Semiconductor device comprising mode register control circuit

Abstract: A mode register control circuit (10) for a semiconductor device (40) includes a first control unit (202) for preventing the content of a mode register (20) from being read, using an initializing signal for instructing latching circuits (103, 104) to be initialized, the initialization being done in a transient occurring after the semiconductor device (40) is turned on; a second control unit (104) for instructing the mode register (20) to execute a mode register read command even if a mode register set command has not been executed, on the condition that an external command other than the mode register read command is detected when the semiconductor device (40) is turned on; or a third control unit (206) for instructing the mode register (20) to execute the mode register read command on the condition that the mode register set command is executed after the semiconductor device (40) is turned on.

Description: The present invention relates to a semiconductor device having mode register control circuitry for controlling a mode register of the device, and more particularly to mode register control circuitry for use in a semiconductor memory device such as a synchronous dynamic RAM (SDRAM), a type of high-band dynamic RAM (high-band DRAM) capable of achieving a data transfer speed of, for example, 100 Mbyte/sec. Fig. 1A is a circuit diagram showing a conventional mode register control circuit 9 and a mode register 2 controlled by the mode register control circuit 9. A conventional mode register control circuit 9 may be built into an integrated SDRAM chip (semiconductor device) capable of high-speed outputting of data in response to an externally supplied high-speed clock. Such a semiconductor may be considered to comprise a mode register for storing operating mode data representing an operating mode of the semiconductor device; and mode register control circuitry operable, in response to an externally-applied mode register read command, to cause the stored operating mode data to be read out from the mode register and output by output circuitry of the device. The mode register control circuit 9 (indicated as MRGCTL in Fig. 1A) is provided in an SDRAM 1 so as to control operation of reading from the mode register 2 within tnhe SDRAM 1, in synchronism with an external command (mrspz) and external address data (a0 - a6, a8 and a9) fed via a terminal of the SDRAM 1. The mode register 2 latches an operating mode of the SDRAM 1 in response to a mode register set command and a mode register read command which are external commands. The mode register read command causes an operating mode of the SDRAM 1 chip to be output via an output terminal DQ provided on the SDRAM 1 chip. More specifically, the mode register read command causes a mode selection signal (indicated as MRDZ in Fig. 1A) to be output via an output transistor 3 (indicated as outTr in Fig. 1A) connected to the output terminal DQ. A description will now be given, with reference to Fig. 1A, of an operation effected by external commands in the mode register control circuit 9. The mode register control circuit 9 is constructed to be capable of executing the mode register read command whether the mode register control circuit 9 is in an idle state or an active state. Since a normal reading operation is conducted after the chip becomes active, data is not output in an idle state. The mode register set command initiates an operating mode which controls the SDRAM 1 to be set to a desired operating mode by setting a CAS latency (CL) operating mode, a burst length (BL) operating mode and a burst type in the mode register 2. Setting of an operating mode (an operating mode initiated by the mode register set command, or an operating mode initiated by the mode register set command) in the mode register control signal 9 is effected by raising an operating mode signal (more specifically, CL1 - CL3 signals) selected when the mode register set command is executed. More specifically, as shown in Fig. 1A, address data a0 - a6, a8 and a9 input to the mode register 2 via an address input terminal ADD on the chip and specifying mode addresses are used to set the operating mode signal. In the conventional mode register control circuit 9 as shown in Fig. 1A, the output transistor 3 (indicated as outTr in Fig. 1A) of the SDRAM 1 may be put in a low-impedance state when the mode register read command is input to the SDRAM 1 when the SDRAM 1 is turned on, or when it is determined that the mode register read command is latched in internal latching means in the SDRAM 1 (more specifically, an external command latching part or an external address latching part). The low-impedance state presents problems described later. In the conventional control circuit, the mode register read command allows data to be output when the SDRAM 1 is idle. Hence, if it is determined that the data is output via the output terminal DQ according to the mode register read command, mode data inside the mode register can be read from the output transistor of the SDRAM 1. A description will now be given, with reference to Fig. 1B, of how the mode register set command and the mode register read command are executed in the mode register control circuit 9. As shown in the timing chart of Fig. 1B, predetermined data (a0 - a6, a8 and a9 shown in (b-2) of Fig. 1B) for selecting between the mode register set command and the mode register read command is input to the mode register 2 in synchronism with an internal clock of the SDRAM 1. The internal clock is a signal indicated as clkiz in (b-1) of Fig. 1B externally supplied via an external clock terminal CLK and an internal clock generating unit. A difference between the data setting for the mode register set command and that for the mode register read command is found only in a mode setting signal, that is, an a08 pin signal a8 supplied via the address input terminal ADD on the chip as address data specifying the mode address. When the a8 signal is L, the mode register set command is specified; when H, the mode register read command is specified. Subsequently, the mode register control circuit 9 generates a register read signal (rgrz) (see (b-4) of Fig. 1B) which is a composite signal composed of the mode setting signal a8 and a mrspz signal (see (b-3) of Fig. 1B) which is generated in the external command latching part in the SDRAM 1 when the mode register set command or the mode register read command is latched, in synchronism with the external clock signal clkiz. Subsequently, the mode register control circuit 9 generates a driving signal mrrz (see (b-5) of Fig. 1B) which is the register read signal latched. The driving signal mrrz is latched until the next external clock clkiz is generated. In response to the driving signal mrrz, the mode selection signal MRDZ stored in the mode register 2 is output (see (b-6) of Fig. 1B). However, in the conventional mode register control circuit 9, if it is determined that data is output, when the SDRAM 1 is turned on, via the external output terminal DQ of the SDRAM 1 according to the mode register read command, or if it is determined that data is output in an idle state before the SDRAM 1 is turned on via the external output terminal DQ of the SDRAM 1 according to the mode register read command, the mode data inside the mode register is read from the output transistor of the SDRAM 1, causing the output transistor of the SDRAM 1 to be put in a low-impedance state. Therefore, the conventional mode register control circuit has a problem in that an abnormal current may flow when the SDRAM 1 is turned on or in an idle state occurring after the SDRAM 1 is turned on. A semiconductor device embodying the present invention is characterised in that the said mode register control circuitry comprises a control unit for preventing such reading out from the mode register in a power-up period in which the device begins to be supplied with power. In one embodiment of the present invention the mode register control circuitry is capable of preventing an abnormal current from flowing in the SDRAM when the SDRAM is turned on or in an idle state occurring after the SDRAM is turned on, by providing such a control unit in the SDRAM (more specifically, in the mode register control circuit MRGCTL) for preventing the output transistor of the SDRAM from being in a low-impedance state even if it is determined that data is output, when the SDRAM is turned on, via an external output terminal of an SDRAM according to the mode register read command, or it is determined that data is output in an idle state before the SDRAM is turned on via the external output terminal DQ of the SDRAM according to the mode register read command. In one embodiment the control unit uses internal means to disable, when the semiconductor device is turned on, execution of the mode register read command, using an initialising signal for specifying initialisation of a latch part for latching an external command and/or an external address externally fed to the semiconductor device, the initialisation being done during a transient state occurring after the semiconductor device is turned on. In this way, an abnormal current is prevented from flowing in an output transistor of the semiconductor device by preventing the mode register read command from being executed when the semiconductor device is turned on. In another embodiment, the mode register read command is enabled on the condition that the mode register set command is executed after the semiconductor device is turned on, so that an abnormal current is prevented from flowing in the output transistor of the semiconductor device. In an idle state occurring after the semiconductor device is turned on, an abnormal current is prevented from flowing in the output transistor of the semiconductor device. In another embodiment the mode register control circuitry may include an override circuit which, upon determining that an external command detected when the semiconductor device is turned on is other than a mode register read command, overrides the control unit so as to permit the mode register to execute a subsequent mode register read command even if a mode register set command has not ben executed after a power supply voltage becomes stable. By providing the override circuit and the control unit, an abnormal current is prevented from flowing in the output transistor of the semiconductor device, by disabling execution of the mode register read command when the semiconductor device is turned on. In the idle state occurring after the semiconductor device is turned on, an abnormal current is prevented from flowing in the output transistor of the semiconductor device, by ensuring that execution of the mode register read command is enabled on the condition that the mode register set command is executed. In one embodiment the semiconductor device further comprises: a starter signal generating circuit for generating an initializing signal for initializing a latching circuit when the semiconductor device is turned on; an internal clock generating unit for generating an internal clock signal in correspondence with an external clock signal; a memory cell array; a read/write circuit for reading data from and writing data to the memory cell array; and an input/output circuit for inputting and outputting data addresses and commands. Reference will now be made, by way of example, to the accompanying drawings, in which: Fig. 1A is a circuit diagram showing a conventional mode register circuit and a mode register controlled by the same; Fig. 1B is a timing chart which explains an operation of the circuit of Fig. 1A; Fig. 2 is a circuit diagram showing parts of a semiconductor device embodying the present invention; Fig. 3A is a circuit diagram showing a first embodiment of the present invention; Fig. 3B is a timing chart which explains an operation of the first embodiment; Fig. 4A is a circuit diagram showing a second embodiment of the present invention; and Figs. 4B and 4C are timing charts which explain an operation of the second embodiment. A description will now be given, with reference to the associated drawings, of a first and second embodiment of the present invention. A mode register control circuit 10 according to the first and second embodiments of the present invention controls a semiconductor device 40 (SDRAM) capable of a data transfer speed higher than 100 Mbyte/sec. A description will be given of a basic construction of an SDRAM 40. Fig. 2 is a block diagram showing the mode register control circuit 10. An external clock signal clkiz is a signal input via an external clock terminal (CLK) 116 on the chip. An initializing signal sttz is a signal generated by a starter signal generating unit 101 on the chip so as to instruct latching circuits (specifically, an external command latching circuit 103 and an external address latching circuit 104) in the chip to be initialized when the SDRAM is turned on. A mrspz signal is generated by the external command latching circuit 103 in synchronization with the external clock signal clkiz, when the mode register set command or the mode register read command is fed via a command input terminal (CTL) 117 and latched in the external command latching circuit 103. A mode setting signal a8 is generated by the external address latching circuit 104 in correspondence with address data specifying mode address and input via the address input terminal (ADD) 118 on the chip. The external clock terminal (CLK) 116, the command input terminal (CTL) 117, the address input terminal (ADD) 118 and an output terminal (DQ) 119 are external terminals provided on the SDRAM 40 chip. The starter signal generating circuit 101 generates an initializing signal sttz for instructing latching circuits (more specifically, the external command latching circuit 103 and the external address latching circuit 104) to be initialized. The internal clock generating circuit 102 generates a clock signal for internal use in accordance with the external clock signal clkiz supplied via the external clock terminal (CLK) 116. The external address latching circuit 104 latches an external address signal (signal fed to the address input terminal (ADD) 118) in synchronism with the external clock signal clkiz supplied by the internal clock generating circuit 102. The external address signal is embodied by the address data a0 - a6, a9 and the mode setting signal a8. An external address decoding circuit 109 generates a mode type out of the external address signal. The external command latching circuit 103 latches an external command signal fed via the command input terminal (CTL) 117, that is, the mode register set command or the mode register read command, in synchronism with the external clock signal clkiz supplied from the internal clock generating circuit 102. An external command decoding circuit 110 generates internal commands such as a mode register set command out of an external command signal. The mode register control circuit 10 (indicated as MRGCTL in Fig. 2) executes the mode register set command or the mode register read command in accordance with the external address signal (a0 - a6, a9), the mode setting signal (a8) and the mrspz signal. The mode register control circuit 10 generates a register set signal rgwz for instructing a mode register 20 to be set and also generates the register read signal rgrz for controlling the reading from the mode register. A register read control circuit 111 generates, in accordance with a register read signal rgrz, a driving signal mrrz for instructing the output transistor control circuit 120 to drive an output transistor (outTr) 30. The mode register 20 generates the mode selection signal (MRDZ) stored in the mode register 20, in accordance with the register read signal rgrz, the external address signal (a0 - a6, a9). The output transistor control circuit 120 controls the output transistor (outTr) 30 in accordance with the mode selection signal MRDZ. A memory cell array 113 is constructed such that memory cells each storing data in units of a bit are formed into layers in a predetermined manner. The memory cell array 113 stores write data from a write amplifier circuit 107. A sense amplifier 112 reads data stored in a memory cell of the memory cell array 113, temporarily stores the same, and transfers the same to a sense buffer 114. The sense amplifier 112 also temporarily stores the write data supplied by the write amplifier circuit 107 and stored in the sense buffer 114. The sense buffer 114 temporarily stores data read by the sense amplifier 112 or the write data transferred by the write amplifier circuit 107. A pipeline structure constructed of a pipeline 115 and a pipeline 108 one on top of the other executes a pipeline process for reading data stored in the sense buffer 114 in a parallel manner, in synchronism with an external clock signal clkiz. A write data latching circuit 105 temporarily stores data input via an external input terminal (DQ) 119B. The write amplifier circuit 107 writes the buffer latched in the write data latching circuit 105 in the sense buffer 114 in synchronism with the external clock signal clkiz. A write control circuit 106 controls the latching action of the write data latching circuit 105 and the write operation of the write amplifier circuit 107, in synchronism with the external clock signal clkiz. A description will now be given of modes of operation of the SDRAM 40. The SDRAM 40 is capable of outputting data at a data transfer speed of higher than 100 Mbyte/sec, in synchronism with a high-speed clock supplied via the external clock terminal 116. In the SDRAM 40, two operating modes are available: the mode register set mode and the mode register read mode. The mode register set command initiating the mode register set mode specifies the operating mode in which the SDRAM 40 is used. The mode register set command may set the CAS latency operating mode (CL), the burst length operating mode (BL) or the burst type in the mode register 2 in order to specify a specific operating mode. Setting of the operating mode is effected by raising the operating mode signal (specifically, CL1 - CL3 signals) selected when the mode register set command is executed. Specifically, as shown in Figs. 3A and 4A, predetermined data a0 - a6 and a9 is input to the mode register 20 so that the operating mode is set. The mode register read command causes an operating mode of the SDRAM 40 chip to be output via a terminal DQ (an output terminal 119A or the output terminal 119B) provided on the SDRAM 40 chip. More specifically, the mode register read mode is such that the mode selection signal (MRDZ shown in Figs. 3A and 4A) is output via the output transistor (outTr) 30 shown in Figs. 3A and 4A and connected to the terminal DQ (the output terminal 119A or the input terminal 119B). A description will now be given of the SDRAM 40. When the SDRAM 40 receives the external clock signal clkiz generated by the internal clock generating circuit 102, or a signal supplied via the external clock terminal (CLK) 116 on the chip and the internal clock generating circuit 102, predetermined data (a0 - a6, a8 and a9) for selection between the mode register set command and the mode register read command is input to the mode register 20 in synchronism with the reception. A difference between the data setting for the mode register set command and that for the mode register read command is found only in a mode setting signal, that is, an a08 pin signal a8 supplied via the address input terminal (ADD) 118 on the chip as address data specifying the mode address. When the a8 signal is L, the mode register set command is specified; when H, the mode register read command is specified. Subsequently, the mode register control circuit 10 generates a register read signal rgrz which is a composite signal composed of the mode setting signal a8 and a mrspz signal which is generated in the external command latching circuit 103 in the SDRAM 40 when the mode register set command or the mode register read command is latched, in synchronism with the external clock signal clkiz. Subsequently, the mode register control circuit 10 generates a driving signal mrrz which is the register read signal rgrz latched. The driving signal mrrz is latched until the next external clock clkiz is generated. In response to the driving signal mrrz, the mode selection signal MRDZ stored in the mode register 20 is output via the output transistor (outTr) 30. A description will now be given of individual embodiments of the present invention. Fig. 3A is a circuit diagram showing a first embodiment of the present invention; and Fig. 3B is a timing chart which explains an operation according to the first embodiment. The mode register control circuit (indicated as MRGCTL in Fig. 3A) 10 according to the first embodiment is provided in the SDRAM 40 so as to control operation of reading from the mode register within the SDRAM 40 in synchronism with the external command signal and the external clock signal. The mode register control circuit comprises a first control unit 202. The first control unit 202 disables, when the SDRAM 40 is turned on, execution of the mode register read command so as to prevent the content of the mode register from being read. When the SDRAM 40 is turned on, the first control unit 202 uses an initializing signal sttz for instructing a transient state which occurs at power-on in the external command latching circuit 103 and the external address latching circuit 104 to be initialized, so that execution of the mode register read command is internally disabled. A starter signal sttz is used to set logical elements like flip-flop circuits constituting the external command latching circuit 103 and the external address latching circuit 104 at a predetermined potential level (more specifically, at logical H or logical L) so as to put the elements out of an unstable state occurring after the power is on. A prohibiting signal setz generated by the first control unit 202 is set to logical H (active) so as to prevent the mode register read command from being executed at the power-on. That is, no command for reading from the mode register 20 is output. In this way, the mode register read command is prevented by internal means from being executed when the SDRAM 40 is turned on, and the output transistor (output circuitry) 30 is prevented from being in a low-impedance state. The prohibiting signal setz generated by the first control unit 202 is set to logical (L) inactive when the mode register set command is executed after the SDRAM 40 is turned on, so as to enable reading from the mode register. More specifically, a signal similar to (derived from the same source as) the register set signal rgwz output to set the mode register is used to reset the flip-flop circuits of the first control unit 202 which were initially set by the starter signal sttz. In this way, the prohibiting signal setz is maintained at logical L. Thus, the first control unit 202 enables execution of the mode register read command after execution of the mode register set command. A description will now be given, with reference to Fig. 3B, of a specific operation according to the first embodiment. In phase (1) shown in Fig. 3B, execution of the mode register read command is attempted. However, since the prohibiting signal setz generated by the first control unit 202 is at logical H, the register read signal rgrz generated by the mode register control circuit 10 is at logical L so that the output transistor 30 is maintained at a high-impedance state. In phase (2) shown in Fig. 3B, the mode setting signal a8 is set to logical L in order for the mode register set command to be executed. Subsequently, the mode register control circuit 10 outputs the register set signal rgwz so as to set the mode register 20. At the same time, the prohibition signal setz generated by the first control unit 202 is latched at logical L so that subsequent generation of the driving signal mrrz is enabled. Thus, subsequent execution of the mode register read command is enabled. In phase (3) shown in Fig. 3B, a bus that carries the register read signal rgrz generated by the mode register control circuit 10 is activated since the prohibition signal setz generated by the first control unit 202 is set at logical L as a result of the operation in phase (2). The mode setting signal a8 is set so that the mode register read command is executed. The mrspz signal in synchronism with the internal clock CLK is generated. The register read signal rgrz generated by the mode register control circuit 10 is output. Thus, the mode selection signal MRDZ which carries a content of the mode register 20 is output from the output transistor 30. In phase (4) shown in Fig. 3B, the output transistor 30 is reset by the internal clock CLK to a high-impedance state so that the whole operation is completed. As has been described, according to the mode register control circuit 10 provided with the first control unit 202, execution of the mode register read command is prevented when the SDRAM 40 is turned on. In this way, an abnormal current is prevented from flowing in the output transistor 30 of the SDRAM 40. A description will now be given of the SDRAM 40 according to the first embodiment. The SDRAM 40 comprises the mode register control circuit 10, the starter signal generating circuit 101 for generating an initializing signal sttz for instructing latching circuits to be initialized at power-on, the internal clock generating unit 102 for generating an internal clock clkiz in accordance with the external clock, the memory call array 113, a read/write circuit 130 responsible for reading data from the memory cell 113 and writing data into the same, an input/output circuit 140 responsible for inputting and outputting data addresses and commands, the mode register 20 storing the operating mode of the input/output circuit, and the register read control circuit 111 for controlling reading from the mode register in synchronism with the external clock. The read/write circuit 130 comprises the write data latching circuit 105, the write control circuit 106, the write amplifier circuit 107, the pipeline 108, the sense amplifier 112, the sense buffer 114, and the pipeline 115 which are described above. The input/output circuit 140 comprises the external command latching circuit 103, the external address latching circuit 104, the external address decoding circuit 109, the external command decoding circuit 110, and the output transistor control circuit 120. According to the SDRAM 40 provided with the mode register control circuit 10, it is possible to prevent an abnormal current from flowing in the output transistor (outTr) 30 by prohibiting execution of the mode register read command when the SDRAM 40 is turned on. In the idle state occurring after the SDRAM 40 is turned on, it is ensured that execution of the mode register read command is enabled on the condition that the mode register set command is executed so that an abnormal current is prevented from flowing in the output transistor (outTr) 30. A description will now be given of a second embodiment. Fig. 4A is a circuit diagram showing the second embodiment, and Figs. 4B and 4C are timing charts which explain the operation according to the second embodiment. The mode register control circuit 10 (indicated as MRGCTL in Fig. 4A) according to the second embodiment comprises a control unit (override circuit) 204 and a third control circuit (control unit) 206, as shown in Fig. 4A. Upon detecting that the external command detected when the SDRAM 40 is turned on is other than the mode register read command, the second control unit 204 enables the mode register to execute the mode register read command even if the mode register set command has not been executed after the power supply voltage is stabilized. The third control circuit 206 enables the mode register to execute the mode register read command when it is determined that the mode register set command is executed after the SDRAM 40 is turned on. Since the mode register control circuit 10 according to the second embodiment is provided with the second control unit 204 and the third control circuit 206, its added advantage over the mode register control circuit 10 according to the first embodiment is that under some circumstances operating mode type of the external command, latched in the mode register 20, can be read after power-up even if the mode register set command has not yet been executed. In particular in the second embodiment the mode register read command can be executed on the condition that an external command other than the mode register read command is executed after power-on. In this way, a normal internal operation can be effected. A description will now be given, with reference to Fig. 4B, of a specific operation according to the second embodiment executed when it is determined that the mode register read command is specified, that is, it is determined that a8 = H. In phase (1) shown in Fig. 4B, the mrspz signal and the mrsqz signal in synchronism with the internal clock CLK, and the mode setting signal a8 are set to logical H, so that the second control unit (override circuit) 204 sets a set signal (responsive-setting signal) setR to logical L (inactive). The set signal setR set to logical L by the second control unit 204 is maintained at logical L after power-on. Still in phase (1) shown in Fig. 4B, the third control circuit 206 latches the prohibiting signal setz to logical H (inactive), in response to the starter signal sttz and the set signal setR set to logical L by the second control unit 204. As a result, the output transistor 30 is prevented from being put in a low-impedance state at power-on and an abnormal current is prevented from flowing. In phase (2) shown in Fig. 4B, when the mode register set command is executed, the mode register control circuit 10 outputs the register set signal rgwz. At the same time as the mode register 20 is set, the prohibiting signal setz generated by the third control circuit 206 is latched at logical L so that subsequent generation of the driving signal mrrz is enabled. Thus, subsequent execution of the mode register read command is enabled. In phase (3) shown in Fig. 4B, the prohibiting signal setz is set by the third control circuit 206 at logical L as a result of the operation in phase (2). Therefore, the bus carrying the register read signal rgrz generated by the mode register control circuit 10 is activated. Further, the mode setting signal a8 is set to logical H to enable execution of the mode register read command, the mrspz signal in synchronism with the internal clock CLK is generated, and the register read signal rgrz is generated and output by the mode register control circuit 10. As a result, the mode selection signal MRDZ stored in the mode register 20 is output via the output transistor 30. A description will now be given, with reference to Fig. 4C, of a specific operation according to the second embodiment executed when at the time of power-on it is determined that the mode register read command is not specified, that is, a8 = L. In phase (1) shown in Fig. 4C, the mrspz signal or the mode setting signal a8 is set to logical L, so that the second control unit (override circuit) 204 sets the set signal (responsive-setting signal) setR to logical H (active). The set signal setR set to logical H by the second control unit 204 is maintained at logical H after power-on for the duration of phase (1). Still in phase (1) shown in Fig. 4C, the third control circuit 206 latches the prohibiting signal setz to logical L. As a result, the gate (read signal generating circuit) which receives the prohibiting signal setz is made responsive to the mode setting signal a8 and the mrspz signal, from which it is determined that the mode register read command is not specified (a8=L, mrspz=L). Thus, the output transistor 30 is prevented from being put in a low-impedance state. In phase (2) shown in Fig. 4C, the prohibiting signal setz generated by the third control circuit 206 remains latched at logical L so that in the second embodiment execution of the mode register read command is enabled and from this time onward the content of the register can be read after power-on even though no mode register setting command has yet been executed. As has been described, according to the mode register control circuit 10 provided with the second control unit 204 and the third control circuit 206, execution of the mode register read command is disabled when the SDRAM 40 is turned on. By maintaining the output transistor 340 in a high-impedance state, an abnormal current is prevented from flowing into the output transistor 30 of the SDRAM 40. In an idle state occurring after power-on, execution of the mode register read command is enabled by the third control circuit 206 subsequent to at least one execution fo the mode register set command. Thus, an abnormal current is prevented from flowing in the output transistor 30 of the SDRAM 40. A description will now be given of the SDRAM 40 according to the second embodiment. As shown in Fig. 2, the SDRAM 40 comprises the mode register control circuit 10, the starter signal generating circuit 101 for generating an initializing signal sttz for instructing latching circuits to be initialized at power-on, the internal clock generating unit 102 for generating an internal clock clkiz in accordance with the external clock, the memory cell array 113, the read/write circuit 130 responsible for reading data from the memory cell 113 and writing data into the same, the input/output circuit 140 responsible for inputting and outputting data addresses and commands, the mode register 20 storing the operating mode of the input/output circuit, and the register read control circuit 111 for controlling reading from the mode register in. synchronism with the external clock. The read/write circuit 130 comprises the write data latching circuit 105, the write control circuit 106, the write amplifier circuit 107, the pipeline 108, the sense amplifier 112, the sense buffer 114, and the pipeline 115 which are described above. The input/output circuit 140 comprises the external command latching circuit 103, the external address latching circuit 104, the external address decoding circuit 109, the external command decoding circuit 110, and the output transistor control circuit 120. According to the SDRAM 40 provided with the mode register control circuit 10, it is possible to prevent an abnormal current from flowing in the output transistor (outTr) 30 by prohibiting execution of the mode register read command when the SDRAM 40 is turned on. In the idle state occurring after the SDRAM 40 is turned on, it is ensured that execution of the mode register read command is enabled on the condition that the mode register set command is executed so that an abnormal current is prevented from flowing in the output transistor (outTr) 30. As described above, in a semiconductor device embodying the present invention, an abnormal current is prevented from flowing in the output transistor of the semiconductor device, by disabling execution of the mode register read command when the semiconductor device is turned on. In the idle state occurring after the semiconductor device is turned on, an abnormal current is prevented from flowing in the output transistor of the semiconductor device, by ensuring that execution of the mode register read command is enabled on the condition that the mode register set command is executed. The present invention is not limited to the above described embodiments, and variations and modifications may be made without departing from the scope of the present invention.

Claims: A semiconductor device comprising: a mode register (20) for storing operating mode data (MRDZ) representing an operating mode of the semiconductor device (40); and mode register control circuitry (10) operable, in response to an externally-applied mode register read command, to cause the stored operating mode data to be read out from the mode register and output by output circuitry (30) of the device; characterised in that the said mode register control circuitry (10) comprises a control unit (202; 206) for preventing such reading out from the mode register in a power-up period in which the device begins to be supplied with power. A device as claimed in claim 1, further comprising: latch circuitry (103, 104) for latching a command and/or address applied externally to the device, said latch circuitry being initialised in said power-up period by an initialising signal (sttz) generated by a starter signal generating circuit (101) of the device; wherein the said control unit (202; 206) is also operable to employ the said initialising signal to make the mode register control circuitry non-responsive to such an externally-applied mode register read command in said power-up period. A device as claimed in claim 1 or 2, wherein: said mode register control circuitry (10) is switchable from a non-responsive state, in which that circuitry is non-responsive to such an externally-applied mode register read command, to a responsive state in which that circuitry is responsive to such an externally-applied mode register read command; and said control unit (202; 206) is operable to set said mode register control circuitry (10) into said non-responsive state when the device begins to be supplied with power, and to switch said mode register control circuitry into said responsive state when an externally-applied mode register set command is executed after the device is supplied with power, said mode register set command serving to register such operating mode data in the said mode register (20). A device as claimed in claim 3, wherein: said mode register control circuitry (10) further comprises a read signal generating circuit having a control input connected to said control unit (202; 206) for receiving therefrom a prohibiting signal (setz) and operable, when said prohibiting signal is inactive (L), to generate a register read signal (rgrz), during execution of such a mode register read command, for activating said output circuitry (30), generation of said register read signal (rgrz) being inhibited when said prohibiting signal (setz) is active (H); and said control unit (202; 206) is operable to make said prohibiting signal (setz) active (H) when the device is to have said non-responsive state and to make said prohibiting signal inactive (L) when the device is to have said responsive state. A device as claimed in claim 4, wherein the said control unit (202; 206) is connected for receiving a reset signal generated by the said mode register control circuitry (10), which reset signal is derived from the same source as a register set signal (rgwz) applied to the mode register (20) during execution of such a mode register set command, and said control unit (202; 206) is operable in response to said reset signal to make said prohibiting signal (setz) inactive (L). A device as claimed in claim 4 or 5 when read as appended to claim 2, wherein said control unit (202; 206) is operable in response to said initialising signal (sstz) to make said prohibiting signal (setz) active (H). A device as claimed in any one of claims 3 to 6, wherein said mode register control circuitry (10) further comprises an override circuit (204) connected to said control unit (206) for preventing said control unit from selecting said non-responsive state when the device begins to be supplied with power provided that at that time no such mode register read command is applied to the device, whereby subsequent execution of such a mode register read command is permitted after power up even if no such mode register set command has yet been executed. A device as claimed in claim 7, wherein said override circuit (204) is operable to detect, when the device begins to be supplied with power, whether or not such a mode register read command is applied and, if so, causes said control unit (206) to select said non-responsive state and, if not, causes said control unit (206) to select said responsive state. A device as claimed in claim 8, wherein: said control unit (206) has a control input connected to said override circuit (204) for receiving therefrom a responsive-setting signal (setR) and operable, in the event that said responsive-setting signal (setR) is active (H) when the device begins to be supplied with power, to select said responsive state, and to select said non-responsive state in the event that said responsive-setting signal is inactive (L) when the device begins to be supplied with power; and said override circuit (204) is operable to make said responsive-setting signal (setR) active (H) upon detecting that such a mode register read command is applied when the device begins to be supplied with power and, otherwise, to make the responsive-setting signal inactive (L). A semiconductor device as claimed in any preceding claim, further comprising: an internal clock generating unit (102) for generating an internal clock signal in correspondence with an external clock signal; a memory cell array (113); a read/write circuit (130) for reading data from and writing data to the memory cell array (113); and an input/output circuit (140) for inputting and outputting data addresses and commands. A device as claimed in any preceding claim, being a synchronous dynamic random access memory (SDRAM) device.

Inventors: KANDA TATSUYA, SHINOZAKI NAOHARU, KANDA, TATSUYA, SHINOZAKI, NAOHARU, Kanda, Tatsuya, c/o Fujitsu Limited, Shinozaki, Naoharu, c/o Fujitsu Limited
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1996
Main classification: G11C7
IPC codes: G11C7, G11C11