RTT and defmt

RTT moves data between the target and the host through ring buffers in the target's RAM, read by the probe while the firmware runs. defmt is a compact logging format on top of it. probe-web decodes defmt in the browser, from the firmware's ELF.

Monitor a running target

// Also on `@probe-web/client/elf`, which loads no wasm if a symbol lookup is all you need.
import { elfHasRtt } from '@probe-web/client';

if (elfHasRtt(elf)) {
  // The ELF's _SEGGER_RTT symbol gives the exact address, so no RAM scan is needed.
  await session.createRttClient({ elf, defaults: { dataFormat: 'Defmt' } });
  session.setDefmtElf(elf); // the defmt string table
}

const boot = await session.flash({ image: elf, format: 'elf' });
const exit = await session.monitor(boot, (event) => {
  switch (event.kind) {
    case 'rtt-discovered': console.log('channels', event.up, event.down); break;
    case 'text':           console.log(event.channel, event.text); break;
    case 'defmt':          event.lines.forEach((l) => console.log(l.level, l.message, l.location)); break;
    case 'bytes':          plot(event.channel, event.bytes); break;
    case 'semihosting':    console.log(event.stream, event.data); break;
  }
});

monitor(boot, …) resets into the image and streams output until you call session.cancel(), the firmware exits through semihosting, or the core stops. It resolves with the reason. To attach to firmware that is already running without resetting it, pass 'attach' instead of the boot info.

Call createRttClient before flashing if the image contains the control block, so the flash does not leave a stale one behind.

Channel formats

Each up channel has a data format, set through defaults or per channel:

await session.createRttClient({
  elf,
  channels: [
    { channelNumber: 0, dataFormat: 'Defmt' },
    { channelNumber: 1, dataFormat: 'BinaryLE' }, // raw bytes: 'bytes' events
  ],
});

Format	Events	Use
String (default)	text	Plain logging, rtt-target's rprintln!
Defmt	defmt	defmt logging; needs setDefmtElf
BinaryLE	bytes	Samples, telemetry, anything not text

Send data to the target

Down channels carry data to the firmware:

await session.rttWrite(0, 'hello\n');

<probe-rtt-terminal> sends what the user types to down channel 0.

While debugging

With a Debugger, RTT keeps flowing across halts:

await dbg.enableRtt({ elf, channels: [{ channelNumber: 1, dataFormat: 'BinaryLE' }] });
dbg.addEventListener('output', (e) => console.log((e as CustomEvent).detail));    // text
dbg.addEventListener('rtt-bytes', (e) => console.log((e as CustomEvent).detail)); // binary channels

Plot a binary channel

SampleDecoder from @probe-web/ui turns a byte stream into numbers (u8 through f32, little-endian) across chunk boundaries. <probe-rtt-plot> draws them live:

import { SampleDecoder } from '@probe-web/ui';

const decoder = new SampleDecoder('i16');
dbg.addEventListener('rtt-bytes', (e) => chart.push(...decoder.push((e as CustomEvent).detail.bytes)));