Running the Program

Setting the log level

Enter the appropriate command into the terminal you're using. This will set the log level for this session.

MacOS & Linux

export DEFMT_LOG=warn

PowerShell

$Env:DEFMT_LOG = "warn"

Windows Command Prompt

set DEFMT_LOG=warn

Inside VS Code

To get VS Code to pick up the environment variable, you can either:

set it as above and then open VS Code from inside the terminal (ensuring it wasn't already open and hence just getting you a new window on the existing process), or
add it to your rust-analyzer configuration, by placing this in your settings.json file:
```
"rust-analyzer.runnables.extraEnv": {
    "DEFMT_LOG": "warn"
}
```
This will ensure the variable is set whenever rust-analyzer executes cargo run for you.

Running from VS Code

✅ Open the nrf52-code/radio-app/src/bin/hello.rs file, go to the "Run and Debug" button on the left, and then click the "Run" triangle next to Debug Microcontroller.

Note: you will get the "Run" button if the Rust analyzer's workspace is set to the nrf52-code/radio-app folder. This will be the case if the current folder in VS code (left side panel) is set to nrf52-code/radio-app.

If you are not using VS code, you can run the program out of your console. Enter the command cargo run --bin hello from within the nrf52-code/radio-app folder. Rust Analyzer's "Run" button is a short-cut for that command.

Expected output

NOTE: Recent version of the nRF52840-DK have flash-read-out protection to stop people dumping the contents of flash on an nRF52 they received pre-programmed, so if you have problems immediately after first plugging your board in, see this page.

If you run into an error along the lines of "Debug power request failed" retry the operation and the error should disappear.

$ cargo run --bin hello
   Compiling radio_app v0.0.0 (/Users/jonathan/Documents/ferrous-systems/rust-exercises/nrf52-code/radio-app)
    Finished dev [optimized + debuginfo] target(s) in 0.28s
     Running `probe-rs run --chip nRF52840_xxAA target/thumbv7em-none-eabihf/debug/hello`
     Erasing sectors ✔ [00:00:00] [######################################################] 8.00 KiB/8.00 KiB @ 26.71 KiB/s (eta 0s )
 Programming pages   ✔ [00:00:00] [######################################################] 8.00 KiB/8.00 KiB @ 29.70 KiB/s (eta 0s )    Finished in 0.59s
Hello, world!
`dk::exit()` called; exiting ...

What just happened?

cargo run will compile the application and then invoke the probe-rs tool with its final argument set to the path of the output ELF file.

The probe-rs tool will

flash (load) the program on the microcontroller
reset the microcontroller to make it execute the new program
collect logs from the microcontroller and print them to the console
print a backtrace of the program if the halt was due to an error.

Should you need to configure the probe-rs invocation to e.g. flash a different microcontroller you can do that in the .cargo/config.toml file.

[target.thumbv7em-none-eabihf]
runner = "probe-rs run --chip nRF52840_xxAA" # <- add/remove/modify flags here
# ..

🔎 How does flashing work?

The flashing process consists of the PC communicating with a second microcontroller on the nRF52840 DK over USB (J2 port). This second microcontroller, which is a J-Link Arm Debug Probe, is connected to the nRF52840 through a electrical interface known as SWD (Serial Wire Debug). The SWD protocol specifies procedures for reading memory, writing to memory, halting the target processor, reading the target processor registers, etc.

🔎 How does logging work?

Logging is implemented using the Real Time Transfer (RTT) protocol. Under this protocol the target device writes log messages to a ring buffer stored in RAM; the PC communicates with the J-Link to read out log messages from this ring buffer. This logging approach is non-blocking in the sense that the target device does not have to wait for physical IO (USB comm, serial interface, etc.) to complete while logging messages since they are written to memory. It is possible, however, for the target device to run out of space in its logging ring buffer; this causes old log messages to be overwritten or the microcontroller to pause whilst waiting for the PC to catch up with reading messages (depending on configuration).

Rust Exercises