6.1.3 Hybrid Types

As memory technology has matured in recent years, the line between RAM and ROM devices has blurred. There are now several types of memory that combine the best features of both. These devices do not belong to either group and can be collectively referred to as hybrid memory devices. Hybrid memories can be read and written as desired, like RAM, but maintain their contents without electrical power, just like ROM. Two of the hybrid devices, EEPROM and Flash, are descendants of ROM devices; the third, NVRAM, is a modified version of SRAM.

EEPROMS are electrically-erasable-and-programmable. Internally, they are similar to EPROMs, but the erase operation is accomplished electrically, rather than by exposure to ultraviolet light. Any byte within an EEPROM can be erased and rewritten. Once written, the new data will remain in the device forever — or at least until it is electrically erased. The tradeoff for this improved functionality is mainly higher cost. Write cycles are also significantly longer than writes to a RAM, so you wouldn't want to use an EEPROM for your main system memory.

Flash memory is the most recent advancement in memory technology. It combines all the best features of the memory devices described thus far. Flash memory devices are high density, low cost, nonvolatile, fast (to read, but not to write), and electrically reprogrammable. These advantages are overwhelming and the use of Flash memory has increased dramatically in embedded systems as a direct result. From a software viewpoint, Flash and EEPROM technologies are very similar. The major difference is that Flash devices can be erased only one sector at a time, not byte by byte. Typical sector sizes are in the range of 256 bytes to 16 kilobytes. Despite this disadvantage, Flash is much more popular than EEPROM and is rapidly displacing many of the ROM devices as well.

The third member of the hybrid memory class is NVRAM (nonvolatile RAM). Nonvolatility is also a characteristic of the ROM and hybrid memories discussed earlier. However, an NVRAM is physically very different from those devices. An NVRAM is usually just an SRAM with a battery backup. When the power is turned on, the NVRAM operates just like any other SRAM. But when the power is turned off, the NVRAM draws just enough electrical power from the battery to retain its current contents. NVRAM is fairly common in embedded systems. However, it is very expensive — even more expensive than SRAM — so its applications are typically limited to the storage of only a few hundred bytes of system-critical information that cannot be stored in any better way.

Table 6-1 summarizes the characteristics of different memory types.

Table 6-1. Memory Device Characteristics