# Debug probes and bench instruments

Tools for seeing inside a running chip and on the wires around it: JTAG, SWD, GDB, logic analysers and oscilloscopes.

Page: https://dharanova.com/learn/debugging-hardware · Track: Run · Level: Core · Facts last checked 6 October 2026

Read first: [Bare metal: the super-loop and interrupts](https://dharanova.com/learn/bare-metal.md)

Diagram: Top: GDB and a tool such as OpenOCD on your computer connect to a debug probe, which connects to the target chip over SWD or JTAG, giving breakpoints, single-stepping and memory inspection. SWD needs two signals, clock and data; JTAG (IEEE 1149.1) was created for board testing. Bottom left: a logic analyser records five digital lines and decodes protocols such as I²C and SPI. Bottom right: an oscilloscope shows the analog shape of a few signals: ringing after a rising edge, a slow falling edge, and noise, which a logic analyser hides.

## Key points

1. JTAG (IEEE 1149.1) was created for board testing and is widely used for debugging; Arm's Serial Wire Debug (SWD) provides debug access with just two signals, clock and data. [1][2]
2. Open-source tools such as OpenOCD connect a debug probe to GDB, giving breakpoints, single-stepping and memory inspection on the target chip. [3]
3. A logic analyser records many digital lines and decodes protocols such as I²C and SPI; an oscilloscope shows the analog shape of a few signals, revealing ringing, slow edges and noise that a logic analyser hides. [4][5]

## Sources

1. [IEEE 1149.1-2013: Standard for Test Access Port and Boundary-Scan Architecture](https://standards.ieee.org/ieee/1149.1/4484/), IEEE
2. [Arm Debug Interface Architecture Specification ADIv5.0 to ADIv5.2 (IHI 0031E)](https://documentation-service.arm.com/static/5f900b1af86e16515cdc0642), Arm
3. [OpenOCD User's Guide](https://openocd.org/doc/html/index.html), OpenOCD project
4. [sigrok: Protocol decoders](https://sigrok.org/wiki/Protocol_decoders), sigrok project
5. [Logic Analyzer vs Oscilloscope](https://www.keysight.com/used/us/en/knowledge/guides/logic-analyzer-vs-oscilloscope), Keysight Technologies