  are respectively  20  set at L, L, H levels thereof and L, H, L levels thereof. Therefore, the output signals d 1 , d 2 , d 9  of the decoder  44  are respectively set at L, L, . . . , L, H, L level thereof. The mode selecting circuit  46  responds to the mode setting signal MRS being high, to turn the output signals outputted from the decoder  44  into mode selecting signals MI, M 2 , . . . , M 9  and output them. 
     In other words, the semiconductor device of the present invention shown in FIG. 4 internally generates  9  different mode selecting signals, if input signals of three levels are transmitted to two input pins. If three levels of input signals are transmitted to three input pins,  27  different mode selecting signals can be internally generated. 
     Therefore, if the number of test items increases at the test operational mode, a variety  30  of mode selecting signals can be generated by a small number of pins. 
     Furthermore, it becomes possible to generate output signals PA 2 B, PA 2 , PA 3 B, . . . , PAnB, PAn from the buffers  56 - 1 ,  56 - 2 , . . . ,  56 -n and output signals PA 2 , PPA 2 , PSAI, SBI, PAnB, PPAn, PSA(n-I), SB(n-l) from the scrambling circuits  62 - 1 ,  62 - 2 , . . . ,  62 -n at the normal operational mode. That is, at the normal operational mode, the output signals of the buffers are generated into addresses or data if input signals of two or three levels are transmitted to pins. 
     FIG. 8 is a block diagram of a signal generating circuit in accordance with another embodiment of the present invention. The circuit includes input pins  50 ,  54 - 1 ,  54 - 2 , . . . ,  54 -n, buffers  52 ,  56 - 1 ,  56 - 2 , . . . ,  56 -n, first high voltage detectors  58 - 1 ,  58 - 2 , . . . ,  58 -n, second high voltage detectors  60 - 1 ,  60 - 2 , . . . ,  60 -n, scrambling circuits  62 - 1 ,  62 - 2 , . . . ,  62 -n, a high sensing circuit  64  and a mode selecting circuit  68 . 
     In FIG. 8, the input pin  50  is used as a pin for transmitting a-signal to select a test or normal operational mode, and the input pins  54 - 1 ,  54 - 2 , . . . ,  54 -n are used as pins for transmitting mode selecting signals to select test items at the test operational mode. Furthermore, the buffers  52 ,  56 - 1 ,  56 - 2 , . . . ,  56 -n respectively buffer the input signals, to generate mutually complementary levels of two signals PAi, PAIB (i= 1 ,  2 , . . . n). 
     The first high voltage detectors  58 - 1 ,  58 - 2 , . . . ,  58 -n respectively compare the input signals and the reference voltage thereof At this time, if a level of voltage higher than the reference voltage is applied, the first high voltage detectors generate signals SAi (i= 1 ,  2 , . . . , n-l). 
     The second high voltage detectors  60 - 1 ,  60 - 2 , . . . ,  60 -n in general output a higher second level signal indicative of whether the input signal has a second extra high voltage level, which is higher than the first extra high voltage level of the first high voltage detectors. In particular, they respectively compare the input signals and the reference voltage thereof. At this time, if a level of voltage higher than the reference voltage is applied, the second high voltage detectors generate signals SBi (i=l,  2 , . . . , n-i). 
     The scrambling circuits  62 - 1 ,  52 - 2 , . . . ,  62 -n respectively scramble input signals PAiB, PAi, SAj, SBj (i= 2 ,  3 , . . . , n) (= 1 ,  2 , . . . , n-i), to generate signals PAiB, PPAi, PSAj, SBj (i= 2 , 3 , . . . , n) (= 1 , 2 , . . . , n-I). The high voltage sensing circuit  64  senses the high voltage inputted from the input pin  50 , to generate a signal of a high level-at the test operational mode, and a low level at the normal operational mode, respectively. 
     The decoder  66  decodes the output signals outputted from the scrambling circuits  621 ,  62 - 2 , . . . ,  62 -n to generate mode selecting signals dX. 
     The mode selecting circuit  68  responds to a high level of the signal MRS to select and latch the mode selecting signals outputted from the decoder  66 . The mode selecting signals generated as such make the internal state of the semiconductor device set up for testing specifically designated test items at the test operational mode. 
     In the case of the semiconductor device shown in FIG. 8, signals of four levels can be inputted from the tester at the test operational mode, and signals of two, three or four levels can be inputted from an external system out of the semiconductor device at the normal operational mode. 
     The various levels are now explained in more detail with reference to FIG.  9 . On the input test pins  54 -i there is applied an input signal IS 4 . Generally, input signal IS 4  has one of  4  preselected levels, namely an ordinary low (OL), an ordinary high (OH), a first high (SHi), also known as a first extra high (SH 1 ), and a second high (SH 2 ), also known as a second extra high (SH 2 ). As in the previous case, other names are also applicable. What is important is that different levels can convey different information. Moreover, additional levels, such as a third extra high voltage, can be defined for the system. 
     In other words, low, high, first high voltage or second high voltage levels of signals can be inputted at the test operational mode, and signals of low or high level or low, high, first high voltage or second high voltage levels can be inputted at the normal operational mode. 
     If four levels of signals can be inputted to address input pins at the normal operational mode, the number of address input pins can be significantly reduced. 
     FIG. 10 is a circuit diagram for illustrating the structure of another embodiment of the block diagram shown in FIG. 8, wherein input signals of four levels are respectively transmitted to two input pins, to generate  16  mode selecting signals M 1 , M 2 , . . . , M 16 . 
     The high voltage detectors  58 - 1 ,  60 - 1  can be made by separate circuits, similar to those taught with reference to the  3  level signal. If the high voltage detector shown in FIG. 5 is applied to the first and second high voltage detectors of FIG. 10, the level of the reference voltage Vrefl of the first high voltage detectors  58 - 1 ,  58 - 2  should be set at a level of voltage that is between the high level thereof and the first high voltage level thereof. The level of the reference voltage Vref 2  of the second high voltage detectors  60 - 1 ,  60 - 2  should be set at a level of voltage, higher than the first high voltage level thereof. 
     The scrambling circuit  62 - 1  is constructed with inverters I 20 , I 21 , I 22 , I 23  and NAND gates NA 12 , NA 13 , and the scrambling circuit  62 - 2  is constructed with inverters I 24 , I 25 , I 26 , I 27  and NAND gates NA 14 , NA 15 . The scrambling circuit  62 - 1  in this case can be considered to have produced two control signals, etc. 
     Additionally, the decoder  66  is constructed with NAND gates NA 16 , NA 17 , . . . , NA 31  and inverters I 28 , I 29 , . . . , I 43 . 
     The mode selecting circuit  68  is constructed with CMOS transmission gates C 10 , C 11 , . . . , C 25 , latches L 10 , L 11 , . . . , L 25  and an inverter  144 . 
     Functions of the parts in the circuit thus constructed and shown in FIG. 10 will be described below. 
     The buffers  56 - 1 ,  56 - 2  respectively buffer the signals inputted from the input pin  54   1 ,  54 - 2  to generate complementary output signals (PA 2 B, PA 2 ), (PA 3 B, PA 3 ). The high voltage detectors  58 - 1 ,  58 - 2  respectively detect and buffer the first high voltage inputted from the input pins  54 - 1 ,  54 - 2  to generate a high level of signals SA 1 , SA 2 . The high voltage detectors  60 - 1 ,  60 - 2  respectively detect and buffer the second high voltage inputted from the input pins  54 - 1 ,  54 - 2  to generate a high level of signals SB 1 , SB 2 . The scrambling circuit  62 - 1  outputs signals PA 2 B, SB 1  as they are, inverts signals SA 1 , SB 1  through the inverters I 20 , I 21  and ANDs a signal PA 2  and signals inverted through the inverters I 20 , I 21  through the NAND gate NA 12  and the inverter  122 , to generate a signal PPA 2  and ANDs a signal SA 1  and a signal inverted through the inverter I 21  to generate a signal PSAL. In addition, the scrambling circuit  62 - 2  performs the same operations as the scrambling circuit  62 - 1  to generate signals PA 3 B, PPA 3 , PSA 2 , SB 2 . 
     The decoder  66  ANDs the output signals PA 2 B, PA 2 , SA 1 , SB 1  outputted from the scrambling circuit  62 - 1  and the output signals PA 3 B, PA 3 , SA 2 , SB 2  outputted from the scrambling circuit  62 - 2  through NAND gates NA 16 , NA 17 , . . . , NA 31  and inverters  128 , 129 ,  143  to generate decoded signals d 1 , d 2 , . . . , d 16 . 
     The mode selecting circuit  68  responds to a high level of a mode setting signal MRS to respectively transit the output signals d 1 , d 2 , . . . , d 16  outputted from the decoder  66  through CMOS transmission gates C 10 , C 11 , . . . , C 25 , and the latches L 10 , L 11 , . . . , L 25  respectively latch the output signals outputted from the CMOS transmission gates C 10 , C 11 , . . . , C 25  to thereby output mode selecting signals M 1 , M 2 , . . . , M 16 . 
     The truth table of the circuit of FIG. 10 is shown in a table of FIG.  11 . In that table, L, H. SH 1  and SH 2  respectively symbolize low, high, the first high voltage and the second high voltage levels of voltage. A 2  and A 3  symbolize signals respectively transmitted to input pins  54 - 1 ,  54 - 2 ; PA 3 B and PA 2  output signals outputted from the buffer  56 - 1 ; PA 3 B, PA 3  output signals outputted from the buffer  56 - 2 ; SA 1  an output signal outputted from the high voltage detector  58 - 1 ; SA 2  an output signal outputted from the high voltage detector  58 - 2 ; SB 1  an output signal outputted from the high voltage detector  60 - 1 ; SB 2  an output signal outputted from the high voltage detector  60 - 2 ; PPA 2  and PSAI output signals outputted from the scrambling circuit  62 - 1 ; PPA 3  and PSA  2  output signals outputted from the scrambling circuit  62 - 2 ; and d 1 , d 2 , . . . and d 16  output signals outputted from the decoder  66 . 
     By using the table of FIG. 11, the operations to generate mode selecting signals M 1 , M 2 , M 3 , M 4 , M 5 , M 6 , M 7 , M 8 , M 16  will be described in accordance with the semiconductor device of the present invention below. 
     In the table of FIG. 11, if all the level of the input signals A 2 , A 3  is set at L level thereof, the output signals PA 2 B, PA 2 , SA 1 , SB 1  outputted from the buffer  56 - 1  and the high voltage detectors  58 - 1 ,  60 - 1  are respectively set at H, L, L, L levels thereof, and the output signals PA 3 B, PA 3 , SA 2 , SB 2  outputted from the buffer  56 - 1  and the high voltage detectors  58 - 2 ,  60 - 2  are respectively set at H, L, L, L levels thereof Furthermore, the output signals (PA 2 B, PPA 2 , PSA 1 , SB 1 ), (PA 3 B, PPA 3 , PSA 2 , SB 2 ) are respectively set at H, L, L, L levels thereof Therefore, the output signals d 1 , d 2 , . . . , d 16  outputted from the decoder  66  are respectively set at H, L, . . . L levels thereof The mode selecting circuit  68  responds to the mode setting signal MRS to turn the output signals outputted from the decoder  66  into mode selecting signals M 1 , M 2 , . . . , M 16  and output them. 
     In addition, in the table of FIG. 11, if the input signals A 2 , A 3  are respectively set at SH 2 , H levels thereof, the output signals PA 2 B, PA 2 , SA 1 , SB 1  of the buffer  56 - 1  and high voltage detectors  58 - 1 ,  60 - 1  are respectively set at L, H, H, H levels thereof, and the output signals PA 3 B, PA 3 , SA 2 , SB 2  of the buffer  56 - 2  and the high voltage detectors  58 - 2 ,  60 - 2  are respectively set at L, H, L, L levels thereof. Furthermore, the output signals (PA 2 B, PPA 2 , PSA 1 , SB 1 ) (PA 3 B, PPA 3 , PSA 2 , SB 2 ) of the scrambling circuits  62 - 1 ,  62 - 2  are respectively set at L, L, L, H levels thereof and L, H, L, L levels thereof Therefore, the output signals d 1 , d 2 , . . . , d 16  of the decoder  66  are respectively set at L,.L, . . . , L, H, L, L levels thereof The mode selecting circuit  68  responds to the mode setting signal MRS to output mode selecting signals M 1 , M 2 , . . . , M 16 . 
     In other words, the signal generating circuit of the semiconductor device of the present invention shown in FIG. 10 internally generates  16  mode selecting signals if input signals at four levels are transmitted to two input pins. If input signals of four levels are transmitted to three input pins,  64  mode selecting signals can be internally generated. 
     Therefore, it becomes possible to generate output signals PA 2 B, PA 2 , PA 3 B, PA 3 , . . . , PAnB, PAn of the buffers  56 - 1 ,  56 - 2 , . . . ,  56 -n and output signals PA 2 B, PPA 2 , PSA 1 , SB 1 , PANB, PPAn, PSA(n- 1 ), SB(n- 1 ) of the scrambling circuits  62 - 1 ,  62 - 2 , . . . ,  62 -n at the normal operational mode. 
     That is, at the normal operational mode, output signals of the buffers are generated into addresses or data if input signals of two levels are transmitted the pins, and the output signals of the scrambling circuits are generated into addresses or data if input signals of three levels are transmitted to the pins. 
     Therefore, in the signal generating circuit of the semiconductor device of the present invention, n pins are used as the pins for generating test mode selecting signals, and, if the number of levels of the signals inputted into n pins is M, the number of mode selecting signals to be generated can be M n . 
     Thus, if the number of test items to be tested at the test operational mode increases, a small number of pins can generate a variety of mode selecting signals. 
     Most of the aforementioned description has been made in terms of the test operational mode. However, the semiconductor device of the present invention is a memory device, which can be applied to a system. If the mode setting circuit of the present invention is constructed at address input pins, and if the address to be inputted from outside has signals of four levels, a small number of pins can be used for generating a great number of address decoding signals. Therefore, it becomes possible to overcome the problem that the number of pins is restricted in spite of reduction in the size of a chip. 
     Therefore, the signal generating circuit of the semiconductor device of the present invention generates and transits more than three levels of signals from a tester to input pins at a test operational mode, thereby enabling to test a larger variety of test items than the prior art. 
     Furthermore, if more than three levels of signals are transmitted from an external device at the normal operational mode, a small number of pins can internally generate a great number of signals, thereby overcoming the limitation that the number of pins should be increased against the tendency of reducing the size of a chip. 
     The method of the invention is now described in more detail with reference to FIG.  12 . The method is for invoking a test mode in a circuit of a semiconductor device. 
     According to box  122 , coded input signals are applied to input test terminals of the circuit. At least one of the input signals has more than two possible levels. 
     Then indicator signals are generated in response to the coded input signals. The indicator signals being of only two levels. This is accomplished by a number of ways. The preferred way is shown in FIG.  12 . 
     According to box  124 , regular signals are generated if a coded input signal has a preset ordinary low level. 
     According to box  126 , higher first level signals are generated if the input signal has a first extra high level that is preset to be higher than the ordinary low level. 
     According to box  128 , control signals are produced in response to the regular signals and to the higher first level signals. The control signals are produced if the input signal has an ordinary high level that is preset to be higher than the ordinary low level and lower than the first extra high level. 
     The above description works well if the input signals have three possible levels. If they have more levels, then more signals are produced. For example, according to box  130 , higher second level signals are generated if the input signal has a second extra high voltage level that is preset to be higher than the first extra high voltage level. Then the control signal is produced in response also to the higher second level signal. According to a next box  132 , the indicator signals are decoded to generate decoded signals. According to a next box  134 , the decoded signals are optionally latched. The remaining portion of the method becomes apparent from the remainder of this description. 
     While the signal generating circuit of the semiconductor device of the present invention has been described in terms of preferred embodiments, those skilled in the art will recognize that the invention is not limited at those embodiments, and can be practiced with modification within the spirit and scope of the appended claims.