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Tool

JTAGulator

Joe Grand's open-source hardware tool and Parallax Propeller firmware for locating on-chip debug interfaces among an unfamiliar board's test points, vias, component pads or connectors. Driven from a serial terminal, its 24 level-shifted channels try pin permutations to identify JTAG pinouts by IDCODE or BYPASS scans, SWD pinouts by IDCODE and UART pinouts with their baud rates. With a known JTAG pinout the firmware can also read device IDs, discover instruction and data registers and map a chip's pins to boundary-scan positions with EXTEST, and other modes pass UART traffic through, act as a SUMP-compatible logic analyzer or emulate a Bus Pirate so OpenOCD can use the board as a JTAG adapter.

Targets
Firmware
Category
Firmware tools
Relationship
Foundation tool
Sources reviewed

Requirements & limits

  • Firmware 1.12, tagged 2023-06-05, is the newest version; the repository publishes tags but no GitHub releases, and the README calls master a development branch whose interim commits may be unstable. Later commits, up to the reviewed one of 2026-09-02, update links and the license text and add the Rev. C hardware design files, and since July 2025 the README names EXPLIoT as maintainer and supporter.
  • Requires JTAGulator hardware, for which the repository commits the Rev. C design files (Altium project, Gerbers, schematic, bill of materials, assembly drawing and test procedure), a serial terminal on the host and physical wiring to the target's candidate pins. The target I/O voltage, 1.4–3.3 V, must be set before scanning, and the firmware warns that use may affect target behavior; it also warns that its front-end circuitry is incompatible with many SWD-based targets and points to hardware modifications described in the wiki.
  • The firmware is Spin code for the Propeller P8X32A: the README recommends Propeller Tool 1.3.2 for Windows and calls other environments untested and unsupported, while also giving OpenSpin and PropLoader commands for compiling, loading to RAM or writing to EEPROM. The repository commits the compiled image JTAGulator.eeprom (32 KB), last updated for 1.12. The OpenOCD and logic-analyzer modes need OpenOCD's Bus Pirate driver or an Openbench Logic Sniffer client, and both persist across resets and firmware updates until exited manually.
  • The README distributes the design under CC BY-SA 4.0 and says the JTAGulator name and logo are registered trademarks of Grand Idea Studio that may not be used on derived works without permission; the Spin source headers still state CC BY 3.0 United States, and GitHub detects no license. No hardware was tested for this entry, and the reviewed sources establish no REA connection.

About this entry

The description was reviewed against primary sources. This does not establish runtime testing, security auditing or verified compatibility.

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Primary sources

  1. README.markdown GitHub · grandideastudio/jtagulator · Revision 6a764cd
  2. CHANGES.markdown GitHub · grandideastudio/jtagulator · Revision 6a764cd
  3. JTAGulator.spin#L2796-L2869 GitHub · grandideastudio/jtagulator · Revision 6a764cd
  4. JTAGulatorCon.spin GitHub · grandideastudio/jtagulator · Revision 6a764cd
  5. PropOCD.spin GitHub · grandideastudio/jtagulator · Revision 6a764cd
  6. PropSUMP.spin GitHub · grandideastudio/jtagulator · Revision 6a764cd
  7. Hardware/Rev.%20C GitHub · grandideastudio/jtagulator · Revision 6a764cd
  8. https://github.com/grandideastudio/jtagulator/tags
  9. https://expliot.io/jtagulator/

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