Debugging with CMSIS-DAP

This tutorial covers setting up hardware debugging for NanoVNA-H firmware development using a CMSIS-DAP debug probe and OpenOCD.

What You Will Learn

• Connecting a CMSIS-DAP debug probe
• Configuring OpenOCD for NanoVNA-H
• Using GDB for firmware debugging
• Setting breakpoints and inspecting variables

Prerequisites

• Build Environment Setup completed
• Building and Flashing working
• A CMSIS-DAP compatible debug probe

Debug Probe Options

CMSIS-DAP is a standard debug interface. Compatible probes include:

Probe	Notes
DAP-Link	Common, inexpensive
DAPLink (official)	Open-source reference design
J-Link	Professional, also supports CMSIS-DAP mode
ST-Link v2	Can be converted to CMSIS-DAP with firmware
Black Magic Probe	Built-in GDB server, no OpenOCD needed

Tip

Inexpensive DAP-Link probes are available for under $10 and work well for NanoVNA development.

Hardware Connection

NanoVNA-H Debug Pins

The NanoVNA-H has SWD (Serial Wire Debug) pads on the PCB:

Pin	Function	Connect To
SWDIO	Data	Probe SWDIO
SWCLK	Clock	Probe SWCLK
GND	Ground	Probe GND
3V3	Power (optional)	Probe VRef

Caution

The debug pads are small and may require soldering wires. Work carefully to avoid shorts.

Connection Diagram

NanoVNA-H                Debug Probe
----------               -----------
  SWDIO  <------------->  SWDIO
  SWCLK  <------------->  SWCLK
  GND    <------------->  GND
  3V3    <--- optional ->  VRef

1. Locate the debug pads

   Look for small test points or pads labeled SWD, SWDIO, SWCLK on the NanoVNA PCB.

2. Solder connection wires

   Use thin wire (30 AWG recommended) to connect the debug pads to your probe.

3. Connect grounds first

   Always connect GND before other signals.

4. Verify connections

   Use a multimeter to check for shorts between SWDIO, SWCLK, and GND.

Installing OpenOCD

Linux (Debian/Ubuntu)

sudo apt install openocd

Linux (Arch)

sudo pacman -S openocd

macOS

brew install openocd

Windows

Download from OpenOCD releases or use Chocolatey:

choco install openocd

Verify installation:

openocd --version

OpenOCD Configuration

The NanoVNA-H repository includes a configuration file: NanoVNA_DAP.cfg

If not present, create it with this content:

NanoVNA-H (F072)

# NanoVNA-H (STM32F072) OpenOCD configuration

source [find interface/cmsis-dap.cfg]
transport select swd

source [find target/stm32f0x.cfg]

adapter speed 4000

# Reset configuration
reset_config srst_nogate

init
targets

NanoVNA-H4 (F303)

# NanoVNA-H4 (STM32F303) OpenOCD configuration

source [find interface/cmsis-dap.cfg]
transport select swd

source [find target/stm32f3x.cfg]

adapter speed 4000

# Reset configuration
reset_config srst_nogate

init
targets

Starting OpenOCD

1. Connect the debug probe

   Connect the probe to your computer via USB and to the NanoVNA via SWD.

2. Power the NanoVNA

   Connect the NanoVNA via USB or battery.

3. Start OpenOCD

   openocd -f NanoVNA_DAP.cfg

4. Verify connection

   Successful output:

   Open On-Chip Debugger 0.11.0
   Info : CMSIS-DAP: SWD supported
   Info : CMSIS-DAP: Atomic commands supported
   Info : CMSIS-DAP: FW Version = 1.0
   Info : SWCLK/TCK = 1 SWDIO/TMS = 1 TDI = 0 TDO = 0 nRSET = 1
   Info : CMSIS-DAP: Interface ready
   Info : clock speed 4000 kHz
   Info : SWD DPIDR 0x0bb11477
   Info : stm32f0x.cpu: hardware has 4 breakpoints, 2 watchpoints
   Info : starting gdb server for stm32f0x.cpu on 3333

OpenOCD now runs a GDB server on port 3333.

Note

Leave OpenOCD running in this terminal. Open a new terminal for GDB.

Using GDB

Starting GDB

1. Open a new terminal

2. Navigate to build directory

   cd /path/to/NanoVNA-H

3. Start GDB with the ELF file

   arm-none-eabi-gdb build/H.elf

4. Connect to OpenOCD

   In GDB:

   (gdb) target extended-remote localhost:3333

5. Reset and halt the target

   (gdb) monitor reset halt

Common GDB Commands

Command	Description
target extended-remote localhost:3333	Connect to OpenOCD
monitor reset halt	Reset and stop at beginning
monitor reset run	Reset and run
load	Flash the firmware (alternative to dfu-util)
continue or c	Continue execution
break main	Set breakpoint at main()
break ui.c:100	Set breakpoint at line 100 of ui.c
step or s	Step into function
next or n	Step over function
print variable	Print variable value
info registers	Show CPU registers
backtrace or bt	Show call stack
quit or q	Exit GDB

Example Debug Session

(gdb) target extended-remote localhost:3333
Remote debugging using localhost:3333
0x08001234 in main () at main.c:150

(gdb) monitor reset halt
target halted due to debug-request, current mode: Thread

(gdb) break sweep
Breakpoint 1 at 0x8002468: file main.c, line 256.

(gdb) continue
Continuing.
Breakpoint 1, sweep () at main.c:256
256     static bool sweep(bool break_on_operation, uint16_t ch_mask) {

(gdb) print sweep_points
$1 = 101

(gdb) backtrace
#0  sweep () at main.c:256
#1  0x08001a34 in Thread1 () at main.c:243
#2  0x08000f90 in _port_thread_start ()

(gdb) continue

Flashing via Debug Probe

You can flash firmware without entering DFU mode:

(gdb) monitor reset halt
(gdb) load
Loading section .text, size 0x1c234 lma 0x8000000
Loading section .data, size 0x2e4 lma 0x801c234
Start address 0x8000000, load size 115992
Transfer rate: 42 KB/sec, 9666 bytes/write.
(gdb) monitor reset run

This is faster for iterative development than DFU mode.

IDE Integration

VS Code

1. Install Cortex-Debug extension

   Search for “Cortex-Debug” in VS Code extensions.

2. Create launch.json

   Create .vscode/launch.json:

   {
     "version": "0.2.0",
     "configurations": [
       {
         "name": "NanoVNA-H Debug",
         "type": "cortex-debug",
         "request": "launch",
         "servertype": "openocd",
         "cwd": "${workspaceFolder}",
         "executable": "${workspaceFolder}/build/H.elf",
         "configFiles": [
           "${workspaceFolder}/NanoVNA_DAP.cfg"
         ],
         "svdFile": "${workspaceFolder}/STM32F0x2.svd",
         "runToEntryPoint": "main",
         "showDevDebugOutput": "raw"
       }
     ]
   }

3. Download SVD file (optional)

   SVD files provide register definitions for the Peripherals view. Download from STM32 SVD files.

4. Start debugging

   Press F5 or click the Debug icon.

Eclipse

Similar configuration is possible with Eclipse CDT and the GNU MCU Eclipse plugins.

Troubleshooting

”Error: unable to find a matching CMSIS-DAP device”

• Check USB connection
• Verify probe appears in lsusb (Linux) or Device Manager (Windows)
• Try different USB port
• Check probe firmware

”Error: connect failed”

• Verify SWD connections (especially GND)
• Check for shorts
• Ensure NanoVNA is powered
• Try reducing adapter speed in config file

”Error: target not halted”

• The MCU may be running and ignoring halt request
• Try monitor reset halt before other commands
• Check reset line connection if available

Breakpoints not working

• The F072 has only 4 hardware breakpoints
• Software breakpoints require writable flash (not always possible)
• Remove unused breakpoints

Variables show as “optimized out”

• The default build uses -O2 optimization

• For debugging, edit Makefile to use -O0:

  USE_OPT = -O0 -ggdb ...

• Rebuild and reflash

Advanced Topics

Semihosting

Enable printf output via the debug probe:

(gdb) monitor arm semihosting enable

Requires semihosting support in firmware (not enabled by default).

Real-time Tracing

The F303 (NanoVNA-H4) supports ITM/SWO tracing for non-intrusive debugging. This requires additional hardware setup.

Watchpoints

Monitor memory access:

(gdb) watch measured[0][0][0]
Hardware watchpoint 2: measured[0][0][0]
(gdb) continue
Hardware watchpoint 2: measured[0][0][0]
Old value = 0.5
New value = 0.75

The F072 has 2 watchpoints, F303 has 4.

Next Steps

• Build Environment Setup - Toolchain configuration
• Building and Flashing - DFU flashing method