1. Field of the Invention
The present invention relates to a semiconductor memory system and, more particularly, to a nonvolatile semiconductor memory system using an electrically programmable read only memory (EPROM).
2. Description of the Related Art
In a read only memory (ROM) such as a mask ROM an EPROM (erasable and programmable ROM), or an EEPROM (electrically erasable and programmable ROM), arbitrary addresses are allowed to be accessed in an arbitrary order. Since, however, a data read operation is a main or only operation, a data write (program) timing in each of the above ROMs is limited as follows. That is, the data write operation is performed during a wafer process in the mask ROM. In the EPROM, although a user can write data by an electrical means, ultraviolet radiation processing must be performed to erase previous data in order to rewrite data, or a rewrite operation cannot be performed. In the EEPROM, a user can arbitrariy perform data write/read operations by using an electrical means.
As described above, a user can electrically write data in the EPROM or EEPROM and can perform electrical write/erase operations especially in the EEPROM.
For this reason, the EEPROM can be used as a static random access memory (SRAM). In addition, the EEPROM can provide a high additional value, i.e., it requires no battery backup for data storage. Since the EEPROM is expensive, however, it is unsuitable for constructing an inexpensive system.
When a data rewrite operation need not be performed during an operation of a system of interest or a data write operation need only be performed once, it is preferable to use the EPROM which is less expensive than the EEPROM. In addition, when a data write operation need only be performed once, one time PROM (OTPROM) is obtained by omitting a transparent window from a package of the EPROM and therefore a less expensive one can be used. Although a system arrangement using this OTPROM is the same as that using the EPROM, a user cannot erase data since no transparent window for ultraviolet radiation is formed in a package of an integrated circuit. Therefore, a user can perform a data write operation only once.
In order to perform a data rewrite operation in a system using the EPROM as described above, an EPROM integrated circuit is temporarily removed from a board mounting the EPROM, ultraviolet rays are radiated on an EPROM chip through a transparent window of a package of the EPROM to erase data, and a data rewrite operation is performed by using an exclusive EPROM data writer.
In a system using an EPROM as a program memory, a program need only be written once if the program is systematically completed. If, however, a function is to be added to the system by using a program or the system is incomplete, a rewrite operation must be performed several times. In addition, as another application of the EPROM, the EPROM is sometimes used as a fine adjustment data memory for machine control or a password registration memory for system security. Fine adjustment data for machine control need only be rewritten once to several times during machine assemble/adjustment steps, and a password for security need only be written upon initial password registration and rewritten several times when the password must be changed for some reasons.
When a write operation must be performed several times as described above, an EPROM having an ultraviolet radiation transparent window for data erasure in a package of an integrated circuit or an EEPROM capable of performing electrical erase/write operations is used. Since, however, a package having a transparent window or an integrated circuit chip of the EEPROM is expensive, a realized system becomes expensive too.