a2 United States Patent
`US 6,483,771 B2
`(0) Patent No.:
`Nov. 19, 2002
`(45) Date of Patent:
`Shin
`
`US006483771B2
`
`(54) SEMICONDUCTOR MEMORY DEVICE AND
`METHOD OF OPERATION HAVING DELAY
`PULSE GENERATION
`
`(75)
`
`Inventor: Tae-jeen Shin, Seoul (KR)
`
`9/1999 Idei et al. ee 327/160
`5,955,905 A *
`ee 327/158
`5,973,525 A * 10/1999 Fujii...
`
`5,999,483 A * 12/1999 Ttou wee
`eee 365/233
`
`6,005,825 A * 12/1999 Lee et al. we 365/233
`6,014,339 A *
`1/2000 Kobayashietal. ......... 365/233
`6,239,631 B1 *
`5/2001 Fujioka et al... 327/144
`
`(73) Assignee: Samsung Electronics Co., Ltd., Suwon
`(KR)
`
`* cited by examiner
`
`(*) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`US.C. 154(b) by 0 days.
`
`Primary Examiner—David Nelms
`Assisiant Examiner—Thong Le
`(74) Attorney, Agent, or Firm—Volentine T'rancos, P.L.L.C.
`
`(57)
`
`ABSTRACT
`
`(21) Appl. No.: 09/875,001
`
`(22)_Filed: Jun. 7, 2001
`
`A semiconductor memory device including a memory core
`block,
`a
`logic circuit and a direct access circuit which
`
`control the memorycore block, and a delay pulse generation
`circuit. The logic circuit generates first and second internal
`clock signals responsive to first and second external clock
`signals, and operates the memory core block at high speed
`during a normal operation. The direct access circuit gener-
`ates first and secondinternal clock signals responsive to first
`and second external clock signals, to test the memory core
`block during a direct access operation. The delay pulse
`generation circuit generates a pulse signal corresponding to
`the delay difference between the first and second internal
`clock signals generated by the direct access circuit. The
`delay difference is used by a tester to compensate for actual
`delay of the internal clock signals when the memorycore
`block is tested during the direct access operation.
`
`(65)
`
`Prior Publication Data
`US 2002/0001252 Al Jan. 3, 2002
`
`(30)
`
`Foreign Application Priority Data
`
`Jun. 30, 2000
`
`(KR) oe cece ce ee eneeenneees 2000-37398
`
`Tint, C07 ec cee ee ccecceeeteeeeeeteeeeeeereeneennees G11C 8/00
`(SV)
`(52) U.S. Cho... 365/233; 365/194; 365/230.03
`(58) Field of Search oo... 365/233, 201,
`365/194, 225, 191, 230.03
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`5,083,299 A
`
`1/1992 Schwankeet al.
`
`.......... 368/113
`
`18 Claims, 3 Drawing Sheets
`
`3 300
`
`310
`
`550
`
`
`
`o4
`
`02
`
`
`FREQUENCY
`
`
`MEMORY
`LOGIC CIRCUIT
`
`
`CORE
`
`BLOCK
`
`
`
`
`FREQUENCY
`DA
`
`GENERATION
`CIRCUIT
`
` DELAY PULSE
`
`APPLE 1011
`
`APPLE 1011
`
`1
`
`

`

`U.S. Patent
`
`Nov.19, 2002
`
`Sheet 1 of 3
`
`US 6,483,771 B2
`
`FIG.
`
`1
`
`(PRIOR ART)
`
`100
`
`110
`130
`
` %1
`HIGH|| FREQUENCY 7
`
`
`7
`LOW
`_
`
`
`
`G2
`
`LOGIC CIRCUIT
`
`FREQUENCY
`DA
`
`120
`
`;
`
`MEMORY
`CORE
`BLOCK
`
`
`
`FIG. 2A (PRIOR ART)
`
`
`
`6——}|L—_______—
`
`|
`
`,
`
`%)
`
`go?
`
`yO
`1y
`1
`
`|
`
`I
`I
`1/
`
`\
`
`Aoi’ = Ag2’
`
`2
`
`

`

`U.S. Patent
`
`Nov.19, 2002
`
`Sheet 2 of 3
`
`US 6,483,771 B2
`
`FIG. 2B (PRIOR ART)
`
`FREQUENCY
`DA
`
`JtO 5350
`
` 04
`
`
`| FREQUENCY
`HIGH
`
` MEMORY
`
`
`LOGIC CIRCUITtet
`02
`
`CORE
`
`BLOCK
` GENERATION
`
`CIRCUIT
`
`mo
`
`3
`
`

`

`U.S. Patent
`
`Nov.19, 2002
`
`Sheet 3 of 3
`
`US 6,483,771 B2
`
`FIG. 4
`
`401
`
`PAD
`
`
`
`FIG
`
`oO
`
`4
`
`

`

`US 6,483,771 B2
`
`1
`SEMICONDUCTOR MEMORY DEVICE AND
`METHOD OF OPERATION HAVING DELAY
`PULSE GENERATION
`
`The present application claims priority under 35 U.S.C.
`§119 to Korean Application No. 00-37398 filed on Jun. 30,
`2000, which is hereby incorporated by reference in its
`entirety for all purposes.
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`
`10
`
`2
`The internal clock signals p1' and 2’ are importantto the
`operation of the memorycore block 130. When the internal
`clock signals 1’ and $2' are generatedat different times,that
`is, when Agl' is different from Aq2', there may be an error
`in the operation of memory core block 130. In other words,
`the memory core block 130 operates normally depending on
`the internal clock signals @1' and 2' simultaneously gener-
`ated (ic. Apl'=Ag2') while the semiconductor memory
`device 100 is operating at high speed in relation with the
`high frequency logic circuit 110. However, the memory core
`block 130 does not operate properly due to the internal clock
`signals p1' and 2' sequentially generated with a delay time
`therebetween (i.e. Ap1'#Aqg2') while the semiconductor
`memory device 100 is operating at low speedin relation with
`the low frequency DA 120. Even if the memory core block
`130 does not operate in error while it is being operated by
`the low frequency DA 120, the memory core block 130 does
`not satisfy the conditions of normal operation.
`Consequently, complete operating conditions required for
`testing memory cells within the memory core block 130
`using a direct access method cannot be achieved.
`Therefore, a memory device which can operate a memory
`core block under the same conditions both when the memory
`core block operates depending on a high frequency logic
`circuit and when the memorycore block operates depending
`on a low frequency DA,is desired.
`SUMMARY OF THE INVENTION
`
`The present invention is therefore directed to a semicon-
`ductor memory device and method of operation which
`substantially overcomes one or more of the problems due to
`the limitations and disadvantages of the related art.
`To solve the above problems,it is an object of the present
`invention to provide a semiconductor memory device
`capable of operating a memory core block under the same
`conditions during high speed operation and during a direct
`access operation without a delay between internal clock
`signals.
`to achieve the above objects of the
`Accordingly,
`invention,there is provided a semiconductor memory device
`including a memory core block including a plurality of
`memory cells; a logic circuit that generates a first internal
`clock signal and a second internal clock signal in response
`to a first external clock signal and a second external clock
`signal, respectively, and that operates the memory core
`block at high speed, during normal operation; a direct access
`circuit that generates the first internal clock signal and the
`second internal clock signal in response to the first external
`clock signal and the second external clock signal,
`respectively, to test the memory cells within the memory
`core block during a direct access operation; and a delay
`pulse generation circuit that generates a pulse signal corre-
`sponding to the delay difference between the first internal
`clock signal and the second internal clock signal generated
`by the direct access circuit.
`The delay pulse generation circuit may include a first
`internal pulse generator that receives the first internal clock
`signal and that generatesa first internal pulse signal having
`a predetermined pulse width; a second internal pulse gen-
`erator that receives the second internal clock signal and that
`generates a second internal pulse signal having a predeter-
`mined pulse width; and a pulse signal generator that receives
`the first and second internal pulse signals and that generates
`the pulse signal. The pulse signal is transmitted to a pad in
`response lo a third internal clock signal.
`According to the semiconductor memory device of the
`present invention, the time period of the pulse signal gen-
`
`The present invention relates to a semiconductor memory
`device and method of operation thereof, and more
`particularly,
`to delay pulse generation for measuring the
`delay between internal control signals.
`2. Description of the Related Art
`As computer systems deliver higher and higher
`performance,
`it
`is necessary for semiconductor memory
`devices to have a large capacity and operate at high speed.
`Semiconductor memory devices can have a large capacity
`by including memory blocks each having a plurality of
`memory cells, and can operate at a high speed through a
`logic circuit operating at a high frequency.
`FIG. 1 is a schematic diagram illustrating a conventional
`semiconductor memory device. Referring to FIG. 1, a semi-
`conductor memory device 100 includes a high frequency 2
`logic circuit 110, a low frequencydirect access unit (DA)
`120 and a memorycore block 130. The high frequency logic
`circuit 110 is generally controlled by external clock signals
`ol and 2, and generates internal clock signals 1’ and 92!
`to control the operation of the memory core block 130.
`Therefore,
`the memory core block 130 operates at high
`speed according ta the operation specifications of an actual
`semiconductor memory device. The low frequency DA 120
`is used for testing for defects of memory cells within the
`memory core block 130. The low frequency DA 120 ;
`receives the external clock signals 1 and $2 without opera-
`tion of the high frequency logic circuit 110, and generates
`internal clock signals p1' and #2' to control the operation of
`the memory core block 130. Since it is not necessary to
`operate the memory core block 130 at high speed during
`testing for defects of memory cells, the low frequency DA
`120 operates at low speed.
`The internal clock signals p1' and 2' generated by the
`high frequency logic circuit 110 have different delay times
`than the internal clock signals $1' and ¢2' generated bythe
`low frequency DA 120. This will be described with refer-
`ence to the timing chart shown in FIGS. 2A and 2B. FIG. 2A
`illustrates the internal clock signals $1' and $2' generated by
`the high frequency logic circuit 110 in response to the
`external clock signals @1 and 2. Since the high frequency
`logic circuit 110 operates in synchronousrelation to a clock
`signal, the delay time Apl' betweenthe first external clock
`signal 1 andthefirst internal clock signal ¢1' is almost the
`same as the delay time Ag2' between the second external
`clock signal 2 and the second internal clock signal ¢2'.
`FIG. 2B shows the internal clock signals o1' and @2'
`generated by the low frequency DA 120. Unlike the high
`frequency logic circuit 110,
`the low frequency DA 120
`asynchronously operates, and a load on a path through which
`the first internal clock signal 1' is generated is different
`from a load on a path through which the second internal
`clock signal 2' is generated in the low frequency DA 120.
`Accordingly, the delay time A1' between the first external
`clock signal $1 and the first internal clock signal 1’ is
`different from the delay time Ag2' between the second
`external clock signal $2 and the second internal clock signal
`$2".
`
`15
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`20
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`;
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`40
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`45
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`65
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`5
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`

`

`US 6,483,771 B2
`
`10
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`20
`
`4
`3
`high speed operation of the memory device 300, to interface
`erated by the delay pulse generation circuit receiving the
`with memory core block 330. The high frequency logic
`internal clock signals having different delay times is mea-
`circuit 310 may include high frequency logic circuits such as
`sured. A tester compensates for the measured time period
`delay lock loops (DLL) and may receive and transmit an
`before the direct access test. Therefore, the memory core
`interface protocol, as would be understood by one ofordi-
`block is allowed to operate depending on internal clock
`nary skill.
`signals having the same conditions as those of internal clock
`The low frequency DA 320 directly tests memory cells
`signals generated by the logic circuit, even during the direct
`within the memory core block 330 during a direct access
`access operation, so that errors in the operation of the
`operation without operating the high frequency logic circuit
`memory core block can be prevented.
`310,
`to detect defects of the memory cells. The low fre-
`The above objects of the invention may also be achieved
`quency DA 320 may be any direct access unit as would be
`by a method of operating a semiconductor memory device
`well known,that operates to test memorycell (core) param-
`having a memory core block that includes a plurality of
`eters at low frequencies of 100 MHz for example, although
`memory cells, the method including generatinga first inter-
`other frequencies may be used. Memory core block 330
`nal clock signal and a second internal clock signal respec-
`15
`operates at a high frequency of 400 MHz for example,
`tively responsivetoafirst external clock signal and a second
`although other frequencies mayalso be used. Accordingly,it
`external clock signal; operating the memory core block
`is to be understood that the low frequency DA 320 operates
`based on the first and second internal clock signals during a
`the memory core block 330 at low speed. As well knownto
`normal operation mode; generating a third internal clock
`one of ordinary skill in the art, the direct access operation is
`signal and a fourth internal clock signal respectively respon-
`a test for defects in the memorycells, that occur because of
`sive to the first external clock signal and the second external
`bridges or variation in parameters due to a change in
`clock signal; testing the plurality of memorycells based on
`fabrication processes. It should be apparent that high speed
`the third and fourth internal clock signals during a direct
`operation is thus not necessary during the test. The memory
`access operation mode; and generating a pulse signal cor-
`core block 330 reads data from or writes data to memory
`respondingto a delay difference betweenthe third and fourth
`cells depending on internal clock signals @l' and 2' gen-
`internal clock signals.
`erated by the high frequency logic circuit 310 or the low
`Further scope of applicability of the present invention will
`frequency DA 320.
`become apparent from the detailed description given here-
`The delay pulse generation circuit 340 receives the inter-
`inafter. However, it should be understood that the detailed
`nal clock signals $1' and 2' and generates a pulse signal
`description and specific examples, while indicating pre-
`PULcorrespondingto the difference between the delay time
`ferred embodiments of the invention, are given by way of
`of thefirst internal clock signal @1' and the delay time of the
`illustration only, since various changes and modifications
`second internal clock signal ¢2'. The pulse signal PUL is
`within the spirit and scope of the invention will become
`transmitted to a pad PAD, andthe period of the pulse signal
`apparent to those skilled in the art from this detailed descrip-
`PUL can be externally measured using an oscilloscope. The
`tion.
`period of the pulse signal PUT. indicates the difference “d”
`between the delay time ofthe first internal clock signal ol'
`and the delay time of the second internal clock signal 2’.
`The delay pulse generation circuit 340 is illustrated in detail
`in FIG. 4.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The present invention will become more fully understood
`from the detailed description given hereinbelow and the
`accompanying drawings which are given by way ofillus-
`tration only, and thus are not
`limitative of the present
`invention, and wherein:
`FIG. 1 is a schematic diagram illustrating a conventional
`semiconductor memory device;
`FIGS. 2A and 2B are timing diagramsillustrating internal
`control signals in the semiconductor memory device of FIG.
`1;
`
`FIG. 3 is a diagram illustrating a semiconductor memory
`device according to an embodimentof the present invention;
`FIG. 4 is a diagram illustrating the delay pulse generation
`circuit of FIG. 3; and
`FIG. 5 is a timing diagram illustrating the operation of the
`delay pulse generation circuit of FIG. 4.
`DETAILED DESCRIPTION OF THE PRESENT
`INVENTION
`
`40
`
`45
`
`Referring to FIG. 4, the delay pulse generation circuit 340
`includes a first
`internal pulse generator 401, a second
`internal pulse generator 405 and a pulse signal generator
`410. The first internal pulse generator 401 includes a two-
`input NAND gate 403 that receives the first internal clock
`signal @1' and the output of a first delay unit 402 and that
`generatesa first internal pulse signal A. The first delay unit
`402 receives the first internal clock signal 1', and inverts
`and delaysthe first internal clock signal $1' a predetermined
`period of time. The second internal pulse generator 405
`includes a two-input NAND gate 407 that receives the
`second internal clock signal @2' and the output of second
`delay unit 406 and that generates a second internal pulse
`signal B, in the same mannerasin the first internal pulse
`generator 401. The second delay unit 406 receives the
`second internal clock signal 2', and inverts and delays the
`second internal clock signal ¢2' a predetermined period of
`time.
`
`Hereinafter, an embodimentof the present invention will
`be described in detail with reference to the attached draw-
`
`ings. In the drawings, the same reference numerals denote
`the same members.
`
`Referring to FIG. 3, a semiconductor memory device 300
`includes a high frequency logic circuit 310, a low frequency
`direct access unit (DA) 320, a memory core block 330 and
`a delay pulse generation circuit 340. The high frequency
`logic circuit 310 operates depending on external clock
`signals 1 and 2 during normal operation and controls the
`
`60
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`65
`
`The pulse signal generator 410 receives the first internal
`pulse signal A and the secondinternal pulse signal B through
`a two-input NAND gate 411 and generates a third internal
`pulse signal C. A third internal clock signal 3' is generated
`from the MRS (Mode Register Set), and both directly and as
`provided through the illustrated inverter, controls the trans-
`mission gate at the output of NAND gate 411. The third
`internal pulse signal C thus is transmitted to the pad in
`response to the third internal clock signal »3' and appears as
`the pulse signal PUL.
`
`6
`
`

`

`US 6,483,771 B2
`
`5
`FIG. 5 is a timing diagram illustrating the operation of the
`delay pulse generation circuit 340 of FIG. 4. Referring to
`FIG. 5, the first and second internal clock signals p1' and 92!
`are generated with different delay times. The first internal
`pulse signal A is generated as a logic “low” level, and as
`having a pulse width corresponding to the delay time of the
`first delay unit 402 of FIG. 4, at the moment whenthefirst
`internal clock signal p1' is generated. The second internal
`pulse signal B is generated as a logic “low” level, and as
`having a pulse width corresponding to the delay time of the
`second delay unit 406 of FIG. 4, at the moment when the
`second internal clock signal 2' is generated.
`The third internal pulse signal C is generated as a logic
`“high” level corresponding to the pulse width of the logic
`“low”level of the first internal pulse signal A or the second
`internal pulse signal B. The third internal pulse signal C
`appears as consecutive two pulse signals. The time interval
`“d” between the two consecutive pulse signals corresponds
`to the difference between the time delay of the first internal
`clock signal o1' and the time delay of the second internal
`clock signal $2". In this embodiment, the third internal pulse
`signal C appears as two consecutive pulse signals, but in an
`alternative embodiment,
`it may appear as a pulse signal
`having a pulse width corresponding to the interval from
`generation of the first internal clock signal l' and the
`generation of the second internal clock signal $2’.
`The third internal pulse signal C is transmitted to the pad
`in response to the third internal clock signal 3’. It appears
`in the waveform shownat the pad of FIG. 3. This waveform
`is measured using an oscilloscope (not shown), and the
`measured valueis input to a tester as a delay parameter when
`the memorycore block 330 of FIG. 3 is tested. Therefore,
`the tester compensates for the actual delay of the internal
`clock signals so that the memory core block 330 is tested
`depending on delay compensated internal clock signals. In
`other words, the memory core block 330 operates depending
`on internal clock signals having the same conditionsas those
`of internal clock signals generated by the high frequency
`logic circuit 310, even during a direct access operation, so
`that an error in the operation of the memory core block 330
`can be prevented.
`The invention being thus described,it will be obvious that
`the same may be varied in many ways. Such variations are
`not to be regarded as a departure from the spirit and scope
`of the invention, and all such modifications as would be
`obvious to one skilled in the art are intended to be included
`
`within the scope of the following claims.
`What is claimedis:
`
`a
`
`15
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`6
`2. The semiconductor memory device of claim 1, wherein
`said delay pulse generation circuit comprises:
`a first internal pulse generator that receives the third
`internal clock signal and generatesa first internal pulse
`signal having a first pulse width;
`a second internal pulse generator that receives the fourth
`internal clock signal and generates a second internal
`pulse signal having a second pulse width; and
`a pulse signal generator that receives the first and second
`internal pulse signals and generates the pulse signal.
`3. The semiconductor memory device of claim 2, wherein
`said first internal pulse generator comprises:
`a delay unit that delays the third internal clock signal a
`predetermined period of time; and
`a NANDgate that receives the third internal clock signal
`and an output of said delay unit, and that outputs the
`first internal pulse signal.
`4. The semiconductor memory device of claim 2, wherein
`said second internal pulse generator comprises:
`a delay unit that delays the fourth internal clock signal a
`predetermined period of time; and
`a NANDgate that receives the fourth internal clock signal
`and an output of said delay unit, and that outputs the
`second internal pulse signal.
`5. The semiconductor memory device of claim 2, wherein
`said pulse signal generator comprises:
`a NAND gate that receives the first and second internal
`pulse signals and generatesa third internal pulse signal;
`and
`
`a transmission gate that outputs the third internal pulse
`signal as the pulse signal in response to a fifth internal
`clock signal.
`6. The semiconductor memory device of claim 1, wherein
`said delay pulse generation circuit generates a first pulse
`signal corresponding to generation of the third internal clock
`signal and generates a second pulse signal corresponding to
`gencration of the fourth internal clock signal.
`7. The semiconductor memory device of claim 1, wherein
`said delay pulse generation circuit generates a pulse signal
`corresponding to an interval between generation of the third
`internal clock signal and generation of the fourth internal
`clock signal.
`8. The semiconductor memory device of claim 1, wherein
`said logic circuit providesthe first and second internal clock
`signals to said memory core block to operate said memory
`core block.
`
`9. The semiconductor memory device of claim 1, wherein
`said direct access circuit provides the third and fourth
`internal clock signals to said memory core block to test the
`plurality of memorycells.
`10. A method of operating a semiconductor memory
`device having a memary core block including a plurality of
`memory cells, comprising:
`generating a first
`internal clock signal and a second
`internal clock signal respectively responsive to a first
`external clock signal and a second external clock
`signal;
`operating the memory core block based on the first and
`secondinternal clock signals during a normal operation
`mode;
`generating a third internal clock signal and a fourth
`internal clock signal respectively responsiveto the first
`external clock signal and the second external clock
`signal;
`testing the plurality of memory cells based on the third
`and fourth internal clock signals during a direct access
`operation mode; and
`
`1. Asemiconductor memory device comprising:
`a memory core block including a plurality of memory
`cells;
`a logic circuit that generates a first internal clock signal
`and a second internal clock signal respectively in
`response to a first external clock signal and a second
`external clock signal, and that operates said memory
`core block at high speed during normal operation;
`a direct access circuit that generates a third internal clock
`signal and a fourth internal clock signal respectively in
`response to the first external clock signal and the
`second external clock signal,
`to test the plurality of
`memory cells within said memory core block during a
`direct access operation; and
`delay pulse generation circuit that generates a pulse
`signal having a pulse periodor interval cqual to a delay
`difference between the third internal clock signal and
`the fourth internal clock signal generated by said direct
`access circuit.
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`US 6,483,771 B2
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`a
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`14. The method of operating a semiconductor memory
`generating a pulse signal having a pulse period or interval
`equal to a delay difference between the third and fourth
`device of claim 11, wherein said generating the pulse signal
`internal clock signals.
`comprises:
`11. The method of operating a semiconductor memory
`performing a NANDlogical operation on the first and
`device of claim 10, wherein said generating pulse signal
`second internal pulse signals to provide a third internal
`comprises:
`pulse signal; and
`generating a first internal pulse signal havingafirst pulse
`gating the third internal pulse signal with a fifth internal
`width based on the third internal clock signal;
`clock signal to provide the pulse signal.
`generating a secondinternal pulse signal having a second
`15. The method of operating a semiconductor memory
`pulse width based on the fourth internal clock signal;
`device of claim 10, wherein said generating a pulse signal
`and
`comprises:
`generating a first pulse signal corresponding to generation
`of the third internal clock signal; and
`generating a second pulse signal corresponding to gen-
`eration of the fourth internal clock signal.
`16. The method of operating a semiconductor memory
`device of claim 10, wherein said generating a pulse signal
`comprises generating a pulse signal corresponding to an
`interval between generation of the third internal clock signal
`and generation of the fourth internal clock signal.
`17. The method of operating a semiconductor memory
`device of claim 10, wherein the first and second internal
`clock signals are provided to the memory core block during
`said operating the memory core block.
`18. The method of operating a semiconductor memory
`device of claim 10, wherein the third and fourth internal
`clock signals are provided to the memory core block during
`said testing the plurality of memory cells.
`*
`*
`*
`*
`*
`
`generating the pulse signal based on the first and second
`internal pulse signals.
`12. The method of operating a semiconductor memory
`device of claim 11, wherein said generating a first internal
`pulse signal comprises:
`delaying the third internal clock signal a predetermined
`period of time; and
`performing a NAND logical operation on third internal
`clock signal and the delayed third internal clock signal,
`to provide the first internal pulse signal.
`13. The method of operating a semiconductor memory
`device of claim 11, wherein said generating a secondinternal
`pulse signal comprises:
`delaying the fourth internal clock signal a predetermined
`period of time; and
`performing a NAND logical operation on the fourth
`internal clock signal and the delayed fourth internal
`clock signal, to provide the second internal pulse sig-
`nal.
`
`;
`
`8
`
`