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Caravel
gpio testbench
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This testbench exercises the fundamental use of the Caravel
management SoC to drive the I/O in the user area as general
purpose I/O on startup.

On startup, all GPIO are configured as input to the management
region (so as to be high impedence to the external world) and
decoupled from the user project area.

To configure any GPIO as output, the appropriate memory-mapped
location for the I/O must be properly configured.  Since the
I/O configuration is stored in two places, in the SoC, but
also locally at each I/O pad, the "transfer" bit must be
applied, which initiates a transfer of the configuration data
around the padframe.

The testbench takes 16 pins from the user area and checks
functionality by applying input values on 8 of these pins from
the testbench verilog, detecting them in the C program, then
copying the values to the other 8 pins, and detecting those
values in the testbench verilog.

If any of that does not work, then the testbench will fail.