SWO (Serial Wire Output) for printf in ARM Architecture - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to enable SWO output for printf in ARM Cortex-M.

ARM Architecture

CoreDebug->DEMCR |= [1];

Drag options to blanks, or click blank then click option'

ACoreDebug_DEMCR_TRCENA_Msk

BCoreDebug_DEMCR_VC_CORERESET_Msk

CCoreDebug_DEMCR_MON_EN

DCoreDebug_DEMCR_SWOENA

Attempts:

3 left

💡 Hint

Common Mistakes

Using bits unrelated to trace enablement.

Confusing SWO enable bits with other debug bits.

The DEMCR register's TRCENA bit enables the trace and debug blocks, which is necessary for SWO output.

2fill in blank

medium

Complete the code to unlock the ITM stimulus port for SWO printf output.

ARM Architecture

ITM->LAR = [1];

Drag options to blanks, or click blank then click option'

A0xC5ACCE55

B0x00000001

C0xFFFFFFFF

D0x12345678

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect unlock keys.

Not unlocking ITM before writing to stimulus ports.

The ITM Lock Access Register (LAR) must be written with the key 0xC5ACCE55 to unlock ITM registers for writing.

3fill in blank

hard

Fix the error in enabling the ITM stimulus port 0 for printf output.

ARM Architecture

ITM->TER |= [1];

Drag options to blanks, or click blank then click option'

A0x00000002

B0xFFFFFFFF

C0x00000001

D0x00000000

Attempts:

3 left

💡 Hint

Common Mistakes

Enabling wrong stimulus port bits.

Using zero which disables all ports.

Setting bit 0 in the ITM Trace Enable Register (TER) enables stimulus port 0, which is used for printf output via SWO.

4fill in blank

hard

Fill both blanks to configure the TPIU for SWO output at 2 MHz.

ARM Architecture

TPI->ACPR = [1]; // Set prescaler
TPI->SPPR = [2]; // Select protocol

Drag options to blanks, or click blank then click option'

A39

B1

C2

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong prescaler values.

Selecting incorrect protocol in SPPR.

The prescaler divides the core clock to get 2 MHz SWO speed; for 80 MHz core clock, prescaler is 39 (80/2 -1). SPPR=1 selects NRZ protocol for SWO.

5fill in blank

hard

Fill all three blanks to enable ITM, configure stimulus port, and unlock ITM for SWO printf.

ARM Architecture

CoreDebug->DEMCR |= [1];
ITM->LAR = [2];
ITM->TER |= [3];

Drag options to blanks, or click blank then click option'

ACoreDebug_DEMCR_TRCENA_Msk

B0xC5ACCE55

C0x00000001

D0x00000002

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up register bits and keys.

Forgetting to unlock ITM before enabling ports.

First, enable trace in DEMCR with TRCENA bit. Then unlock ITM with key 0xC5ACCE55. Finally, enable stimulus port 0 by setting bit 0 in TER.

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