The probe for beginners
Never plugged a probe into a board? Here, in a few minutes, is what a debug probe is, what this one does, and why it does without a USB cable.
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1. Looking inside a running chip
The Eliott65 board is an Olimex Neo6502kbd: a small computer whose brain is an RP2040 microcontroller. When developing its software, you need two things: to install a new program on it (to “flash” it), and to look at what happens inside while it runs (read its memory, stop it, step through it).
A debug probe does that. It is a small device placed between the PC and the board: on one side, it receives orders from software on the PC, here OpenOCD; on the other, it translates them into electrical signals on two debug pins of the chip.
2. Two languages: SWD and CMSIS-DAP
- SWD (Serial Wire Debug) is the chip’s language: two wires, a clock (SWCLK) and a data line (SWDIO), through which any address of the chip can be read and written, even while it runs.
- CMSIS-DAP is the language between the PC and the probe: commands such as “read this word” or “write this block”, which OpenOCD knows how to send.
The probe is therefore a translator: it receives CMSIS-DAP commands and carries them out in SWD.
3. What changes: Wi-Fi instead of USB
Usually, the probe is connected to the PC by a USB cable. This one is a Raspberry Pi Pico W, which
has a Wi-Fi chip: it joins the home network, and OpenOCD reaches it over the local network. All you
need is the probe’s name, eliott65sonde.local.
4. A probe that works on its own
The probe can read a list of the board’s addresses by itself, at a fixed interval, and store the values in its own memory. It can stop when a value no longer moves, or reaches a chosen value, keeping a copy of the memory areas of interest at that moment. The PC then reads everything back in one go.
This is useful for a fault that only appears after several minutes: no need for an attentive PC during all that time.
5. And to change it?
The probe updates itself over the network, without unplugging it. If the new firmware does not work, it goes back on its own to the old one. See Updates.
In short
| Question | Short answer |
|---|---|
| What is it? | A debug probe for the Eliott65 board, on a Pico W. |
| What does it replace? | A probe connected over USB; this one goes over Wi-Fi. |
| With which software? | OpenOCD, and the project’s e65s.py tool for the log and updates. |
| Does it work? | Yes, on the board: OpenOCD connects to it, and Eliott65 was flashed over Wi-Fi. |
| What next? | See Where the probe stands. |
Technical words are explained in the glossary.