1. Field of the Invention
The present invention relates to a semiconductor memory device having a low power consumption mode.
2. Description of the Related Art
In recent years, the cellular phone has been given not only a function to have a vocal communication but also a function to transmit character-string data or image data. Moreover, the cellular phone has been expected in the future to become a kind of information terminal (for example, a portable type personal computer) as the internet services are diversified. Thus, the information volume of data to be handled by the cellular phone has been drastically increasing. Conventionally, the cellular phone has employed as its work memory SRAMs having a memory capacity of about 4 Mbit. The work memory is a memory for retaining the necessary data during the operation of the cellular phone. It is obvious that the memory capacity of the work memory will be short in the future.
On the other hand, the transmission speed of the cellular phone has been heightening. The smaller the cellular phone becomes, the smaller the battery to be mounted becomes. Therefore, the work memory to be employed in the cellular phone is required to have a high speed, low power consumption and a large capacity. In the cellular phone serious price competitions, it is necessary to make the costs for parts as low as possible. Therefore, the work memory has to be at low price.
The conventional SRAMs as employed in the work memory are higher per bit in cost than DRAMs. The production number of SRAMs is smaller than that of the DRAMs so that it is difficult to lower its price. Moreover, there have never been developed SRAMs having a large memory capacity (for example, 64 Mbit).
In this situation, it has been considered to replace the SRAMs by flash memories and DRAMs in the work memory of the cellular phone.
The flash memory has a power consumption as low as several μW during a standby state but requires several μs to several tens μs for writing data. When the flash memory is employed as the work memory of the cellular phone, therefore, it is difficult to transmit/receive massive data at high speed. The flash memory performs the write operation at the unit of a sector so that it is not suitable for rewriting bit by bit image data such as the data of a moving image.
On the contrary, the DRAMs can execute both the read operation and the write operation within several tens ns and can process the data of the moving image easily. The power consumption during the standby state is higher than that of the flash memories. In the present DRAMs, the power consumption in the standby state is about 1 mW during a self-refresh mode for retaining written data and about 300 μW during a standby mode not required for retaining written data.
If the power consumption during the standby mode could be reduced to that of the flash memories, the DRAMs could be employed as the work memory of the cellular phone, but such circuit technology has never been proposed.
The power consumption of the DRAMs can be reduced to zero by stopping the power supply to the DRAMs. However, since the address terminals, the data terminals and the like of the DRAMs are connected with the terminals of other electronic parts through the wiring patterns on a circuit board, it is required to drastically change the system of the cellular phone (the pattern change of the circuit board, re-layout and so on) for the termination of the power supply to the DRAMs.
Besides, there has not been proposed a technology which realizes exit from the standby mode without the malfunction of an internal circuit after the power supply is terminated to stop the operation of the internal circuit during the standby mode.
Where the internal voltage to be used in the internal circuit is generated inside of the device, it has to be quickly returned to a predetermined voltage when a release is made from a standby mode (a low power consumption mode). However, this technique has never been proposed.