Using SWD (Serial Wire Debug) for debugging STM32 microcontrollers is a powerful way to monitor and control code execution, inspect registers, and analyze faults. Here’s a step-by-step guide to set up and use SWD effectively:

1. Hardware Setup
Required Components:
STM32 Board (e.g., STM32F4 Discovery, Nucleo, or custom PCB).

Debug Probe:

• ST-Link (built-in on Nucleo boards) or J-Link (faster, more features).
• CMSIS-DAP (open-source alternative).

Connections:

4-Wire SWD:

Note: Some boards omit VCC if powered separately.

Wiring Example:

ST-Link (V2)    →   STM32  
SWDIO           →   PA13  
SWCLK           →   PA14  
GND             →   GND  
3.3V (optional) →   VDD  

2. Software Configuration
IDE Setup (STM32CubeIDE/Keil/IAR):

1. Install Drivers:

• ST-Link drivers (from ST’s website).
• J-Link drivers (if using Segger).

2. Project Configuration:

In STM32CubeIDE:

• Right-click project → Debug As → STM32 Cortex-M C/C++ Application.
• Select ST-Link (SWD) as the debug probe.

In Keil/IAR:

Set debugger to ST-Link and interface to SWD in project options.

3. Enable Debug in Code:

• Ensure debug pins (PA13/PA14) are not accidentally configured as GPIOs in HAL_Init().
• Use __HAL_AFIO_REMAP_SWJ_DISABLE() only if SWD is needed without JTAG.

3. Basic Debugging Features
A. Breakpoints & Step Execution

• Set breakpoints by clicking the gutter in your IDE.
• Use Step Into (F5), Step Over (F6), and Resume (F8) to control flow.

B. Register/Memory Inspection

• View peripheral registers (e.g., GPIOA->ODR) in Register View.
• Monitor variables in Live Watch or Expressions tab.

C. Peripheral Debugging
Use STM32CubeMonitor to visualize ADC, PWM, or UART data in real-time.

4. Advanced Techniques
A. Serial Wire Viewer (SWV) for printf Debugging

1. Enable ITM (Instrumentation Trace Macrocell) in STM32CubeMX:

Enable Trace Asynchronous Sw under System Core > SYS.

2. Add this code to redirect printf:

c
#include <stdio.h>
int _write(int fd, char *ptr, int len) {
    ITM_SendChar(*ptr);  // Send via SWO
    return len;
}

3. View output in:

• STM32CubeIDE: Window → SWV → SWV ITM Data Console.
• Teraterm: Configure SWO viewer with correct baud rate.

B. Fault Analysis
HardFault Debugging:

• Check SCB->CFSR (Configurable Fault Status Register) for fault cause (e.g., IMPRECISERR = bus access error).
• Use STM32CubeIDE’s Fault Analyzer to decode the call stack.

C. RTOS Debugging (FreeRTOS/ThreadX)
Enable RTOS-aware debugging in IDE:

View task states, queues, and stacks in System Viewer.

5. Common Issues & Fixes

6. Tools & Alternatives
OpenOCD: Open-source debugging (useful with ST-Link/V2).

bash
openocd -f interface/stlink.cfg -f target/stm32f4x.cfg

J-Link Commander: Advanced memory access (Segger tools).

Black Magic Probe: SWD over USB-UART.

Example Workflow

1. Connect ST-Link to STM32 via SWD.
2. Load firmware and start debugging.
3. Set breakpoints in critical code (e.g., ISRs).
4. Inspect variables and peripherals.
5. Use SWV for real-time logging.

Pro Tip: Combine SWD with logic analyzers (Saleae) to debug timing-sensitive protocols (SPI/I2C) simultaneously.