EPROM and OTP
EPROM is used to store information such as programs and data that must be retained when the system is switched off. It is used within PCs to store the Toolbox and BIOS routines and power on software in the MAC and IBM PC that is executed when the computer is switched on. These devices are read only and cannot be written to, although they can be erased by ultraviolet (UV) light and have a transparent window in their case for this purpose. This window is usually covered with a label to prevent accidental erasure, although it takes 15–30 minutes of intense exposure to do so.
There is a different packaged version of EPROM called OTP (one time programmable) which is an EPROM device packaged in a low cost plastic package. It can be programmed once only because there is no window to allow UV light to erase the EPROM inside. These are becoming very popular for small production runs.
Flash memory is a non-volatile memory which is electri-cally erasable and offers access times and densities similar to that of DRAM. It uses a single transistor as a storage cell and by placing a higher enough charge to punch through an oxide layer, the transistor cell can be programmed. This type of write operation can take several milliseconds compared to sub 100 ns for DRAM or faster for SRAM. Reads are of the order of 70–100 ns.
FLASH has been positioned and is gaining ground as a replacement for EPROMs but it has not succeeded in replacing hard disk drives as a general-purpose form of mass storage. This is due to the strides in disk drive technology and the relatively slow write access time and the wearout mechanism which limits the number of writes that can be performed. Having said this, it is frequently used in embedded systems that may need remote software updating. A good example of this is with modems where the embedded software is stored in FLASH and can be upgraded by downloading the new version via the Internet or bulletin board using the modem itself. Once downloaded, the new version can be transferred from the PC to the modem via the serial link.
Electrically erasable programmable read only memory is another non-volatile memory technology that is erased by apply-ing a suitable electrical voltage to the device. These types of memory do not have a window to allow UV light in to erase them and thus offer the benefits of plastic packaging, i.e. low cost with the ability to erase and reprogram many times.
The erase/write cycle is slow and can typically only be performed on large blocks of memory instead of at the bit or byte level. The erase voltage is often generated internally by a charge pump but can be supplied externally. The write cycles do have a wearout mechanism and therefore the memory may only be guaranteed for a few hundred thousand erase/write cycles and this, coupled with the slow access time, means that they are not a direct replacement for DRAM.