Openocd Jtag

mohitmishra786/low-level-dev-skills/skills/embedded/openocd-jtag

by mohitmishra786bdc58472fa9fNo license253 starsListed Oct 9, 2026Updated Oct 9, 2026Repository updated 3 months ago

OpenOCD skill for embedded hardware debugging. Use when configuring OpenOCD for JTAG or SWD targets, flashing firmware, connecting GDB to bare-metal targets, setting hardware watchpoints, or using J-Link with OpenOCD. Activates on queries about OpenOCD, JTAG, SWD, GDB remote target, flash programming, hardware breakpoints, J-Link, or connecting GDB to an MCU.

Instructions onlySoftware Development
AI-generated overview

Guides configuring OpenOCD for JTAG/SWD targets, flashing firmware, and attaching GDB for bare-metal debugging.

What it does
This skill provides instructions for configuring OpenOCD against JTAG and SWD targets, including interface and target config files for CMSIS-DAP, ST-Link, J-Link, and common MCUs. It covers flashing firmware via OpenOCD telnet, GDB, or script mode, connecting GDB to the OpenOCD remote target, and setting hardware breakpoints and watchpoints. It also lists common OpenOCD commands and error/fix tables, plus J-Link and J-Link GDB Server setup.
When to use it
Use it when setting up OpenOCD for JTAG or SWD debugging, flashing firmware to a microcontroller, attaching GDB to a bare-metal target, setting hardware watchpoints, or configuring J-Link or CMSIS-DAP adapters. It also helps when diagnosing OpenOCD connection and flash errors.
Requirements
Instructions only; no scripts are shipped. It assumes OpenOCD, a GDB build for the target (for example arm-none-eabi-gdb), and a hardware debug adapter such as CMSIS-DAP, ST-Link, or J-Link, plus the relevant OpenOCD interface and target config files.

OpenOCD / JTAG Debugging

Purpose

Guide agents through configuring OpenOCD for JTAG and SWD targets, flashing firmware to microcontrollers, attaching GDB for bare-metal debugging, setting hardware watchpoints, and configuring J-Link and CMSIS-DAP adapters.

Triggers

  • "How do I connect GDB to my MCU with OpenOCD?"
  • "How do I flash firmware using OpenOCD?"
  • "How do I set up J-Link with OpenOCD?"
  • "What's the difference between JTAG and SWD?"
  • "How do I set a hardware watchpoint in GDB?"
  • "OpenOCD says 'Error: unable to find JTAG device' — how do I fix it?"

Workflow

1. JTAG vs SWD

FeatureJTAGSWD (Serial Wire Debug)
Pins4+ (TCK, TMS, TDI, TDO, TRST)2 (SWCLK, SWDIO)
Multi-targetYes (daisy chain)No (one target)
SpeedUp to 30 MHzUp to 10 MHz
AvailabilityFull JTAG: Cortex-A/R, RISC-VSWD: Cortex-M only
Cable costMore complexSimpler 2-wire

Most Cortex-M microcontrollers support both. Use SWD when pin count is limited.

2. OpenOCD configuration

tcl
# openocd.cfg — CMSIS-DAP (ST-Link v2, DAPLink)source [find interface/cmsis-dap.cfg]source [find target/stm32f4x.cfg]adapter speed 4000              # kHz
# For SWD explicitlytransport select swd
# J-Link adaptersource [find interface/jlink.cfg]jlink serial 123456789          # optional: select by serialsource [find target/nrf52.cfg]adapter speed 8000
bash
# Run OpenOCD (keeps running, serves GDB on port 3333)openocd -f openocd.cfg
# Common interface config filesls $(openocd --help 2>&1 | grep "scripts" | head -1)/interface/# cmsis-dap.cfg, jlink.cfg, ftdi/olimex-arm-usb-ocd.cfg, stlink.cfg ...
# Common target config filesls $(openocd --help 2>&1 | grep "scripts" | head -1)/target/# stm32f4x.cfg, nrf52.cfg, esp32.cfg, rp2040.cfg, at91sam4s.cfg ...

3. Connecting GDB

bash
# In terminal 1: start OpenOCDopenocd -f openocd.cfg
# In terminal 2: start GDBarm-none-eabi-gdb firmware.elf
# GDB commands(gdb) target extended-remote :3333   # connect to OpenOCD(gdb) monitor reset halt              # reset and halt target(gdb) load                            # flash ELF to target(gdb) monitor reset init             # re-initialize after flash(gdb) break main                      # set software breakpoint(gdb) continue
# One-liner for quick debuggingarm-none-eabi-gdb -ex "target extended-remote :3333" \                  -ex "monitor reset halt" \                  -ex "load" \                  -ex "break main" \                  -ex "continue" \                  firmware.elf

4. Flashing firmware

bash
# Flash via OpenOCD telnet interfacetelnet localhost 4444
# OpenOCD telnet commands> reset halt> program firmware.elf verify reset> exit
# Or via GDB(gdb) monitor flash write_image erase firmware.bin 0x08000000(gdb) monitor reset run
# Flash only (no debug) — script modeopenocd -f openocd.cfg \        -c "program firmware.elf verify reset exit"
# For raw binaryopenocd -f openocd.cfg \        -c "program firmware.bin 0x08000000 verify reset exit"

5. Hardware breakpoints and watchpoints

Software breakpoints (break) patch instruction memory with trap instructions — they don't work in flash-execute-in-place without debug registers. Use hardware breakpoints instead:

bash
# Hardware breakpoint (uses debug register, limited count: 4-8 on Cortex-M)(gdb) hbreak function_name(gdb) hbreak *0x08001234
# Hardware watchpoint — triggers on memory read/write(gdb) watch global_variable         # write watchpoint(gdb) rwatch some_buffer            # read watchpoint(gdb) awatch sensor_value           # read OR write watchpoint
# List breakpoints/watchpoints(gdb) info breakpoints
# Cortex-M typically has: 4–8 hardware breakpoints, 2–4 watchpoints

6. OpenOCD commands reference

bash
# Via telnet (port 4444) or GDB monitormonitor reset halt          # reset and hold at reset vectormonitor reset init          # reset and run init scriptsmonitor reset run           # reset and run freelymonitor halt                # halt running targetmonitor resume              # resume executionmonitor mdw 0x20000000      # memory display word at addressmonitor mww 0x40021000 0x1  # memory write wordmonitor reg r0              # read registermonitor arm disassemble 0x08000000 16  # disassemble 16 instructions
# Flash operationsmonitor flash listmonitor flash erase_sector 0 0 0   # erase sector 0monitor flash write_bank 0 firmware.bin 0

7. Common errors

ErrorCauseFix
unable to find JTAG deviceWrong interface, cable, or powerCheck USB connection; power target; verify interface config
JTAG scan chain interrogation failedWrong target config or bad SWD modeMatch config to exact MCU; try transport select swd
flash 'stm32f4x' is not supportedWrong targetCheck MCU part number; use correct .cfg
Error: timed out while waiting for target haltedTarget running, not haltedmonitor halt; check BOOT pins
Cannot access memory at addressMMU, memory protection, or wrong addressCheck MPU config; use correct flash/RAM address
Warn: target not examined yetOpenOCD not connected to targetCheck power and connections

8. J-Link configuration

tcl
# openocd.cfg for J-Linksource [find interface/jlink.cfg]jlink usb 0          # use first J-Link foundtransport select swd  # or jtagadapter speed 4000
source [find target/stm32l4x.cfg]

J-Link GDB Server (alternative to OpenOCD):

bash
# Segger's own GDB serverJLinkGDBServer -if SWD -device STM32L476RG -port 3333 &arm-none-eabi-gdb -ex "target remote :3333" firmware.elf

Related skills

  • Use skills/embedded/freertos for FreeRTOS-aware debugging with OpenOCD
  • Use skills/embedded/zephyr for Zephyr's west debug integration
  • Use skills/debuggers/gdb for GDB session management
  • Use skills/embedded/linker-scripts for understanding memory map for flash addresses

Source and attribution

Source:mohitmishra786/low-level-dev-skillsinskills/embedded/openocd-jtagat commitbdc5847

License: No license

Content belongs to its original authors. SourceWeft indexes it from a public repository.

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