Semiconductor memory circuit

Abstract: A semiconductor memory circuit according to the present invention comprises a decode circuit (T2, T3; T12, T13; T22, T23; T32, T33) responsive to an internal address ( Ax0 , Ax1 ) for generating a signal of a predetermined level, a drive circuit (T5, T6, T15, T16, T25, T26, T35, T36) responsive to a word line activating signal øw for generating a word line selection signal øwl corresponding to an output level of the decode circuit and a shutdown circuit (T7, T17, T27, T37) responsive to a control signal øs for disconnecting the decode circuit from a ground terminal. With this construction, since the decode circuit is disconnected from the ground terminal during a time period for which a memory sense amplifier operates, an output level of the word line selection signal is not affected by noise even if the latter is generated on the ground line during that period. Therefore, it is possible to keep the memory cell in a stable selection state.

Description: The present invention relates to a semiconductor memory circuit and, particularly, to a word selection circuit for activating a predetermined word line selection signal according to an address internal signal. In a semiconductor memory circuit having memory cells arranged in an array, a word line selection circuit is one of circuit construction for selecting a memory cell corresponding to an input address. The word line selection circuit receives an internal address corresponding to an address inputted externally and activates one of a plurality of word line selection signals in order to select a predetermined word line. Generally, the word line selection circuit includes, for each word line selection signal, a decode circuit responsive to an internal address for generating a signal having a predetermined level, a drive circuit responsive to a word line activating signal for outputting a signal corresponding to an output level of the decode circuit as a word line selection signal and a precharge circuit for precharging an input point of the drive circuit to a power source voltage. Therefore, the number of such word line selection circuits corresponds to the number of word line selection signals. With an internal signal supplied to the respective decode circuits of the word line selection circuits, an output of one of the decode circuits is maintained at high level (power source voltage level) while outputs of the remaining decode circuits are changed to low level (ground voltage level). Then, when the word line activating signal becomes high level, one of the drive circuits generates a high level, activated word line selection signal and the remaining drive circuits generate low level word line selection signals. Thus, the word line signal corresponding to the internal address is activated. When the word line selection signal is activated in this manner, a word line corresponding thereto is driven and a memory cell connected to the word line is selected. Then, information read out from the thus selected memory cell is amplified by a sense amplifier and readout data is outputted. In this case, when the sense amplifier amplifies the input information, a large current flows through the ground line. As a result, a condition where a potential of the ground line rises from 0V occurs, that is, noise is generated in the ground line. When such noise is generated, the output levels of the decode circuits which are low are raised. Therefore, output levels of the associated drive circuits are also raised while the level of the more activated word line selection signal is lowered, causing a memory cell selection to become unstable. A typical memory circuit is described in US-3980899. This publication discloses a word line driver circuit, for driving of memory, connected to the word driver line of the memory cells, including a resistive connection, connected between the word line and ground potential, for preventing a potential of the word line from floating. The driver circuit includes an enhancement-type switching transistor. An object of the present invention is to provide a word line selection circuit by which a level of a word line selection signal is not influenced by noise generated in a ground line while a sense amplifier amplifies input information from a selected memory cell. According to the present invention there is provided a semiconductor memory circuit comprising a decode circuit (T2, T3; T12, T13; T22, T23; T32, T33) responsive to an internal address (Ax0, Ax1) for generating a signal of a predetermined level, and a drive circuit (T5, T6; T15, T16; T25, T26; T35, T36) responsive to a word line activating signal (øw1,øw2,øw3,øw4) corresponding to an output level of said decode circuit, a shutdown circuit (T7; T17; T27; T37) responsive to a control signal (øs) for disconnecting said decode circuit from a ground terminal, characterized in that said control signal changes its state during a time period from a time (t11) at which said word line selection signal is outputted, to a time (t12) at which a sense amplifier starts to operate. There may be a precharge circuit for precharging an input point of said drive circuit to a source voltage. Also described is a semiconductor memory circuit comprising a plurality of word line selection signal generating circuits each comprising a decode circuit responsive to an internal address for generating a signal of a predetermined level, a drive circuit responsive to a word line activating signal for generating a word line selection signal corresponding to an output level of said decode circuit, a precharge circuit for precharging an input point of said drive circuit to a source voltage and a shutdown circuit responsive to a control signal øs for disconnecting said decode circuit from a ground terminal for a predetermined time period corresponding to amplification of a sense amplifier. The decode circuit comprises a plurality of transistors connected in parallel to each other between an input terminal of the drive circuit and a node. The transistors have gates which receive the internal address. The shutdown circuit comprises a transfer gate provided between the node and the ground terminal. The transfer gate has a control terminal to which the control signal is supplied. The drive circuit comprises a first transistor and a second transistor which are connected in series with each other between an input terminal of the word line activating signal at voltage level not less than a power source voltage and the ground terminal. The output of the decode circuit is supplied to the gate of the first transistor and an inverted output of the decode circuit is supplied to the gate of the second transistor. A junction between the first and second transistors constitutes an output terminal of the word line selection signal. The above-mentioned and other objects, features and advantages of the present invention will become more apparent by way of example and by reference to the following detailed description of the present invention taken in conjunction with the accompanying drawings, wherein: Fig. 1 is a circuit diagram showing a construction of a word line selection circuit according to the present invention; and Fig. 2 shows waveforms useful to understand an operation of the circuit shown in Fig. 1. An embodiment of the present invention will be described with reference to Fig. 1. In this embodiment, it is assumed that an internal address to be generated according to an input address includes 2 bits, Ax0 and ax1, and there are 4 word line selection signals øw1-øw4. The word line selection circuit comprises, for each word line selection signal, a decode circuit responsive to an internal address for generating a signal of predetermined level, a drive circuit responsive to a word line activating signal for outputting a word line selection signal corresponding to an output level of the decode circuit, a precharge circuit for precharging an input point of the drive circuit to a power source voltage and a shutdown circuit responsive to a control signal for cutting off between the decode circuit from a ground terminal. Describing a portion of this. circuit in which the word line selection signal øw1 is generated, the decode circuit includes N channel transistors T2 and T3 which are connected in parallel between nodes N1 and N4 and gates which receive the internal address bits Ax0 and Ax1, respectively. This decode circuit functions as a NOR circuit for the internal address bits Ax0 and Ax1. The precharge circuit includes a P channel transistor T1 connected between a power source terminal Vcc and the node N1 and having a gate to which a precharge signal øp is supplied. The drive circuit includes an N channel transistor T5 having a gate connected through an N channel transistor T4 to the node N1 and an N channel transistor T6 having a gate connected through an inverter circuit 1 to the node N1. The transistors T5 and T6 are connected in series with each other between a ground terminal and a terminal to which the word line activating signal øw is supplied and a junction between the transistors T5 and T6 is used as an output point of the word line selection signal øw1. Further, the shutdown circuit includes an N channel transistor T7 having a gate to which a control signal øs is supplied and connected between a node N4 and the ground line. In the case of this embodiment, the control signal øs is an inverted signal of a sense amplifier activating signal (not shown). A circuit portion for generating the word line selection signals øw2 - øw4 is the same as that mentioned above, except that signals to be supplied to the decode circuits are a combination of the internal address Ax0 and Ax1 and inversions thereof. An operation of the circuit shown in Fig. 1 will be described with reference to Fig. 2. In this description, it is assumed that the word line selection signal øw1 is selected and activated. The internal address bits Ax0 and Ax1 are low level before the start of the selection operation the potentials at nodes N1, N11, N21 and N31 are made high level by means of respective precharge circuits T1, T11, T21 and T31. Therefore, nodes N2, N12, N22 and N32 are high level and nodes N3, N13, N23 and N33 are low level. Further, since the control signal øs is high level, the N channel transistors T7, T17, T27 and T37 which constitute the shutdown circuits are in ON state and, thus, nodes N4, N14, N24 and N34 are low level. P channel transistors T1, T11, T21 and T31 are turned off when the precharge signal øp becomes high level. In this state, the internal address bits Ax0 and Ax1 are made low level to select the word line selection signal øw1. Therefore, the N channel transistors T2 and T3 constituting one decode circuit are turned off and thus the node N1 is kept high level. Therefore, the node N2 and the node N3 are kept high level and low level, respectively. However, since the N channel transistors T12, T23, T32 and T33 are turned on, the nodes N11, N21 and N31 become low level. With these changes of state, the nodes N12, N22 and N32 change their states from high level to low level and the nodes N13, N23 and N33 are changed from low level to high level (see time instant t10 in Fig. 2). Then, when the word line activating signal øw becomes as high as or higher than the source voltage (see time instant t11 in Fig. 2), potential at the node N2 increases to a level equal to or higher than the source voltage due to the self boot effect of the transistor T5. Therefore, the word line selection signal øw1 becomes high level equal to or higher than the source voltage, that is, it is activated. On the other hand, since the nodes N12, N22 and N32 are low level, the transistors T15, T25 and T35 are turned off and, since the nodes N13, N23 and N33 are high level, the transistors T16, T26 and -T36 are turned on, while other word line selection signals are kept low level. Thereafter, a word line corresponding thereto is driven and a memory cell (not shown) connected thereto is selected. As mentioned previously, when data read out from the memory cell is amplified by a sense amplifier (not shown), large current flows through the ground line and thus the potential thereof is raised temporarily from OV resulting in ground line noise. In this invention, however, the control signal øs which is an inverted signal of a sense amplifier activating signal becomes low level when the sense amplifier operates (see time instant t12 in Fig. 2) and thus the N channel transistors T7, T17, T27 and T37 constituting the shutdown circuits are turned off. Therefore, all of the decode circuits are disconnected from the ground line. As a result, the nodes N1, N11, N21 and N31 are disconnected from the ground line electrically and thus voltage levels of the word line selection signals øw1 - øw4 are not changed. When amplified readout data is outputted by the sense amplifier and readout operation is completed, the control signal øs becomes high level (not shown) again and the precharge signal øp becomes low level. Therefore, the nodes N1, N11, N21 and N31 become high level by means of the precharge circuits T1, T11, T21 and T31. As is clear from the operation mentioned above, in the word line selection circuit according to this embodiment, the output level of the word line selection signal is not affected by noise generated in the ground line during operation of the memory sense amplifier, since the decode circuits are shutdown with respect to the ground line. Therefore, it is possible to keep the memory cell in a stable selecting condition. In the described embodiment, the control signal øs is obtained by inverting the sense amplifier activating signal. However, any other signal may be used with the same effect provided that its state is changed (from high to low or vice versa) during a time period from the moment at which any of the word line selection signals øw1 - øw4 becomes fully high level to a start of operation of the sense amplifier. Further, although, in this embodiment, the decode circuit has an NOR circuit construction, the present invention is not limited thereto. For example, the decode circuit may have a NAND circuit construction. Although the present invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that the appended claims will cover any modifications or embodiments as fall within the scope of the invention.

Claims: A semiconductor memory circuit comprising a decode circuit (T2, T3; T12, T13; T22, T23; T32, T33) responsive to an internal address (Ax0, Ax1) for generating a signal of a predetermined level, a drive circuit (T5, T6; T15, T16; T25, T26; T35, T36) responsive to a word line activating signal (øw) for generating a word line selection signal (øw1,øw2,øw3,øw4) corresponding to an output level of said decode circuit, and a shutdown circuit (T7; T17; T27; T37) responsive to a control signal (øs) for disconnecting said decode circuit from a ground terminal, characterized in that said control signal changes its state during a time period from a time (t11) at which said word line selection signal is outputted, to a time (t12) at which a sense amplifier starts to operate. A semiconductor memory circuit according to Claim 1, further comprising a precharge circuit (T1, T11, T21, T31) for precharging an input terminal of said drive circuit to the source voltage. The semiconductor memory circuit according to any preceding claim wherein the decode circuit comprises a plurality of transistors having gates supplied with said internal address and connected in parallel to each other between an input terminal of the drive circuit and a node (N4; N14; N24; N34). A semiconductor memory circuit according to Claim 3, wherein the shutdown circuit comprises a transfer gate having a control terminal supplied with the control signal and provided in between said node and said ground terminal. A semiconductor memory circuit according to any preceding claim, wherein the or each drive circuit comprises a series connection of a first transistor (T5, T15, T25, T35) and a second transistor (16, T16, T26, T36), said series connection being connected between an input terminal for said word line activating signal having a voltage level equal to or higher than a source voltage and said ground terminal, wherein said output of the decode circuit is supplied to a gate of said first transistor and inverted output of the decode circuit is supplied to a gate of said second transistor and wherein a junction of said first transistor and said second transistor constitutes an output terminal for said word line selection signal. A semiconductor memory circuit according to any preceding claim, wherein said control signal is generated in response to a sense amplifier activating signal. A semiconductor memory circuit according to any preceding claim, wherein the decode circuit has a NOR circuit construction having an input for receiving said internal address. A semiconductor memory circuit according to any of Claims 1 to 6, wherein the decode circuit has a NAND circuit construction having an input for receiving said internal address.

Inventors: KANEKO SHOUJI, KANEKO, SHOUJI, Kaneko, Shouji, c/o NEC IC Microcomp. Systems Ltd.
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1992
Main classification: G11C8
IPC codes: G11C8, G11C11