On the Nucleo board there are two jumpers, just above the crystal of the on-board ST-Link. You need to remove them to isolate the SWD port of the ST-Link from the F446. With the jumpers in place, the on-board ST-Link will prevent using the external CMSIS-DAP.
Please reply if this fixes the issue, so we can close the ticket --- **[tickets:#465] CMSIS-DAP SWD: "SWD ack not OK @ 0 JUNK" and "Error connecting DP: cannot read IDR" ** **Status:** new **Milestone:** 0.12.0 **Created:** Thu Feb 12, 2026 03:48 PM UTC by Shreyas Acharya **Last Updated:** Thu Feb 12, 2026 03:48 PM UTC **Owner:** nobody **Attachments:** - [Screenshot 2026-02-12 211553.png](https://sourceforge.net/p/openocd/tickets/465/attachment/Screenshot%202026-02-12%20211553.png) (47.4 kB; image/png) Hi, I’m trying to program/debug an STM32F446RE (Nucleo-F446RE target MCU) using a CMSIS-DAPv2 probe (DAP103 Bluepill CMSIS-DAP). OpenOCD fails to connect over SWD. Symptoms: - Repeated: "SWD ack not OK @ 0 JUNK" - Final: "Error connecting DP: cannot read IDR" What works: - Flashing the same firmware via onboard ST-LINK works (program + verify OK). What fails: - Using CMSIS-DAP from OpenOCD/PlatformIO. Earlier I also saw OpenOCD start without interface cfg and error: "Can't find openocd.cfg" and "Debug Adapter has to be specified" until I forced the command line. OpenOCD version: - xPack OpenOCD 0.12.0-01004-g9ea7f3d64-dirty (2023-01-30) Command used (direct OpenOCD): openocd -d4 -f interface/cmsis-dap.cfg -f target/stm32f4x.cfg -c "transport select swd; adapter speed 100; init; reset halt; exit" Platform/OS: - Windows (MSYS2/MINGW64 terminal) Wiring: - CMSIS-DAP GND -> target GND - SWDIO -> SWDIO - SWCLK -> SWCLK - NRST -> NRST (optional / please advise) - VTref/VTarget -> 3.3V (if required by CMSIS-DAP) Notes/questions: 1) Could ST-LINK be contending on SWD lines on Nucleo-F446RE when using an external probe? If yes, what’s the recommended way to disconnect ST-LINK from the target? 2) Any recommended OpenOCD settings for CMSIS-DAP (connect_under_reset/reset_config, lower speed, etc.)? Full log (debug_level 4) attached below: --- Sent from sourceforge.net because [email protected] is subscribed to https://sourceforge.net/p/openocd/tickets/ To unsubscribe from further messages, a project admin can change settings at https://sourceforge.net/p/openocd/admin/tickets/options. Or, if this is a mailing list, you can unsubscribe from the mailing list.
