Numonyx® PCM combines the best attributes of NOR, NAND, and RAM, simplifying memory and producing more capabilities within a single chip. These attributes are summarized in the chart below.
Like RAM or EEPROM, PCM is bit-alterable. Flash memory technology requires a separate erase step in order to change information. Information stored in bit-alterable memory can be switched from a one to zero, or zero to a one, without a separate erase step.
Like NOR flash and NAND flash, PCM is non-volatile. RAM requires a constant power supply, such as a battery backup system, to retain information. Early testing results conducted by Numonyx on multi-megabit PCM arrays for long-term data retention show excellent results.
Like RAM and NOR flash memory, the technology features fast random access times. This enables the execution of code directly from the memory, without an intermediate copy to RAM. The read latency of PCM is comparable to single bit per cell NOR flash, while the read bandwidth can match DRAM.
PCM will achieve write throughput speeds faster than NAND and with lower latency. These features, when combined with a no separate erase step (bit-alterable), will deliver significant write performance improvement over NOR and NAND flash.
Scaling is the fifth area where PCM offers a difference. Both NOR and NAND rely on floating gate memory structures which are difficult to shrink. As the memory cell shrinks on flash, the number of electrons stored on the floating gate shrinks. It is estimated that a 45nm NAND flash uses fewer than 100 electrons to delineate between a “1” and a “0”. Below 45nm, the number of electrons will continue resulting in higher bit error rates and lower levels of reliability. Because PCM does not store charge (electrons) it is immune to the charge storage scaling issue. In fact smaller lithographies improve PCM technology!