iused to envy CPU designers—the lucky engineers with access to expensive
tools and fabs. But, field-programmable gate arrays (FPGAs) have made
custom-processor and integratedsystem design much more accessible.
20–50-MHz FPGA CPUs are perfect for many embedded applications.
They can support custom instructions and function units, and can be
reconfigured to enhance system-on-chip (SoC) development, testing, debugging,
and tuning. Of course, FPGA systems offer high integration, short
time-to-market, low NRE costs, and easy field updates of entire systems.
FPGA CPUs may also provide new answers to old problems. Consider
one system designed by Philip Freidin. During self-test, its FPGA is
configured as a CPU and it runs the tests. Later the FPGA is reconfigured for
normal operation as a hardwired signal processing datapath. The ephemeral
CPU is free and saves money by eliminating test interfaces.