Page 210 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

Programmable ICs 19 1

As an alternative to the user specifying a particular device, the program can
be instructed to automatically select the best device for the task. The program
can base its selection on a variety of criteria, such as the speed, cost, and power
consumption of the devices. The program may also be used to partition a large
design across several devices, in which case it will output a separate JEDEC file
for each device.
Finally, the designer takes a virgin device of the appropriate type and places
it in a socket on a special tool, which may be referred to as a programmer,
blower, or burner. The main computer passes the JEDEC file to the programmer,
which uses the contents of the file to determine which fuses to blow. The
designer presses the GO button, the burner applies the appropriate signals to
the device’s inputs, and a new device is born (Figure 16-19).

(a) Host computer (b) Device programmer
Figure 16-19. Programming a physical PLD

Reprogrammable PLDs

One consideration with fusible link and antifuse ti .ihnologies is that, once
they have been programmed, there is no going back. This may be of particular
concern with PROMS as the data they store is prone to change.

EPROMs

One alternative is a technology known as erasable programmable read-only
memory (EPROM). An EPROM transistor has the same basic structure as a
standard MOS transistor, but with the addition of a second polysilicon floating
gate isolated by layers of oxide (Figure 16-20).

In its unprogrammed state, the floating gate is uncharged and doesn’t affect