Patent Publication Number: US-6912677-B2

Title: Circuit for inspecting a data error

Description:
TECHNICAL FIELD 
   The present disclosure relates generally to a circuit for inspecting a data error, and more particularly, to a circuit for inspecting a data error that is applied to implement a test mode of a synchronous DRAM (SDRAM) configured to inspect error of each of a plurality of bit data through one bit data line. 
   BACKGROUND 
   Generally, as a method for compressing data of several bits into one bit data can inspect error of data of several bits using data of one bit physically, the inspection time and bit number can be reduced. Thus, this method is spotlighted as a method of testing a DRAM. A data compression method is mainly employed to inspect data of multiple bits using a data line of one bit. 
   Referring now to  FIG. 1 , a conventional circuit for inspecting a data error is described. A clock buffer  1  transfer a clock signal CLK inputted from the outside to a latch unit  3 . A buffer unit  2  transfers a row address select signal/RAS, a column address select signal/CAS, a write enable signal /WE, a chip select signal/CS, a clock enable signal CKE and address signals AN&lt; 0 :m&gt;, all of which are inputted from the outside, to a DRAM. Based on the clock signal iclk outputted from the clock buffer  1 , the latch unit  3  synchronizes an output signal of the buffer unit  2  for receiving the signals /RAS, /CAS, /WE, /CS, CKE and AN&lt; 0 :m&gt;. At this time, the clock signal iclk is a signal that becomes a HIGH level when the clock signal CLK becomes a HIGH level. The clock signal iclk may become a LOW level when the clock signal CLK becomes a LOW level or a LOW level because of a certain delay within the DRAM. 
   Each of the signals irasb, icasb, iweb and icsb outputted from the latch unit  3  becomes a HIGH level when the signals /RAS, /CAS, /WE and /CS become LOW levels. Each of the signals Ai&lt; 0 :m&gt; becomes a LOW level when the signals AN&lt; 0 :m&gt; become LOW levels. 
     FIG. 2  is a circuit diagram of the decoder  4  shown in FIG.  1 . The inverter I 1  inverts a signal icasb from the latch unit  3  to output an inverted signal icas. 
   The signals irasb, icasb, iweb and icsb from the latch unit  3  are inputted into a NAND gate G 1 . A NAND gate G 2  may receive the output signal of the NAND gate G 1 , which is inverted by the inverter I 2 , and the address signal Ai&lt; 10 &gt; to output a reset signal reset. 
   The address signals Ai&lt; 0 &gt;, Ai&lt; 1 &gt;, Ai&lt; 2 &gt; and Ai&lt; 3 &gt; from the latch unit  3  are inputted to a NAND gate G 3 . The signals irasb, icasb, iweb and icsb from the latch unit  3  are inputted to a NAND gate G 5 . Also, an inverted signal of the NAND gate G 5 , which is inverted by the inverter I 4 , and an address signal Ai&lt; 7 &gt; are inputted to a NAND gate G 6 . Further, an output signal of the inverter  13  and an output signal of the NAND gate G 6 , which is inverted by the inverters I 5 , I 6  and I 7 , are inputted into a NAND gate G 4 . A signal outputted from the NAND gate G 4  is outputted to an output terminal tcomp through a latch circuit having NAND gates G 7  and G 8 . 
   The decoder  4  described above operates so that an output signal tcomp of the decoder  4  becomes a HIGH level when the signals irasb, icasb, iweb, icsb, Ai&lt; 7 &gt;, Ai&lt; 0 &gt;, Ai&lt; 1 &gt;, Ai&lt; 2 &gt;, Ai&lt; 3 &gt;, or the like from the latch unit  3  are HIGH levels. The output signal tcomp becomes a LOW level when the signals irasb, iweb, icsb, Ai&lt; 10 &gt;, and the like are HIGH levels and the signal icasb is a LOW level. The decoder  4  operates in a data compression mode when the output signal tcomp is a HIGH level. 
   Although, a signal that is decoded to control the output signal tcomp is shown in  FIG. 2 , persons of ordinary skill in the art will readily appreciate that a decoding of another mode is possible. 
     FIG. 3  is a detailed circuit diagram of the compression unit  6  shown in FIG.  1 . The signals gio&lt; 0 &gt; to gio&lt;n&gt; transferred through a data bus are inputted to a NAND gate G 9  and the NOR gate G 10 . An output signal of the NAND gate G 9  through the inverters I 8  and I 9 , an output signal of the NOR gate G 10  through the inverter I 10 , and the output signal tcomp of the decoder  4  are inputted to the NAND gate G 11 . A signal rdbc is outputted from the NAND gate G 11 . 
   The output signal rdbc becomes a HIGH level when the output signal tcomp of the decoder  4  is a HIGH level, and the signals gio&lt; 0 :n&gt; transferred through the data bus within the DRAM are HIGH levels or LOW levels. For all the case except for the above, the output signal rdbc becomes a LOW level (e.g., the output signal tcomp of the decoder  4  is a LOW level). 
     FIG. 4  is a detailed circuit diagram of the select unit  5  shown in  FIG. 1. A  signal gio&lt;k&gt; and a compressed signal rdbc are transferred to the data line rdb through the transmission means T 1  and T 2  that are driven by the output signal tcomp of the decoder  4  and an inverted signal tcompb. 
   When the output signal tcomp of the decoder  4  is a HIGH level, the signal rdbc outputted from the compression unit  6  is transferred to the one bit data line rdb. When the output signal tcomp of the decoder  4  is a LOW level, one signal gio&lt;k&gt; of the signals gio&lt; 0 :n&gt; transferred through the data bus is transferred to the data line rdb. As a result, in the data compression test mode, the signals gio&lt;o:n&gt; transferred through the data bus are compressed to become the signal rdbc, which in turn, is transferred to the data line rdb. For all other modes, the signal gio&lt;k&gt; is transferred to the data line rdb. 
   As noted above, when the signal rdbc compressed through the compression unit  6  does not contain information on the signals gio&lt; 0 :n&gt; and when the signal rdbc is a LOW level, it is difficult to determine which signal of the data bus signals gio&lt; 0 :n&gt; is a HIGH level and which is a LOW level using only the signal rdbc. In other words, in an error inspection test situation in which only the signal rdb being one bit data is outputted from the DRAM, it is difficult to determine which signal of the data bus signals gio&lt; 0 :n&gt; has an error. To inspect which signal of the data bus signals gio&lt; 0 :n&gt; has an error, there is an error inspecting environment by which the signals rdb of n bit corresponding to the signals gio&lt; 0 :n&gt;, respectively, are output from the DRAM to inspect an error. 
   The conventional error inspecting circuit does not contain data information of respective bits in data transferred through the data line of one bit. Accordingly, if an error of data occurs, it is difficult to determine which data has an error. As a result, in a test environment of DRAM using a conventional data compression method, there is a need for a test environment by which each of data of several bits can be inspected for error. 
   SUMMARY OF THE DISCLOSURE 
   A circuit for inspecting a data error in which information on each of N bit data is transferred to a data transferred through a data line of one bit is disclosed. 
   The circuit for inspecting a data error comprises a clock buffer for receiving a clock signal from the outside; a buffer unit for receiving a plurality of operation control signals and an address signal from the outside; a latch unit for synchronizing the plurality of the operation control signals and the address signal from the buffer unit depending on the clock signal from the clock buffer; a decoder for receiving the plurality of the operation control signals and the address signal from the latch unit to output a data compression signal, a first signal, and a second signal; a compression unit for compressing the data bus signal depending on the data compression signal from the decoder; a counter for sequentially generating count pulses depending on the clock signal from the clock buffer, and the first signal and the second signal from the decoder; a data bus signal latch unit having a latch for storing the data bus signal depending on the first signal from the decoder, for outputting a signal stored at the latch depending on the count pulse from the counter; and a select unit for selectively outputting one of an output signal of the data bus signal latch unit, a signal from the compression unit and the data bus signal, depending on the data compression signal and the second signal outputted from the decoder. 
   The decoder includes a first logic means for logically combining the plurality of the operation control signals and the address signal from the latch unit to generate a reset signal; a second logic means for logically combining the plurality of the operation control signals from the latch unit; a delay means for delaying a signal from the second logic means to generate a first signal; a third logic means for logically combining the address signal from the latch unit; a fourth logic means for logically combining the plurality of the operation control signals and the address signal from the latch unit and then delaying the logically combined signals; a fifth logic means for logically combining the address signal from the latch unit; and a sixth logic means for generating the data compression signal and the second signal depending on a signal from the third, fourth and fifth logic means and the reset signal. 
   The counter includes a delay means for delaying a clock signal from the clock buffer; a logic means for logically combining the first and second signals from the decoder and the clock signal from the delay means to generate first, second and third signals; and a count pulse generator for generating a count pulse depending on the first, second and third signals. 
   The data bus signal latch unit includes a logic means for generating first and second signals of opposite state each other using the first signal from the decoder; a plurality of inverters for inverting each of the plurality of the count signals outputted from the counter; a plurality of latch means for latching each of the data bus signals depending on the first and second signals from the logic means; and a plurality of transfer means for transferring the data bus signals latched in the latch means to the output terminal, depending on a count signal from the counter and an inverted count signal from the inverter. 
   The select unit includes a first transfer means for an output signal of the data bus signal latch unit to the output terminal depending on the second signal and the inverted second signal; a first logic means for logically combining the data compression signal and the inverted second signal; a second transfer means for transferring an output signal of the compression unit to the output terminal depending on an output signal of the first logic means and an inverted output signal of the first logic means; a second logic means for logically combining the data compression signal and the second signal; and a third transfer means for transferring the data bus signal to the output terminal depending on an output signal of the second logic means and an inverted output signal of the second logic means. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This disclosure will be described in terms of several embodiments to illustrate its broad teachings. Reference is also made to the attached drawings. 
       FIG. 1  is a block diagram of a conventional data error inspecting circuit; 
       FIG. 2  is a detailed circuit diagram of a decoder shown in  FIG. 1 ; 
       FIG. 3  is a detailed circuit diagram of a compression unit shown in  FIG. 1 ; 
       FIG. 4  is a detailed circuit diagram of a select unit shown in  FIG. 1 ; 
       FIG. 5  is a block diagram a circuit for inspecting a data error; 
       FIG. 6  is a detailed circuit diagram of a decoder shown in  FIG. 5 ; 
       FIG. 7  is a detailed circuit diagram of a counter shown in  FIG. 5 ; 
       FIG. 8  is a detailed circuit diagram of a data bus signal latch shown in  FIG. 5 ; 
       FIG. 9  is a detailed circuit diagram of a select unit shown in  FIG. 5 ; and 
     FIG.  10 A and  FIG. 10B  are a timing diagram of an operation of the data error inspecting circuit. 
   

   DETAILED DESCRIPTION 
   The present disclosure will be described in detail by way of a preferred embodiment with reference to accompanying drawings, in which like reference numerals are used to identify the same or similar parts. 
   To solve problems inherent in the conventional data compression method, data of a N bit is sequentially transferred through a data line of one bit. A test mode is implemented by which information of each of N bit data can be transferred to data transferred through the data line of one bit. Therefore, the an error of each of data of plural bits can be inspected with only data transferred through the data line of one bit without additional test environment. 
   Referring now to  FIG. 5 , a circuit for inspecting a data error is shown. A clock buffer  11  transfers a clock signal CLK from the outside. A buffer unit  12  receives a plurality of operation control signals: a row address select signal/RAS, a column address select signal/CAS, a write enable signal/WE, a chip select signal/CS, a clock enable signal CKE and address signals AN&lt; 0 :m&gt;. The latch unit  13  synchronizes the plurality of the operation control signals and an address signal from the buffer unit  12  depending on a clock signal iclk outputted from the clock buffer  11 . 
   A decoder  14  receives the plurality of the operation control signals irasb, icasb, iweb and icsb, and the address signals Ai&lt; 0 :m&gt; from the latch unit  13  to output a data compression signal tcomp, a signal rs and a signal tsdout. The compression unit  16  compresses the data bus signals gio&lt; 0 :n&gt; depending on the data compression signal tcomp from the decoder  14 . The counter  18  sequentially generates count pulses ts&lt; 0 :n&gt; depending on the clock signal iclk from the clock buffer  11 , and the signal rs and the signal tsdout from the decoder  14 . The data bus signal latch unit  17  stores the data bus signals gio&lt; 0 :n&gt; with its latch depending on the signal rs from the decoder  14  and then outputs a signal rdbl stored at the latch depending on the count pulses ts&lt; 0 :n&gt; from the counter  18 . The select unit  15  selectively outputs one of the output signal rdbl from the data bus signal latch unit  17 , the signal rdbc from the compression unit  16 , and one signal gio&lt;k&gt; of the data bus signals gio&lt; 0 :n&gt; through the output terminal rdb depending on the data compression signal tcomp and the signal tsdout from the decoder  14 . 
   Referring now to  FIG. 6 , the decoder  14  shown in  FIG. 5  will be described in detail. The inverters I 21  and I 22  output inverted signals icas and iras of the operation control signals icasb and irasb outputted from the latch unit  13 , respectively. The NAND gate G 21  receives the plurality of the operation control signals irasb, icasb, iweb and icsb from the latch unit  13 . The NAND gate G 22  receives an output signal of the NAND gates G 21  that is inverted by the inverter I 23 , and the address signal Ai&lt; 10 &gt; to output a reset signal reset. 
   The NAND gate G 23  receives the data compression signal tcomp outputted from the decoder  14 , the output signal iras from the inverter I 22 , and the output signals icasb and icsb from the latch unit  13 . An output signal of the NAND gate G 23  is inputted to the inverter I 24 . An output signal of the inverter I 24  is inputted to a delay means  20 . The delay means  20  includes inverters I 25 , I 26  and I 27  for inverting an output signal of the inverter I 24 , and a NAND gate G 24  for receiving an output signal of the inverter I 24  and an output signal of the inverter I 27 . An output of the delay means  20  is delayed by the delay means  21  and is then outputted to an output terminal rs through the inverter I 28 . 
   The NAND gate G 29  receives the plurality of the operation control signals irasb, icasb, iweb and icsb from the latch unit  13 . The NAND gate G 30  receives an output signal of the NAND gate G 29  that is inverted by the inverter I 30 , and the address signal Ai&lt; 7 &gt;. An output signal of the NAND gate G 30  is inverted through the inverters I 31 , I 32  and I 33 . The NAND gate G 25  receives the address signals Ai&lt; 0 &gt;, Ai&lt; 1 &gt;, Ai&lt; 2 &gt; and Ai&lt; 3 &gt;. Also, the NAND gate G 26  receives an output signal of the NAND gate G 25  that is inverted by the inverter I 29 , and an output signal of the inverter I 33 . The NAND gate G 31  receives the address signals Ai&lt; 0 &gt;, Ai&lt; 1 &gt; and Ai&lt; 2 &gt;, and the address signal Ai&lt; 3 &gt; inverted by the inverter I 34 . The NAND gate G 32  receives an output signal of the NAND gate G 31  that is inverted by the inverter I 35 , and an output signal of the inverter I 33 . The latch circuit consisting of the NAND gates G 27  and G 28  outputs the data compression signal tcomp depending on an output signal of the NAND gate G 26  and the reset signal reset. Further, the latch circuit consisting of the NAND gates G 33  and G 34  outputs the signal tsdout depending on an output signal of the NAND gate G 32  and the reset signal reset. 
   When the plurality of the operation control signals irasb, icasb, iweb and icsb, and the address signals Ai&lt; 7 &gt;, Ai&lt; 0 &gt;, Ai&lt; 1 &gt; and Ai&lt; 2 &gt; are HIGH levels and the address signal Ai&lt; 3 &gt; is a LOW level, the signal tsdout becomes a HIGH level. When the signals irasb, iweb, icsb and Ai&lt; 10 &gt; are HIGH levels and the signal icasb is a LOW level, the signal tsdout becomes a LOW level. 
   When the signal tsdout is a HIGH level, a test mode by which data of plural bits are outputted through the data line of one bit. The decoding signal used to control the signal tsdout is illustrative and another decoding signal is possible. 
   The signal rs is an active HIGH pulse that is generated by making an active LOW pulse with an adequate width into a signal in which the signals icasb, ics are HIGH levels when the signal rs is a HIGH level. The signals icasb, ics become a HIGH signal when the signal irasb is a LOW level and then by delaying this pulse for a given time. The delay time of the delay means  21  is controlled so that data loaded onto the data bus signals gio&lt; 0 :n&gt; of plural bits can be stored at the latch lat&lt; 0 :n&gt; in  FIG. 8  when the signals icasb and icsb are HIGH levels and the signal irasb is a LOW level. 
     FIG. 7  is a detailed circuit diagram of the counter  18  shown in FIG.  5 . The clock signal iclk outputted from the clock buffer  11  is outputted to the output terminal iclkd through the inverters I 41  and I 42 , the delay means  31  and the inverter I 46 . The delay means  31  includes inverters I 43 , I 44  and I 45  for inverting an output signal of an inverter I 42 , and a NAND gate G 41  for receiving an output signal of the inverter I 45  and an output signal of the inverter I 42 . 
   The latch circuit consisting of NAND gates G 42  and G 43  receives the signals rs and tsdout, and a signal ts&lt;n&gt; that is inverted by the inverters I 48 , I 49  and I 50 . An output signal of the latch circuit is outputted to the output terminal c 1  through an inverter I 51 . A NAND gate G 44  receives an output signal of the latch circuit consisting of NAND gates G 42  and G 43  and the signal iclkd. An output signal of the NAND gate G 44  is outputted to an output terminal c 2  through an inverter I 52 , a delay means  32  and an inverter I 53 . An output signal of the inverter I 53  is outputted to an output terminal c 3  through an inverter I 54 . 
   A count pulse generator  33  is driven depending on the signals c 1 , c 2  and c 3 . The count pulse generator  33  includes a plurality of signal transfer units  34  serially connected, and a plurality of pulse generators  35  for generating a pulse depending on a signal outputted from the signal transfer unit  34 . 
   The signal iclkd is an active HIGH pulse that becomes a LOW pulse in which the clock signal iclk from the clock buffer  11  has an adequate width that is delayed by the delay means  31  and inverted by the inverter I 46 . If the signal rs becomes a HIGH level, the signal c 1  becomes a LOW level. The signal iclkd is delayed for a given time and is then inverted to produce the signal c 2 . The signal c 3  is an inverted signal of the signal c 2 . 
   In the signals ts&lt; 0 :n&gt;, if the signal tsdout is a HIGH level, wherein: 
   1) If the signal rs is outputted with a HIGH level from the decoder  14 , the signal c 1  becomes a LOW level to make only the count pulse ts&lt; 0 &gt; an active HIGH, 
   2) After the first step, if the signal iclkd becomes a HIGH level and a given time elapses, only the count pulse ts&lt; 1 &gt; becomes an active HIGH, and 
   3) After the second step, if the signal iclkd becomes a HIGH level and a given time elapses, only the count pulse ts&lt; 2 &gt; becomes an active HIGH, 
   4) After the first step, the signal iclkd becomes a HIGH level and a given time elapses, only the count pulse ts&lt; 3 &gt; becomes an active HIGH. In this manner, if the pulse is sequentially generated up to a n-th count pulse ts&lt;n&gt;, the signal c 1  becomes a HIGH level so that generation of the count pulse ts&lt; 0 :n&gt; is stopped. 
     FIG. 8  is a detailed circuit diagram of the data bus signal latch unit  17  shown in FIG.  5 . The inverter I 61  outputs an inverted signal tonb of the signal rs outputted from the decoder  14 . The inverter I 62  outputs an inverted signal ton of the signal tonb. The inverter I 63  outputs an inverted signal tsb&lt; 0 &gt; of the count signal ts&lt; 0 &gt;. In the same manner, the inverters I 63  to I 66  output inverted signals tsb&lt; 1 &gt; to tsb&lt;n&gt; of the count signals ts&lt; 1 &gt; to ts&lt;n&gt;. 
   The data bus signals gio&lt; 0 &gt; to gio&lt;n&gt; are inputted to the latch means (one shown as  41 ). A signal outputted from each of the latch means  41  is transferred to the output terminal rdbl sequentially through a transmission gate T 22  driven depending on the count pulse ts&lt;n&gt; and the inverted count pulse tsb&lt;n&gt;, a latch having the inverters I 70  and I 71 , and an inverter I 72 . Each of the latch means  41  includes a transmission gate T 21  for receiving the data bus signals gio&lt; 0 &gt; to gio&lt;n&gt;, a latch connected to the transmission gate T 21  and having inverters I 67  and I 68 , and an inverter I 69  connected to the latch. 
   If the signal rs becomes a HIGH level, data of the data bus signal gio&lt; 0 :n&gt; is stored at the latch means lat&lt; 0 :n&gt;  41 . Next, the count signals ts&lt; 0 :n&gt; become sequentially a HIGH level, wherein: 
   1) If the count signal ts&lt; 0 &gt; becomes a HIGH level, data of the latch lat&lt; 0 &gt; is transferred to the output terminal rdbl, 
   2) If the count signal ts&lt; 1 &gt; becomes a HIGH level, data of the latch lat&lt; 1 &gt; is transferred to the output terminal rdbl, 
   3) If the count signal ts&lt; 2 &gt; becomes a HIGH level, data of the latch lat&lt; 2 &gt; is transferred to the output terminal rdbl, and 
   4) If the count signal ts&lt; 3 &gt; becomes a HIGH level, data of the latch lat&lt; 3 &gt; is transferred to the output terminal rdbl. In this manner, if a n-th count signal ts&lt;n&gt; becomes a HIGH level, data of the latch lat&lt;n&gt; is transferred to the output terminal rdbl. 
     FIG. 9  is a detailed circuit diagram of the select unit  15  shown in FIG.  5 . The NAND gate G 81  receives the data compression signal tcomp outputted from the decoder  14 , and the signal tsdout inverted by the inverter I 81 . The NOR gate G 82  receives the data compression signal tcomp and the signal tsdout. 
   The signal rdbl outputted from the data bus signal latch unit  17  is transferred to the output terminal rdb based on an operation of the transmission gate T 31 , which is driven depending on the signal tsdout and the signal tsdout inverted by the inverter I 82 . The signal rdbc outputted from the compression unit  16  is transferred to the output terminal rdb based on an operation of the transmission gate T 32 , which is driven depending on an output signal of the NAND gate G 81  and an output signal of the NAND gate G 81  that is inverted by the inverter I 83 . The data bus signal gio&lt;k&gt; is transferred to the output terminal rdb based on an operation of the transmission gate T 33 , which is driven depending on an output signal of the NOR gate G 82  and an output signal of the NOR gate G 82  that is inverted by the inverter I 84 . 
   In other words, if the signal tsdout outputted from the decoder  14  is a HIGH level, the signal rdbl outputted from the data bus signal latch unit  17  is transferred to the output terminal rdb. If the signal tsdout is a LOW level and the data compression signal tcomp is a HIGH level, the signal rdbc is transferred to the output terminal rdb. If both the data compression signal tcomp and the signal tsdout are LOW levels, the data bus signal gio&lt;k&gt; is transferred to the output terminal rdb. 
   FIG.  10 A and  FIG. 10B  are timing diagrams illustrating an operation of the data error inspecting circuit when n=3. In a clock CLKO in which the signals /RAS, /CAS, /WE, /CS and AN&lt; 3 &gt; inputted from the outside are LOW levels and the signals AN&lt; 0 &gt;, AN&lt; 1 &gt;, AN&lt; 2 &gt; and AN&lt; 7 &gt; are HIGH levels, if the signals irasb, icasb, iweb, icsb, Ai&lt; 0 &gt;, Ai&lt; 1 &gt;, Ai&lt; 2 &gt; and Ai&lt; 7 &gt; become HIGH levels and the signal Ai&lt; 3 &gt; becomes a LOW level, the output signal tsdout of the decoder  14  becomes a HIGH level. 
   In a clock CLK 1  in which the signals /CAS and /CS are LOW levels and the signals /RAS and /WE are HIGH levels, if the signals icasb and icsb become HIGH levels and the signals irasb and iweb become LOW levels, data is loaded onto the data bus gio&lt; 0 : 3 &gt; depending on an internal operation of the DRAM. The data buses gio&lt; 0 &gt; and gio&lt; 2 &gt; are loaded with data of HIGH levels and the data buses gio&lt; 1 &gt; and gio&lt; 3 &gt; are loaded with data of LOW levels. 
   Further, the signal rs becomes an active LOW pulse and the data of the data bus gio&lt; 0 : 3 &gt; is stored at the latch lat&lt; 0 : 3 &gt;. The count pulse ts&lt; 0 &gt; becomes a HIGH level and data of the latch lat&lt; 0 &gt; is transferred to the output terminals rdbl and rdb. 
   In clocks CLK 2 , CLK 3  and CLK 4 , the count pulses ts&lt; 1 &gt;, ts&lt; 2 &gt; and ts&lt; 3 &gt; sequentially become a HIGH level when a signal /CS of a HIGH level is inputted from the outside and the signal icsb becomes a LOW level. Thus, data of the latches lat&lt; 1 &gt;, lat&lt; 2 &gt; and lat&lt; 3 &gt; are transferred to the output terminals rdbl and rdb. 
   In a clock CLK 5 , the signals /RAS, /WE and /CS of LOW levels and the signals /CAS and AN&lt; 10 &gt; of HIGH levels are inputted from the outside. The signals irasb, iweb, icsb and Ai&lt; 10 &gt; become HIGH levels and the signal icasb becomes a LOW level. Therefore, the signal tsdout becomes a LOW level. 
   As mentioned above, data of N bits is sequentially transferred through a data line of one bit. Thus, a test mode by which information of each of the N bit data is transferred to the data transferred to the data line of one bit can be implemented. Accordingly, error in each of data of plural bits can be inspected with only data transferred through the data line of one bit without additional test environment. Thus, cost in facilitating error inspection can be reduced. 
   Many changes and modifications to the embodiments described herein could be made. The scope of some changes is discussed above. The scope of others will become apparent from the appended claims.