Caravel is the name of the harness provided by Efabless to make it easier to submit a design to the foundry.

Among other things, it provides:

• The padring
• 3kB of memory
• A small RISCV processor called PicoRV32
• GPIOs
• A logic analyser
• A Wishbone bus

For the full details, check the documentation

The simplest way to interface between your project and the PicoRV32 is probably the logic analyser. This gives you 128 ins and outs that you can use to configure or debug your design.

When I was first testing my designs inside Caravel, I was quite confused about all the GPIO options.

• Each pin has a range of options that can be configured by firmware running on the RISCV processor.
• Each pin can be driven from the processor or your custom design.
• The outputs have separate output enable lines for bi-directional signalling.

I put together an experiment where I tried the most important options and checked the results in a simulation. You can watch the video here: