Apparatus for generating data strobe signal applicable to double data rate SDRAM

[0002] The present invention relates to a synchronous memory device; and, more particularly, to an apparatus for generating a data strobe signal applicable to a double data rate SDRAM.

[0003] For achieving a high speed of operation in DRAM (dynamic random access memory), synchronous DRAMs (hereinafter, referred to as SDRAMs) have been developed. The SDRAM operates in synchronization with an external clock signal. The SDRAM includes a single data rate (SDR) SDRAM and a double data rate (DDR) SDRAM and the like.

[0004] The SDR SDRAM operates in synchronization with rising edges of the external clock, so that one data is processed within one period of the external clock. On the contrary, the DDR SDRAM operates in synchronization with rising and falling edges of a data strobe signal, so that two successive data are processed within one period of the external clock. Therefore, compared with the SDR SDRAM, the DDR SDRAM achieves at least twice the operation speed without increasing a frequency of the external clock. At this time, the data strobe signal is a signal instructing reception of a data to a controller.

[0005] FIG. 1 is a timing chart illustrating a data strobe signal generator shown in FIG. 2 at a read operation in case here a burst length is of 2 and a CAS (column address strobe) latency is of 2.5.

[0006] Two data DQ are continuously outputted within one period of an external clock signal CLK, being synchronized with rising and falling edges of the data strobe signal DQS. When a read command is inputted, the data strobe signal DQS maintains a high impedance state. Then, the data strobe signal DQS does not becomes a logic low state until one and half period of the external clock CLK elapses from the input of the read command. That state is called a “preamble” state when the data strobe signal DQS is in a logic low state before the data DQ is outputted.

[0007] If the data DQ is outputted, the data strobe signal DQS is synchronized with a first output data so that the data strobe signal DQS goes from the preamble state to a logic state. With the next data output, the data strobe signal DQS goes from the logic high state to a logic low state. If successive data are outputted when the burst length is of more that two, the data strobe signal DQS is toggled according to a transition from the logic high state to a logic low state, or vice versa.

[0008] When the data output has been completed, the data strobe signal DQS goes again to the high impedance state, informing to an external circuit that there is not data output any longer. At this time, that state is called a “postamble” state when the data strobe signal DQS is in a logic low state before the data strobe signal DQS again becomes the high impedance state.

[0009] At this time, a data strobe enable signal QSEN and a data strobe preamble control signal QSEN_PRE are used to control the data strobe signal DQS.

[0010] FIG. 2 is a schematic diagram illustrating a conventional data strobe signal generator 200 with preamble control portion 202, signal generating portion 204, and signal driving portion 208, with the signal generating portion 204 including a plurality of pairs 206A, 206B, 216A, 216B, 226A, 226B of pull-up/pull-down signal drivers. FIG. 3 is a circuit diagram illustrating a pull-up/pull-down signal driver 206 shown in FIG. 2.

[0011] A structure and operation of the data strobe signal generator 200 and the pull-up/pull-down signal driver is disclosed in a copending U.S Pat. Ser. No. 475,056, filed on Dec. 30, 1999, entitled “DATA STROBE SIGNAL GENERATOR OF SEMICONDUCTOR DEVICE USING TOGGLED PULL-UP AND PULL-DOWN SIGNALS”, which is hereby incorporated by reference.

[0012] As shown in FIG. 3, it should be noted that each of the pull-up/pull-down signal drivers 206 includes inverters INV31 to INV34 for buffering a pull-up signal PU and a pull-down signal PD. That is, in the conventional data strobe signal generator 200, when the pull-up signal PU and the pull-down signal PD are fed back, the pull-up signal PU and the pull-down signal PD are buffered through two inverters INV31 and INV32, INV33 and INV34, respectively. Therefore, junction capacitance looking from a pull-up node N_puand a pull-down node N_pdmay be increased, so that an operation speed is reduced. Additionally, as the number of the pipe counter signals PCNT, e.g., PCNT_EVEN<0:2> and PCNT_ODD<0:2>, increases, the junction capacitance at the pull-up node N_puand the pull-down node N_pdmay be increased much more.

[0013] It is, therefore, an object of the present invention to provide an apparatus for generating a data strobe signal applicable to a double data rate SDRAM, capable of obtaining a stable operation speed regardless of the number of successive output data and reducing a chip size.

[0014] It is an aspect of the present invention to provide a 1. A synchronous memory device having an apparatus for generating a data strobe signal, the apparatus for generating a data strobe signal comprising: a preamble control means for controlling a preamble state of a data strobe signal in response to a data strobe preamble control signal; at least one pair of pull-up/pull-down signal generating means coupled to the preamble control means, for receiving pipe counter signals at a first input terminal to generate pull-up and pull-down signals; a common pull-up signal buffering means for buffering the pull-up signal to generate a buffered pull-up signal, wherein the buffered pull-up signal is commonly inputted to a second input terminal of the pull-up/pull-down signal generating means; a common pull-down signal buffering means for buffering the pull-up signal to generate a buffered pull-down signal, wherein the buffered pull-down signal is commonly inputted to a third input terminal of the pull-up/pull-down signal generating means; and a data strobe signal driving means for outputting the data strobe signal in response to the pull-up signal, the pull-down signals and a data strobe enable signal.

[0015] Other objects and aspects of the invention will become apparent from the following description of the embodiments with reference to the accompanying drawings, in which:

[0016] FIG. 1 is a timing chart of a conventional data strobe signal generator at a read operation, when a burst length is of 2 and a CAS latency is of 2.5;

[0017] FIG. 2 is a schematic diagram illustrating a conventional data strobe signal generator;

[0018] FIG. 3 is a circuit diagram illustrating a pull-up/pull-down signal driver shown in FIG. 2;

[0019] FIG. 4 is a schematic diagram illustrating a data strobe signal generator in accordance with the present invention; and

[0020] FIG. 5 is a circuit diagram illustrating a pull-up/pull-down signal driver shown in FIG. 4.

[0021] FIG. 4 is a schematic diagram illustrating a data strobe signal generator in accordance with the present invention.

[0022] Referring to FIG. 4, the data strobe signal generator 400 includes a preamble control portion 402, a pull-up/pull-down signal generating portion 404, and a data strobe signal driving portion 408.

[0023] The preamble control portion 402 controls a preamble state of a data strobe signal DQS in response to a data strobe preamble control signal QSEN_PRE.

[0024] The pull-up/pull-down signal generating portion 404 includes a plurality of pairs 407, 417 and 427 of pull-up/pull-down signal drivers for generating the pull-up signal PU and the pull-down signal PD in response to pipe count signal PCNT, a pull-down signal buffer unit 405 for buffering the pull-down signal PD, and a pull-up signal buffer unit 406 for buffering the pull-up signal PU. It should be noted that a buffered pull-up signal PD_I and a buffered pull-down signal PD_I are commonly inputted to each input terminal of the pull-up/pull-down signal drivers.

[0025] The data strobe signal driving portion 408 generates the data strobe signal DQS in response to the pull-up signal PU, the pull-down signal PD and a data strobe enable signal QSEN.

[0026] The preamble control portion 402 includes a plurality of inverters INV401 to INV405 for delaying the data strobe preamble control signal QSEN_PRE for a predetermined time, a NAND gate ND401 for NANDing the data strobe preamble control signal QSEN_PRE and a delayed data strobe preamble control signal, and a PMOS transistor P401 whose source is coupled to a power terminal and whose gate receives an output signal from the NAND gate ND401.

[0027] In the pull-up/pull-down signal generating portion 404, each pair, e.g., 407, includes a first pull-up/pull-down signal driver 407A for receiving a first pipe counter signal PCNT_EVEN<0> to generate the pull-up signal PU and the pull-down signal PD, and a second pull-up/pull-down signal driver 407B for receiving a second pipe counter signal PCNT_ODD<0> to generate the pull-up signal PU and the pull-down signal PD. At this time, a pull-up output terminal of the first pull-up/pull-down signal driver 407A is coupled to a pull-down output terminal of the pull-up/pull-down signal driver 407B. Other pairs 417 and 427 of the pull-up/pull-down signal drivers have the same structure as the pair 417 of the pull-up/pull-down signal drivers.

[0028] The pull-down signal buffer unit 405 buffers the pull-down signal PD to generate a buffered pull-down signal PD_I and the pull-up signal buffer unit 406 buffers the pull-up signal PU to generate a buffered pull-up signal PU_I. It should be noted that the buffered pull-down signal PD_I and the buffered pull-up signal PU_I are commonly used as input signals of each pair of the pull-up/pull-down signal drivers.

[0029] Hereinafter, the pull-up/pull-down signal driver will be described in detail with reference to FIG. 5.

[0030] A first pass gate 503A receives the buffered pull-up signal PU_I and outputs the buffered pull-up signal PU_I in response to the pipe counter signal PCNT and the inverted pipe counter signal PCNTZ. A second pass gate 503B receives the buffered pull-down signal PD_I and outputs the buffered pull-down signal PD_I in response to the pipe counter signal PCNT and the inverted pipe counter signal PCNTZ.

[0031] A first latch circuit 505A latches the buffered pull-up signal PU_I in response to the pipe counter signal PCNT and the inverted pipe counter signal PCNTZ to generate a buffered and latched pull-up signal. A second latch circuit 505B latches the buffered pull-down signal PD_I in response to the pipe counter signal PCNT and the inverted pipe counter signal PCNTZ to output a buffered and latched pull-down signal.

[0032] A first pull-up unit 507A drives to pull up the pull-up signal PU in response to the buffered and latched pull-down signal and the inverted pipe counter signal PCNTZ. A first pull-down unit 507B drives to pull down the pull-up signal PU in response to the buffered and latched pull-down signal and the pipe counter signal PCNT.

[0033] A second pull-up unit 509A drives to pull up the pull-down signal PD in response to the buffered and latched pull-up signal and the inverted pipe counter signal PCNTZ. A second pull-down unit 509B drives to pull down the pull-down signal PD in response to the buffered and latched pull-up signal and the pipe counter signal PCNT.

[0034] The first latch circuit 505A is preferably implemented with an inverter INV501 and a tri-state inverter INV502. The inverter INV501 has an input terminal receiving the buffered pull-up signal PU_I. The tri-state inverter INV502 has an input terminal coupled to an output terminal of the inverter INV501, and an output terminal coupled to the input terminal of the inverter INV501. At this time, the tri-state inverter INV502 is driven in response to the pipe counter signal PCNT and the inverted pipe counter signal PCNTZ.

[0035] The latch circuit 505B has the same structure as the latch circuit 505A.

[0036] The first pull-up unit 507A includes a PMOS transistor P501 whose source is coupled to the power terminal and whose gate receives the output signal from the latch circuit 505B, and a PMOS transistor P502 whose source is coupled to a drain of the PMOS transistor P501, whose drain is coupled to a pull-up node N_pu, and whose gate receives the inverted pipe counter signal PCNTZ. An output signal at the pull-up node N_pucorresponds to the pull-up signal PU.

[0037] The second pull-up unit 509A includes a PMOS transistor P503 whose source is coupled to the power terminal and whose gate receives the output signal from the latch circuit 505A, and a PMOS transistor P504 whose source is coupled to a drain of the PMOS transistor P503, whose drain is coupled to a pull-down node N_pd, and whose gate receives the inverted pipe counter signal PCNTZ. An output signal at the pull-down node N_pdcorresponds to the pull down signal PU.

[0038] The first pull-down unit 507B includes an NMOS transistor N501 whose drain is coupled to the drain of the PMOS transistor P502 and whose gate receives the pipe counter signal PCNT, and an NMOS transistor N502 having a drain coupled to a source of the NMOS transistor N501, a source coupled to a ground terminal and a gate receives the output signal from the latch circuit 505B.

[0039] The second pull-down unit 509B includes an NMOS transistor N503 whose drain is coupled to the drain of the PMOS transistor P504 and whose gate receives the pipe counter signal PCNT, and an NMOS transistor N504 having a drain coupled to a source of the NMOS transistor N503, a source coupled to the ground terminal and a gate receiving the output signal from the latch circuit 505A.

[0040] Hereinafter, an operation of the data strobe signal generator will be described in detail with reference to FIGS. 4 and 5.

[0041] First, the preamble control portion 402 senses a rising edge of the data strobe preamble control signal QSEN_PRE to generate a preamble control pulse having a predetermined pulse width. The preamble control pulse is a low enable signal which is activated when a high-to-low logic transition is achieved. The preamble control pulse applied to a gate of the PMOS transistor P401 causes the pull-down signal PD to be in a logic high state.

[0042] At this time, since the pipe counter signal PCNT applied to the pull-up/pull-down signal generating portion 404 is in a low state, the pull-up/pull-down signal generating portion 404 is disabled. Therefore, the buffered pull-up signal PU_I and the buffered pull-down signal PD_I is not outputted as an output signal and the pull-up/pull-down signal generating portion 404 maintains a previous pull-up and pull-down signal.

[0043] Next, when an initial pipe counter signal PCNT_EVEN <0> is inputted as a high state, the activated pull-up/pull-down signal driver 407A outputs a signal having an inverted phase of a previous state. The output signal is buffered through the pull-down signal buffer unit 405 and the pull-up signal buffer unit 406, and the buffered signals PU_I and PD_I are inputted to the input terminals of all the pull-up/pull-down signal drivers. Consequently, all other pipe counters including the next pipe counter 407B have an inverted phase of PU_I and PD_I. And then, a next pipe counter enable signal PCNT_ODD<0> is activated to a high state. Accordingly, the activated pull-up/pull-down signal driver 407B outputs a signal having an inverted phase of a previous state. Furthermore, the pull-up and pull-down signal is again buffered and fed back to the input terminal of the pull-up/pull-down signal driver 417A.

[0044] In the same manner, according as the pull-up and pull-down signal PU and PD are sequentially toggled in response to the pipe counter signal PCNT, the data strobe signal DQS is also toggled. When the operation is completed, the data strobe enable signal QSEN is disabled to a low state. As a result, the pull-up node N_puand the pull-down node N_pdbecome a low state, so that the data strobe signal DQS becomes a high impedance state.

[0045] As described above, by buffering the pull-up signal and the pull-down signal with the common buffer unit, junction capacitance at the pull-up node and the pull-down node is decreased, thereby reducing a chip size and improving an operation speed. Although the number of successive output data is increased, the data strobe signal generator according to the present invention can obtain a constant operation speed.

[0046] Although the preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.