Memory Circuit With Power Supply Voltage Detection Means

The present invention relates to a static type memory circuit having a stand-by mode, or "Data Retention Mode", for allowing stored information to be maintained by a back-up voltage of a smaller voltage value than a normal operation power voltage, and more particularly, to a memory circuit of the mentioned type having means for detecting reduction in a power supply voltage.

As is well known, a static type memory can operate at a high speed, but it requires a relatively large amount of power consumption. In this point of view, when a memory is not accessed from the outside, a value of the power supply voltage applied to the memory is reduced to a value which is enough to maintain information storage in memory cells but is not enough to perform write and read operations to and from the memory cells, introducing "Data Retention Mode" to the memory. The Data Retention Mode is initiated by turning a chip selection signal applied to the memory chip to an inactive level for disenabling peripheral circuits of the memory cell array such as decoder circuits and a read/ write circuit, and by reducing the power voltage. Recently, such a memory has been proposed that is forced to be switched over into the Data Retention Mode even if the chip selection signal is in an active level, when the value of the power supply voltage drops down to a value equal to or near the value to which the power supply voltage is intentionally reduced in the Data Retention Mode. Such a forced Data Retention Mode is called as "Auto Data Retention Mode". The memory is provided with a voltage value detection circuit for detecting a drop in the power supply voltage for the Auto Data Retention Mode, and with a control circuit responsive to the chip selection signal and to the output of the voltage value detection circuit. With these circuits, even if the chip selection signal is in the active level, the state of the memory can be shifted to the Data Retention Mode by intentionally reducing the power voltage and the peripheral circuits are disenabled, thus enhancing the flexibility in controlling the memory. When the power voltage unintentionally drops, the peripheral circuits are automatically disabled by the Auto Data Retention Mode to protect the stored information. The voltage value detection circuit in the prior art memory comprises a voltage divider circuit composed of resistors serially connected between the power voltage and a reference voltage for dividing the power voltage and of an inverter circuit receiving the output of the voltage divider circuit and logically distinguishing the value of the power voltage between the normal operation mode and Data Retention Mode. The output of the inverter of the voltage value detection circuit is then applied to one input terminal of a NOR gate which receives the chip selection signal at the other input terminal. The NOR gate produces an internal chip selection signal for directly controlling the peripheral circuits. In this prior art voltage value detection circuit, however, a current always flows through the serially connected resistors both in the normal operation mode and in the Data Retention Mode, and therefore a considerable amount of power is inevitably consumed. This problem is more serious in the Data Retention Mode, because a battery is generally used as a back-up voltage source and its capacity is limited.

Furthermore, a relatively large number of circuit elements are required to generate the internal chip selection signal in the mentioned prior art.

It is an object of the present invention to provide memory circuit operable with a low power consumption both in the normal operation mode and in the Data Retention Mode.

It is another object of the present invention to provide a memory circuit which can be fabricated with high-density on a semiconductor chip.

It is still another object of the present invention to provide an improved mode control circuit for controlling operation modes of a memory.

A memory circuit according to the present invention comprises a voltage value detection circuit which includes a series connection of a chip selection signal detecting field effect transistor and a voltage value detecting field effect transistor. The gate of the former transistor receives a chip selection signal, and the latter transistor is adapted to be conducting when the potential at a power voltage terminal is in a first value and non-conducting when the potential at the power voltage terminal is in a second value smaller in absolute value than the first value, the second value being not enough to support the read/write operation of the memory circuit but enough to support information storage. This series connection is coupled via a load means between the power voltage terminal, and an output signal of the voltage value detection circuit is derived from the junction of the two transistors and the load means or from the junction between the two transistors.

According to the present invention, the voltage value detecting transistor is made non-conducting when the potential at the power voltage terminal is in the second value which brings the memory circuit into the Data Retention Mode, and hence no DC current flows the series connection in the voltage value detection circuit, resulting in a low power consumption.

Furthermore, the voltage value detection circuit can be fabricated by a small number of circuit elements, e.g. one load element and two transistors, and hence the integration density of the memory circuit can be enhanced.

According to the present invention, there is provided a memory circuit comprising a plurality of memory cells, a selection circuit for selecting at least one of the memory cells based on address information, a read circuit for operatively reading data from the memory cell or cells designated by the selection circuit, a write circuit for operatively writing data to the memory cell or cells designated by the selection circuit and an operation control circuit for controlling at least the read circuit and the write circuit in response both to a value of the power voltage variable between a first value and a second value smaller than the first value and to a chip selection signal having a first and second levels, the first value allowing the memory to normally operate, the second value inhibiting the memory from normally operating but allowing the memory to maintain stored data. The operation control circuit enabling and disenabling the write circuit and the read circuit in response to the first level and to the second level of the chip selection signal, respectively when the value of the power voltage is at the first value while the operation control circuit disenabling the write circuit and the read circuit when the value of the power voltage is at the second value irrespectively of the chip selection signal, the operation control circuit including a load element coupled between a first terminal receiving one of the power voltage and a reference voltage and a second terminal, and a series circuit of a first field effect transistor receiving the chip selection signal at its gate and a second field effect transistor adapted to be conducting only when the power voltage is at the first value between the second terminal and a third terminal receiving the other of the power voltage and the reference voltage in which the controls of the write circuit and the read circuit are conducted in response to a potential at the second terminal.

According to the above memory, only when the power voltage is at the first voltage and the memory is in the normal operation mode, the potential at the second terminal can be controlled by the first transistor in response to the chip selection signal. While when the power voltage is at the second value and memory is to be in Data Retention Mode, the second transistor is non-conducting so that the potential at the second terminal is determined through the load element independently on the chip selection signal so that the memory is automatically set at Data Retention Mode in which stored states of the memory cells are maintained but any access to the memory cells is inhibited. In this instance, no DC current flows the series circuit of the first and second transistors in the operation control circuit at least during Data Retention Mode. Accordingly, the power consumption in the operation control circuit is remarkably reduced. Furthermore, the operation control circuit can be fabricated by a small number of circuit elements so that the integration density of the memory can be enhanced.

FIG. 1 is a block diagram showing a memory circuit according to the prior art;

FIG. 2 is a waveform diagram showing the operations of the memory of FIG. 1;

FIG. 3 is a block diagram showing a memory circuit according to one embodiment of the present invention; and

FIGS. 4 to 6 show other examples of the control circuit 10' of FIG. 3 according to the present invention.

With reference to FIG. 1, a memory according to the prior art will be described.

A memory cell array 22 includes a plurality of memory cells MC which are arranged in a matrix of word lines WL in rows and digit lines DL in columns. A plurality of row address input A.sub.O -A.sub.n are applied to a row decoding circuit 24 through address inverter buffers 16. A column selection circuit 14 selectively connects one of the digit lines DL to a write data line WB and/or a read data line RB under control of a column decoder 15 to which column address inputs A.sub.n+1 -A.sub.m are applied through column address inverter buffers 17. A mode control circuit 10 generates an internal control signal Cs' in response to a chip selection signal CS from the outside and a value of a power voltage V.sub.cc. The signal CS' is applied to an inverting input of a AND gate 21 receiving a write control signal WE at its another input, and to an inverting input of an AND gate 20 receiving the write control signal WE at its other inverting input. The output of the gate 20 is used to operatively enable a data input buffer 18 for supplying data to be written in a write operation. The buffer 18 includes a series circuit of a P-channel transistor Q.sub.22 and N-channel transistors Q.sub.23 and Q.sub.24, and it is enabled when the transistor Q.sub.24 is conducting. The output of the AND gate 21 is used to operatively enable an output buffer 19 receiving a read-out signal via the read data line RB. The buffer 19 includes a P-channel transistor Q.sub.25 and N-channel transistors Q.sub.26 and Q.sub.27, and it is enabled when the transistor Q.sub.27 is conducting. The mode control circuit 10 includes a voltage value detection circuit 11, an inverter 12 and a NOR gate 13 receiving the chip selection signal CS and an output of the inverter 12. The voltage value detection circuit 10 includes a voltage divider composed of resistors R.sub.1 and R.sub.2 connected between the power voltage V.sub.cc and a ground potential and a series circuit of a P-channel field effect transistor Q.sub.1 and a resistor R.sub.3. With reference to FIG. 2, the operation of the memory will be described. When the value of the power voltage V.sub.cc is in a normal operation range, e.g. 4.5-5.5 V, a voltage drop across the resistor R.sub.1, i.e. a voltage difference between the voltage V.sub.cc and a voltage at a node N.sub.1 is larger than a threshold voltage of the transistor Q.sub.1 in absolute value so that a voltage at a node N.sub.2 is larger than a logic threshold of the inverter in absolute value. Accordingly, the output of the inverter 12 necessarily takes logic "0" in the normal operation mode and hence the state of the signal CS' is determined by the chip selection signal. For example, if the chip selection signal CS is at the logic "1", then the signal CS' is at the logic "0" so that the memory is in active state. On the contrary, if the chip selection signal CS is at the logic "0" level, the signal CS' becomes the logic "1" so that the memory takes a stand-by state. In stand-by state, the input buffer 18 and the buffer 19 are inhibited so as not to affect the stored states of the memory cells.

When the value of the power voltage V.sub.cc drops from that in the normal operation mode to a back-up voltage e.g. 3 V, and the voltage drop across the resistor R.sub.1 becomes smaller than the threshold voltage of the transistor Q.sub.1, then the transistor Q.sub.1 becomes non-conducting. Therefore, the potential at the node N.sub.2 becomes the ground level i.e. logic "0". As a result, the signal CS' becomes the logic "1" irrespective of the state of the chip selection signal CS so that the memory enters the stand-by state, i.e. Data Retention Mode. As described above, the memory of FIG. 1 can be automatically shifted to Data Retention Mode by simply reducing the value of the power voltage V.sub.cc. However, in the memory of FIG. 1, a DC current always flows through the resistors R.sub.1 and R.sub.2, both in the normal operation mode and in Data Retention Mode. Accordingly, the power consumption of the memory of the prior art is large. Furthermore, the mode control circuit 10 is composed of a relatively large number of circuit elements and that is a disadvantage in view of the desire to fabricate the memory with high-density.

With reference to FIG. 3, a memory according to one embodiment of the present invention will be described.

The memory circuit of FIG. 3 is fabricated on a semiconductor chip. In FIG. 3, the portions corresponding to those in FIG. 1 are designated by the same reference numbers used in FIG. 1. In the present invention, in place of the mode control circuit 10 in FIG. 1, a mode control circuit 10' is employed. The mode control circuit 10' according to the invention includes a load resistor R.sub.4 connected between the power voltage V.sub.cc and a node N.sub.2 ', and a series circuit of an N-channel field effect transistor Q.sub.20 receiving the chip select signal CS from the outside at a gate thereof and an N-channel field effect transistor Q.sub.21 having a gate and being coupled to the power voltage V.sub.cc coupled between the node N.sub.2 ' and the ground potential. From the node N.sub.2 ' the internal control signal CS' is generated. In this embodiment, the signal CS' is also applied to OR gates 31, each coupled between one of the address inputs A.sub.O -A.sub.n and a corresponding one of the row address inverter buffers 16, and to OR gates 33 each coupled between one of the column address inputs A.sub.n+1 -A.sub.m and a corresponding one of the column address inverter buffers 17. In the row decoding circuit 24', each of the decoding units DU for driving the corresponding one of the word lines WL is composed of a P-channel field effect transistor Q.sub.11 and an N-channel field effect transistor Q.sub.12 coupled between the power voltage V.sub.cc and the ground potential and both receiving the signal CS' at their gates, and a plurality of N-channel field effect transistors Q.sub.13 . . . Q.sub.li connected in parallel with the transistor Q.sub.12. The transistors Q.sub.13 . . . Q.sub.li receive the row address signals from the row address inverter buffers. In the mode control circuit 10', a threshold voltage of the N-channel transistor Q.sub.21 is selected to a value which is lower than the value of the power voltage V.sub.cc in the normal operation mode and larger than the value of the power voltage V.sub.cc in Data Retention Mode. In the normal operation mode, the transistor Q.sub.21 is conducting. Therefore, when the chip selection signal CS is at the logic "1", the transistor Q.sub.20 is conducting so that the potential at the node N.sub.2 ', i.e. signal CS', is at the logic "0" through the conducting transistors Q.sub.20 and Q.sub.21, and the memory operates in the active mode. In the active mode, the OR gates 31 directly transfer the row address input A.sub.O -A.sub.n to the row address inverter buffers 16 and the OR gates 33 directly transfer the column address inputs A.sub.n+1 -A.sub.m to the column address inverter buffers 17. Also, in the row decoding circuit 24', the respective decoding units DU are enabled. When the operation is a read operation, the write control signal WE is at the logic "1", and hence the output of the AND gate 21 is at the logic "1" while the output of the AND gate 20 is at the logic "0". Therefore, the read buffer 19 is enabled while the write buffer 18 is disenabled. When a write operation is to be performed, the write control signal WE is at the logic "0" and hence the write buffer 18 is enabled with the read buffer 19 disenabled.

When the chip selection CS becomes the logic "0" under the power voltage of the normal operation range, the transistor Q.sub.20 becomes non-conducting to make the signal CS' the logic "1" so that the memory takes a stand-by state, in which the outputs of the OR gates 31 and 33 are all logic "1" irrespectively of the address inputs so that changes of the states of the address inverter buffers 16 and 17 are inhibited. Accordingly, the power consumption generated in the address inverter buffers 16 and 17 can be avoided. Simultaneously, the load transistor Q.sub.11 in the decoding unit DU is turned off and both the read buffer 19 and the write buffer 18 are disabled.

When the value of the power voltage V.sub.cc changes from the value e.g. 5 V for the normal operation mode to the value e.g. 3 V for Data Retention Mode, the transistor Q.sub.21 becomes non-conducting. Accordingly, the signal CS' becomes logic "1" irrespective of the chip selection signal CS so that the memory is shifted to the stand-by mode and the peripheral circuits such as the address inverter buffers 16 and 17, the read buffer 19 and the write buffer 18 are disabled. According to the present invention, there is no DC current path in the mode control circuit 10' at least during Data Retention Mode so that the memory operable with a low power consumption is obtained. Furthermore, the control circuit 10' can be fabricated with a small number of circuit elements.

With reference FIGS. 4 to 6, other embodiments of the mode control circuit 10' of FIG. 3 will be explained.

A mode control circuit shown in FIG. 4 is obtained by replacing the resistor R.sub.4 of FIG. 3 with a P-channel field effect transistor Q.sub.41 having a gate connected to the ground potential. The operation of this mode control circuit is substantially the same as that of the circuit 10' of FIG. 3.

A mode control circuit of FIG. 5 is the case where the logic system with respect to the chip selection signal CS and the internal control signal CS' of FIG. 3 is inverted. Namely, a series circuit of P-channel field effect transistors Q.sub.51 and Q.sub.52 is connected between a node N.sub.2 " and the power voltage V.sub.cc and a resistor R.sub.51 is connected between the node N.sub.2 " and the ground potential. In this circuit, when the value of the power voltage V.sub.cc changes from the value in the normal operation mode to that in Data Retention mode, the transistor Q.sub.51 becomes non-conducting so that the signal CS' becomes the inactive level i.e. logic "0", because the threshold voltage of the transistor Q.sub.51 is set slightly larger than the voltage value of the power voltage in Data Retention Mode.

A mode control circuit of FIG. 6 is obtained by replacing the resistor R.sub.51 of FIG. 5 with an N-channel transistor Q.sub.61 having a gate connected to the power voltage. An operation of this circuit is similar to that of FIG. 5.

Although the explanation of the present invention has been performed by way of examples, the present invention is not limited to the mentioned embodiment but applicable to different kind of memory circuits.