Timing and Profiling Techniques on STM32 Microcontrollers

Introduction

Performance tuning and accurate timing measurements are critical in embedded development, especially for real-time systems and power-sensitive applications. STM32 microcontrollers, based on ARM Cortex-M cores, offer a variety of methods to profile code execution and measure timing. This blog explores practical techniques to measure execution cycles and timing on STM32, covering hardware and software approaches.

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Why Timing Matters

Knowing how long a block of code takes to execute—down to the clock cycle—helps you:

• Optimize algorithms for speed or energy efficiency
• Ensure real-time deadlines are met
• Debug performance bottlenecks
• Validate hardware and software integration

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Methods of Measurement

Below, we discuss popular techniques for cycle counting and timing on STM32 platforms, with detailed steps and practical notes for each.

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1. Using DWT_CYCCNT (Data Watchpoint and Trace Unit)

The ARM Cortex-M3, M4, and M7 cores include a built-in cycle counter: DWT->CYCCNT. This register counts CPU clock cycles and is extremely precise for profiling small code sections.

How It Works:

• The DWT unit is part of the ARM debug system. The CYCCNT register increments every clock cycle.
• You can enable or reset it via software.
• Read its value before and after your code to get the cycle count.

Code Example:

// Enable DWT cycle counter (one-time setup)
CoreDebug->DEMCR |= CoreDebug_DEMCR_TRCENA_Msk; // Enable debug
DWT->CYCCNT = 0; // Reset counter
DWT->CTRL |= DWT_CTRL_CYCCNTENA_Msk; // Enable cycle counter

// Start measurement
uint32_t start = DWT->CYCCNT;

// ... code to profile ...

uint32_t end = DWT->CYCCNT;
uint32_t cycles = end - start;

Practical Notes:

• Pros: High precision, minimal overhead, perfect for tight loops or ISR profiling.
• Cons: Not available on Cortex-M0/M0+; may be disabled in low-power modes.
• Tip: If you run code in an RTOS context, consider possible interruptions.

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2. Using Hardware Timers (TIMx)

All STM32 chips offer general-purpose hardware timers. By configuring a timer at a known frequency (ideally matching the CPU clock), you can measure intervals with moderate accuracy.

How It Works:

• Initialize a hardware timer (TIMx) in up-counting mode.
• Set its clock source to a known frequency (e.g., system clock).
• Read its value before and after your code; difference gives elapsed ticks.

Code Example (HAL):

// Timer configuration (done once)
// HAL_TIM_Base_Init(&htimx);
// HAL_TIM_Base_Start(&htimx);

__HAL_TIM_SET_COUNTER(&htimx, 0);        // Reset counter

uint32_t start = __HAL_TIM_GET_COUNTER(&htimx);

// ... code to profile ...

uint32_t end = __HAL_TIM_GET_COUNTER(&htimx);
uint32_t ticks = end - start;

To convert ticks to cycles:

cycles = ticks * (timer_clock_freq / cpu_clock_freq)

Or, if timer runs at CPU frequency:

cycles = ticks

Practical Notes:

• Pros: Works on all STM32 devices, including M0/M0+.
• Cons: Lower resolution than DWT; timer configuration is required; possible overflow for long measurements.
• Tip: Use a 32-bit timer for longer code sections to avoid overflow.

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3. Manual Instruction Counting

Disassemble your code and count the instructions. Multiply by the cycle cost of each instruction (refer to ARM documentation).

How It Works:

• Compile your code and inspect the disassembly (e.g., via objdump, IDE).
• List out instructions for the code section.
• Look up cycle counts for each instruction from the ARM reference manual.
• Sum them for total cycle count.

Example: Suppose your code compiles to:

MOV R0, #1    ; 1 cycle
ADD R1, R0, #2; 1 cycle
STR R1, [R2]  ; 2 cycles (memory access)

Total: 4 cycles

Practical Notes:

• Pros: Useful for extremely simple routines or hand-optimized assembly.
• Cons: Tedious and impractical for complex code, branches, loops, or with interrupts.
• Tip: Use for educational purposes or micro-optimization only.

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4. IDE Profilers and Debuggers

Modern IDEs (STM32CubeIDE, Keil MDK, IAR Embedded Workbench) offer cycle-accurate profiling when connected via SWD/JTAG. These tools typically use the DWT unit or hardware breakpoints.

How It Works:

• Connect your hardware debugger (ST-Link, J-Link, etc.).
• Open your IDE’s profiling or performance analysis tool.
• Set breakpoints at the start/end of your code block.
• Run code and capture profiling statistics (cycles, time, call graphs, etc.).

Example Steps (STM32CubeIDE):

1. Build and flash your project.
2. Open the “Performance Analyzer” or “Instruction Profiling” tool.
3. Mark regions to profile, run the code.
4. View detailed statistics in the IDE.

Practical Notes:

• Pros: Visual, detailed, supports call graphs and code coverage.
• Cons: Requires debugger hardware and IDE; may slow execution slightly.
• Tip: Use for whole-project analysis, not just microbenchmarking.

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5. Measuring Time and Converting to Cycles

If you can measure elapsed time (using SysTick, timers, etc.), you can convert it to cycles.

How It Works:

• Measure the elapsed time for your code (microseconds, milliseconds).
• Multiply by the MCU frequency to get cycles.

Example Calculation: If you measure 50 microseconds on a 72 MHz CPU:

cycles = 50e-6 * 72e6 = 3600 cycles

Practical Notes:

• Pros: Straightforward if you already use time-based measurements.
• Cons: Accuracy depends on timer resolution and the precision of your time measurement.
• Tip: Use for profiling longer code sections or when cycle counting is unavailable.

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6. RTOS Trace Tools

If you use an RTOS (e.g., FreeRTOS), some trace tools can profile task or function execution time, sometimes in cycles.

How It Works:

• Enable trace hooks in your RTOS (often done in configuration).
• Use a trace tool (e.g., FreeRTOS+Trace, Percepio Tracealyzer).
• Run your application; the tool collects timing information for tasks, ISRs, and functions.

Example:

• FreeRTOS+Trace can show task execution time, CPU usage, and sometimes cycles (if DWT or timers are used).

Practical Notes:

• Pros: Great for system-level profiling and optimization.
• Cons: Requires RTOS support and extra software; may add runtime overhead.
• Tip: Ideal for profiling multitasking behavior and system bottlenecks.

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Summary Table

Method	Accuracy	Hardware Needed	Notes
DWT_CYCCNT	High	Cortex-M3/M4/M7	Most precise; not on M0/M0+
TIMx Timer	Medium	Any STM32	Needs timer configuration
Manual Instruction Count	Low	None	Tedious; variable cycle counts
IDE Profiler	High	Debugger + IDE	Visual; needs debug connection
Time Measurement	Medium	Any STM32	Needs precise timer/frequency info
RTOS Trace Tools	Medium/High	RTOS + Trace support	Depends on RTOS/tool

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Best Practices

• Prefer DWT_CYCCNT for quick and accurate measurement if available.
• Use hardware timers as a fallback on simpler cores.

• Leverage IDE tools for deep profiling or when visual feedback is required.

Conclusion

Accurate timing and profiling are essential for robust embedded development on STM32 microcontrollers. Whether you use built-in cycle counters, hardware timers, or advanced IDE tools, understanding these techniques enables you to write faster, more reliable firmware—and deliver on real-time promises.

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