Page 263 - Mechanical Engineers' Handbook (Volume 2)

252 Digital Integrated Circuits: A Practical Application

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GO UT

G
DO UT D X

X D
G

XIN
Figure 14 Simplified I/O block.

resources. Global routing resources connect nonadjacent logic blocks and are used to define
the combinations of channels. Local routing resources connect a logic block to global routing
resources or to adjacent logic blocks. In short, local routing refers to which wire is in each
channel. Programming technologies used include Antifuse, SRAM, and EEPROM.
The Antifuse switch, shown in Fig. 15, is a two-terminal device. It is designed so that
it offers high resistance when not programmed and once programmed the resistance drops
close to 0 . This method offers only one-time programming and any error made during
programming cannot be corrected. The Antifuse array is formed with horizontal and vertical
wires and with an Antifuse switch at every intersection. Based on the required configuration
of the FPGA, high voltage is applied to particular switches. The main disadvantage of this
technology is that it requires a separate on-chip circuitry to deliver the high voltage for
programming. 5
In SRAM-based technology, pass gates are used as switches controlled by RAM cells.
These RAM cells are usually implemented by LUTs and sometimes multiplexers. Some
disadvantages are the huge space requirements needed and the large RC delay to the signal,
6
which makes SRAM-based technology slower than Antifuse. Another major disadvantage
is that it is volatile, or it loses its memory every time it is powered up. The major advantage
is the reprogrammability, which allows for full testing after fabrication.
Lastly, the EEPROM is a viable option. As mentioned previously, it has a control gate
and a floating gate. During normal operation, these transistors look like open circuits, and

High voltage creates
permanent
short

Figure 15 Antifuse technology.