Communicating with the world

As you’ve now seen, blinking an LED for the first time on new hardware actually requires a lot of bring up. But we did it! We now understand our board, a little bit about its SDK, and, most importantly, have our first firmware running! However…our blinky program is really a simple one-bit conversation. Anything more interesting (e.g., a sensor reading, detailed logging, or command from a host) needs the chip to actually talk. This section is about the simplest, oldest, and still most useful way to do that: UART (Universal Asynchronous Receiver-Transmitter).

By the end of this section we’ll be able to enter commands into our VEGAboard over a serial terminal, like so:

vega> led red on
vega> echo hello there
hello there

This will be done by developing a simple and tiny read–eval–print loop (REPL), which we’ll call the VegaConsole After we develop it, we’ll actually keep returning and building on top of it in the following sections.

The plan for this section:

1. UART will walk through the basics of the protocol itself and the LPUART0 peripheral on the RV32M1. We’ll trace a single byte from PRINTF('A') down to the store instruction that puts it on the line. Along the way, we’ll take a short RISC-V specific detour: reading a CSR clock cycle counter to time a UART byte.
2. Building VegaConsole will wrap a busy-polling receive loop, a line buffer, and a small command table around everything to produce a nice interactive REPL.
3. Challenge asks you to add one or more new command(s) of your own.

As before, you can do most things in Renode if you don’t have a board yourself - there will be some limitations though, we’ll point them out as they come up.