nrf_ocd — C reimplementation of pyOCD for nRF54L15 / nRF54LM20A

nrf_ocd is a compact C implementation of CMSIS-DAP programming targeted at the Seeed XIAO nRF54L15 (VID=0x2886 PID=0x0066) and Seeed XIAO nRF54LM20A (VID=0x2886 PID=0x0068) boards. Its nRF54L target, flash-algorithm, and CMSIS-DAP v2 USB implementations include attributed adaptations from pyOCD. It supports Linux, macOS, and Windows.

Status

Feature	Status	Notes
Probe enumeration	✅ Works	Reads serial, product, manufacturer
DAP_Info / DAP_Connect / SWD	✅ Works
DP/AP register read/write	✅ Works
Memory read/write via AHB-AP	✅ Works	32-bit, auto-increment
Flash mass-erase via CTRL-AP	✅ Works
Flash programming via flash algorithm	✅ Works	~14 kB/s with --no-verify on XIAO nRF54LM20A
Flash verify (read-back)	✅ Works	Word-by-word verification, slower but robust
Commander REPL	✅ Works	Interactive memory debug
ELF + Intel HEX loader	✅ Works
Both targets (L15 & LM20A)	✅ Works	Auto-selects flash controller
libusb bulk backend	✅ Works	Recommended for speed

Building

Prerequisites

• Linux: libusb-1.0 (for bulk backend, recommended)
• macOS: IOKit framework (included with Xcode)
• Windows: hid.dll (included with Windows)

Build with libusb bulk backend (recommended)

make USE_LIBUSB=1 CC=/usr/bin/gcc-13
LD_LIBRARY_PATH=/usr/lib/x86_64-linux-gnu ./build/bin/nrf_ocd list

Build with HID backend (no dependencies)

make
./build/bin/nrf_ocd list

The bulk backend uses USB bulk transfers (CMSIS-DAP v2) which are significantly faster than HID. On Seeed XIAO nRF54 boards the HID interface has truncated response issues, so the bulk backend is strongly recommended.

Usage

# List CMSIS-DAP probes
nrf_ocd list

# Show target info (reads UICR.PARTNO)
nrf_ocd -t nrf54l15 -u 761FDE87 info

# Program flash with chip erase, reset, and readback verification
nrf_ocd -t nrf54l15 -u 761FDE87 -e chip -R load firmware.hex

# Fast upload path used by the Arduino core
nrf_ocd -t nrf54l15 -u 761FDE87 -e chip -R --no-verify load firmware.hex

# Mass erase the chip
nrf_ocd -t nrf54l15 -u 761FDE87 erase

# Reset target
nrf_ocd -t nrf54l15 -u 761FDE87 reset

# Read 16 bytes from UICR.PARTNO
nrf_ocd -t nrf54l15 -u 761FDE87 read 0x00FFC31C 16

# Write to RAM (4 bytes)
nrf_ocd -t nrf54l15 -u 761FDE87 write 0x20000000 DEADBEEF

# Interactive memory REPL
nrf_ocd -t nrf54l15 -u 761FDE87 commander

Example output

$ nrf_ocd -t nrf54l15 -u 761FDE87 load firmware.hex
Loaded 1 segment(s), 22764 bytes total
Erasing chip...
Erase complete
Programming 22764 byte(s) in 1 segment(s)
  ... 6144 / 22764 bytes (27.0%)
  ... 12288 / 22764 bytes (54.0%)
  ... 18432 / 22764 bytes (81.0%)
Programmed 22764 bytes in 1 segment(s)
Upload complete

Targets

Target	Board	VID:PID	Flash	RAM	Flash Controller
nrf54l15	XIAO nRF54L15	2886:0066	1.5 MB	256 KB	NVMC @ 0x5004B000
nrf54lm20a	XIAO nRF54LM20A	2886:0068	2036 KB	512 KB	RRAMC @ 0x5004E000

Both share the same CTRL-AP-based mass-erase sequence and flash algorithm. The flash controller is selected automatically based on the target type.

Architecture

Flash programming uses a flash algorithm — position-independent ARM Thumb code loaded into the target's RAM and executed via core register manipulation (DCRSR/DCRDR). This is the same approach pyOCD uses and is required because the nRF54L15's NVMC does not support direct AHB-AP writes to flash addresses.

Key files

src/
  main.c                - Entry point
  cli.c                 - CLI (pyOCD-compatible syntax)
  probe.c               - USB probe enumeration
  hid.h                 - Portable HID abstraction
  hid_linux.c           - /dev/hidraw backend
  hid_macos.c           - IOKit HID backend (macOS)
  hid_windows.c         - hid.dll backend (Windows)
  hid_libusb.c          - libusb bulk backend (USE_LIBUSB=1)
  cmsis_dap.c           - CMSIS-DAP v1/v2 protocol
  swd.c                 - SWD line-level helpers
  dap.c                 - DP/AP register + memory access
  target.c              - Target base class
  target_nrf54l.c       - nRF54L15 target
  target_nrf54lm20a.c   - nRF54LM20A target
  flash.c               - Flash programming engine
  flash_algo_nrf54l.c   - Flash algorithm (runs on target CPU)
  flash_algo_nrf54l.h   - Flash algorithm metadata
  commander.c           - pyOCD-style commander REPL
  hex.c                 - Intel HEX parser
  elf.c                 - ELF parser
  log.c                 - Leveled logger
  util.c                - Endian, hex dump, time helpers
tests/
  test_hex.c            - HEX parser tests
  test_elf.c            - ELF parser tests
  test_target.c         - Target type tests

Tests

make test

Runs unit tests for the Intel HEX parser, ELF parser and target type lookup.

Arduino Core

This tool is the default uploader for the nRF54L15 Clean Arduino Core. Install the core and use Tools → Upload Method → nRF OCD (Native) to flash via nrf_ocd without any Python dependency.

arduino-cli core update-index && arduino-cli core install nrf54l15clean:nrf54l15clean@1.0.0
nrf_ocd -p /dev/ttyACM0 -t nrf54lm20a -e chip -R --no-verify load sketch.hex

Known limitations

• Speed: The fast Arduino upload path uses --no-verify and was measured at about 14 kB/s on XIAO nRF54LM20A with a 104 KB image. Full readback verification is still supported with --verify, but remains slower because the safe verifier uses conservative word reads.
• DAP_TransferBlock on HID: The SAMD11 USB bridge truncates HID responses to 63 bytes. The bulk backend (USE_LIBUSB=1) avoids this issue. When using HID, the code tolerates truncated responses.

License

Project-owned source is available under the MIT License. The target, flash-algorithm, and libusb transport files identified in Third-Party Notices retain pyOCD material under Apache-2.0. Component-specific terms take precedence over the project license.

Release Compliance

Linux x86-64 release binaries use a statically linked libusb 1.0.27 and are therefore not MIT-only artifacts. Releases that contain that binary must also provide the exact libusb corresponding source, LGPL-2.1 text, relink instructions, application source, and checksums. macOS, Windows, and Linux ARM builds use native HID/WinUSB unless their release notes state otherwise.

See the release assets and Third-Party Notices before redistributing a binary.

Current Linux x86-64 releases include version-matched source and exact libusb source assets. To rebuild with the supplied or modified library, follow the relink procedure.

Maintainers reconstructing the historical v0.3.3 compliance bundle can use:

./scripts/create_v0.3.3_compliance_bundle.sh

Support The Project

If nRF OCD saves you development time, please consider supporting its ongoing maintenance, hardware testing, and release work.