SWO (Serial Wire Output) for printf in ARM Architecture - Mini Project: Build & Apply

SWO (Serial Wire Output) for printf

📖 Scenario: You are working with an ARM microcontroller and want to use the Serial Wire Output (SWO) feature to send printf debug messages to your computer. This helps you see debug information without using a full UART connection.

🎯 Goal: Build a simple setup that enables SWO output for printf debugging on an ARM microcontroller. You will create the necessary configuration and code steps to send debug messages via SWO.

📋 What You'll Learn

Create a variable to hold the SWO port number

Set up the SWO clock speed configuration

Write the core logic to send a character via SWO

Complete the setup by enabling the SWO output and connecting it to printf

💡 Why This Matters

🌍 Real World

SWO is used in embedded development to send debug messages from ARM microcontrollers without needing extra UART pins. It simplifies debugging and reduces hardware requirements.

💼 Career

Embedded software engineers often use SWO for efficient debugging. Understanding SWO setup and usage is important for developing and troubleshooting ARM-based embedded systems.

1

DATA SETUP: Define the SWO port number

Create a variable called swo_port and set it to the integer 0. This represents the SWO port number used for output.

ARM Architecture

# Define the SWO port number
# Your code here

Hint

The SWO port number is usually 0 for standard debug output.

2

CONFIGURATION: Set the SWO clock speed

Create a variable called swo_clock_speed and set it to 8000000 to represent an 8 MHz clock speed for the SWO interface.

ARM Architecture

int swo_port = 0;
// Set the SWO clock speed
# Your code here

Hint

The SWO clock speed must match your microcontroller's debug clock frequency.

3

CORE LOGIC: Write a function to send a character via SWO

Write a function called swo_send_char that takes a char c parameter and sends it via SWO. Inside the function, use a loop to wait until the SWO port is ready, then write the character to the SWO port register ITM->PORT[swo_port].

ARM Architecture

int swo_port = 0;
int swo_clock_speed = 8000000;

// Write the swo_send_char function
# Your code here

Hint

The function waits until the SWO port is ready before sending the character.

4

COMPLETION: Enable SWO output and connect to printf

Add code to enable the SWO output by setting the ITM->TER register for swo_port and enable the trace in the Debug Exception and Monitor Control Register (DEMCR). Then, connect the printf output to use swo_send_char by implementing the fputc function.

ARM Architecture

int swo_port = 0;
int swo_clock_speed = 8000000;

void swo_send_char(char c) {
    while (ITM->PORT[swo_port].u32 == 0) {
        // wait until port is ready
    }
    ITM->PORT[swo_port].u8 = (uint8_t)c;
}

// Enable SWO output and connect printf
# Your code here

Hint

Enabling the trace and ITM registers allows SWO output. Overriding fputc connects printf to SWO.

Practice

1. What is the main purpose of using SWO (Serial Wire Output) on ARM Cortex-M devices?

easy

A. To power the device externally

B. To increase the device clock speed

C. To send debug output using a single wire

D. To store data in flash memory

Solution

1. Step 1: Understand SWO function

   SWO is designed to send debug information through a single wire connection.

2. Step 2: Compare with other options

   Increasing clock speed, powering device, or storing data are unrelated to SWO's purpose.

3. Final Answer:

   To send debug output using a single wire -> Option C

4. Quick Check:

   SWO = single-wire debug output [OK]

Hint: SWO is about debug output on one wire [OK]

Common Mistakes:

• Confusing SWO with power or memory functions
• Thinking SWO increases device speed
• Assuming SWO stores data

2. Which of the following is required to redirect printf output to SWO on an ARM Cortex-M device?

easy

A. Configure the debugger and enable SWO output

B. Increase the device clock frequency

C. Connect a UART cable to the device

D. Disable all interrupts

Solution

1. Step 1: Identify SWO setup needs

   Redirecting printf to SWO requires enabling SWO in the debugger and device settings.

2. Step 2: Eliminate unrelated options

   Increasing clock, UART connection, or disabling interrupts are not needed for SWO printf redirection.

3. Final Answer:

   Configure the debugger and enable SWO output -> Option A

4. Quick Check:

   Debugger + SWO config = printf redirection [OK]

Hint: SWO printf needs debugger setup [OK]

Common Mistakes:

• Thinking UART cable is needed for SWO
• Assuming clock speed affects SWO output
• Believing interrupts must be disabled

3. Given the following code snippet on an ARM Cortex-M device with SWO enabled:

printf("Hello SWO!\n");

What will happen when this code runs?

medium

A. The message is sent to UART and displayed on a serial terminal

B. The message "Hello SWO!" is sent over the SWO pin and visible in debugger

C. The device will reset due to unsupported printf

D. Nothing happens because printf is disabled

Solution

1. Step 1: Understand printf with SWO enabled

   When SWO is enabled and configured, printf output is sent through the SWO pin.

2. Step 2: Check other options

   UART output requires UART setup, device reset or no output are incorrect if SWO is properly configured.

3. Final Answer:

   The message "Hello SWO!" is sent over the SWO pin and visible in debugger -> Option B

4. Quick Check:

   printf + SWO enabled = message on SWO [OK]

Hint: printf goes to SWO if enabled and configured [OK]

Common Mistakes:

• Assuming printf always uses UART
• Thinking device resets on printf
• Believing printf is disabled by default

4. A developer tries to use SWO for printf but sees no output in the debugger. Which is the most likely cause?

medium

A. The printf function is missing a semicolon

B. The device clock speed is too high

C. The device memory is full

D. SWO pin is not enabled or configured in the debugger

Solution

1. Step 1: Identify common SWO issues

   No output usually means SWO pin or debugger settings are not enabled or configured properly.

2. Step 2: Exclude other causes

   Clock speed, missing semicolon, or memory full do not directly stop SWO output.

3. Final Answer:

   SWO pin is not enabled or configured in the debugger -> Option D

4. Quick Check:

   No SWO output = check debugger SWO config [OK]

Hint: No SWO output? Check debugger SWO enable [OK]

Common Mistakes:

• Blaming clock speed for no SWO output
• Assuming syntax errors cause SWO failure
• Thinking memory full stops SWO

5. How can a developer optimize SWO printf output to avoid slowing down the ARM Cortex-M device during heavy debugging?

hard

A. Use buffered output and adjust SWO baud rate settings

B. Disable SWO and use UART instead

C. Increase the device clock speed to maximum

D. Remove all printf statements from code

Solution

1. Step 1: Understand SWO performance tuning

   Buffered output reduces blocking delays, and adjusting baud rate improves data flow speed.

2. Step 2: Evaluate other options

   Disabling SWO or removing printf loses debug info; increasing clock speed alone doesn't optimize SWO output.

3. Final Answer:

   Use buffered output and adjust SWO baud rate settings -> Option A

4. Quick Check:

   Buffer + baud rate tuning = faster SWO output [OK]

Hint: Buffer output and set baud rate for fast SWO [OK]

Common Mistakes:

• Thinking disabling SWO is optimization
• Assuming clock speed fixes SWO delays
• Removing printf loses debug info