Page 213 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

194 Chapter Sixteen

In-System Programmable

One advantage of E2PROM and FLASH devices is that they can be repro-
grammed while still on the circuit board, in which case they may be referred to
as in-system pogramdle (ISP).14 A large proportion of FLASH and E2PROM

devices use hot (high energy) electron injection similar to EPROM and require
12V signals for programming. Some designers see this as an advantage, because
these devices cannot be accidentally erased by means of the 5.0V (or lower,
such as 3.3V, 3.0V, etc.) signals used during the circuit’s normal operation.
However, an increasing number of devices use cold (low energy) Fowler-
Nordheim electron tunneling and can be programmed using the standard signal
voltage (5.0V, 3.3V, 3.0V, etc.).
In addition to their use as memories, EPROM, E2PROM, and FLASH

technologies are also employed in other PLD types, which are generically
known as EPLDs, E2PLDs, and FLASH PLDs. For example, each fusible link
in a standard PLA is replaced by an EPROM cell in an EPLA, an E2PROM cell
in an E2PLA, or a FLASH cell in a FLASH PLA. Regardless of the technology
used-from fusible link to FLASH-all of the components are created on the
surface of a single piece of silicon substrate. However, it may be useful to
visualize the device as comprising two distinct strata, with the AND and OR

arrays on the lower level and the links “floating” above them (Figure 16-2 1).
In yet another alternative, some PLDs are constructed with each link being
replaced with an SRAM cell. Unlike the other reprogrammable options, PLDs

Figure 16-21. A fusible-link PLA versus an EPLA

14 In conversation, ISP is spelled out as “I-S-P.”