The Raspberry Pi Foundation has a new gadget: a cheap, easy USB probe for debugging bare-metal code on a Pi Pico… but it should work with several other devices too.
The Debug Probe is a device to help debug bare-metal software on the Raspberry Pi Pico. The Pico is the Foundation's $4 single-board computer for microcontroller type roles. In embedded computers like this, there's often no display output, and in typical use, no operating system – both of which make it tricky to monitor what it's doing, or more to the point, work out why it's not doing it.
A common way to peer into the memory of an embedded Arm device and work out what it's doing is Arm's SWD interface, or Serial Wire Debug – broadly, Arm's three-wire version of the traditional four- or five-wire JTAG interface.
However, to use those SWD signals, you need a device to read and translate them into something another computer can read and interpret, using software such as the FOSS OpenOCD or pyOCD. (In this context, OCD stands for On-Chip Debugger.) This device is called a debug probe.
Conveniently, the normal Raspberry Pi's GPIO interface can talk SWD over a few of its pins [PDF]. This means you can use a Pi to debug something else – including another Pi.
The OCD software runs on the Pi, and you connect a few wires from the Pi's GPIO interface to probe the other device. These days, you'd probably want to use a Pi 4 for this, but in principle any model can do it, down to the Pi Zero. For example, here is how to debug an STM32, and this tells you how to reprogram a Particle Xenon.
For clarity: Yes, that does even include the Pi Pico. A Pico hasn't got enough power to run a full OCD suite on the device itself, but the Pico can be used as a probe for a larger computer running the OCD software, thanks to some special firmware called Picoprobe. This is a little cheaper than a dedicated hardware debug probe, but the snag is that you'll need to wire up some circuits, as this blogpost describes.
Even if it's versatile – for instance, you can also use SWD to upload software into a Pi Pico from another Pi – it's not that easy, and if you want to use a bigger, faster computer, such as a PC or Mac, you then need to interface your Pi-based probe to that.
Which is the selling point of the new Pi Debug Probe. You plug one end into your Pi Pico, and the other end into a free USB port, and you're ready to go. It's a very low-end offering, but it should do the job – and it's $12. For comparison, a professional tool such as a Segger J-Link comes in at $60 a pop in the States and €576 for a 12-pack (€48 per unit) in the EU. Even an open-source hardware device such as an ORBtrace Mini – for instance from this store – is around 10 times the price.
Better still, the Pi Debug Probe should be able to talk to any embedded device with an Arm Cortex M series and an SWD interface, such as the Arm-powered Arduino Due, or the 32-bit variants of the Teensy development boards. These are popular for tasks like equipping vintage keyboards with a USB interface. ®