RTOS Tick Calculator

This tool calculates the FreeRTOS tick period (in microseconds), required SysTick timer prescaler and reload values, total RAM footprint for task stacks (including idle task), and estimates timing resolution vs. power trade-offs. It also validates whether the selected tick frequency is feasible given the CPU clock and hardware timer constraints.

A higher tick rate (e.g., 1000 Hz) improves timing precision and task responsiveness but increases CPU overhead and power consumption. A lower rate (e.g., 10–100 Hz) saves power and reduces interrupt load but limits minimum delay accuracy and may affect real-time behavior. This calculator helps balance those trade-offs quantitatively.

Tickless mode (also called low-power or deep-sleep mode) disables the periodic SysTick interrupt when the system is idle, allowing the MCU to enter low-power states between tasks. Enabling it reduces average power consumption significantly—especially in battery-powered applications—but requires careful handling of timekeeping during sleep.

CPU Clock is your microcontroller’s core clock frequency (e.g., 168 MHz for STM32F4, 80 MHz for ESP32). SysTick Frequency is your desired OS tick rate—common values are 100 Hz (10 ms period), 500 Hz (2 ms), or 1000 Hz (1 ms). Ensure the resulting tick period yields an integer SysTick reload value within your timer’s 24-bit range (≤ 16,777,215).

FreeRTOS allocates stack space in words (typically 4-byte units on 32-bit MCUs), not bytes. Specifying size in words avoids portability errors across architectures. For example, 256 words = 1024 bytes on ARM Cortex-M, but matches FreeRTOS’s internal xTaskCreate() parameter expectations.

The calculator flags this as an error—SysTick’s 24-bit reload register cannot hold values ≥ 16,777,216. To fix it, either reduce your tick frequency (increase tick period) or increase your CPU clock (if hardware allows). Alternatively, use a different timer peripheral with wider bit depth for tick generation.

The tool sums stack allocations for all user tasks plus the idle task (which runs when no other task is ready), then adds ~20–40 bytes per task for TCB (Task Control Block) overhead. It does *not* include heap allocation for queues, semaphores, or dynamic objects—those must be sized separately using FreeRTOS heap analysis tools.

While optimized for FreeRTOS conventions (e.g., SysTick-based timing, word-aligned stacks, idle task behavior), the core timing and prescaler math applies to any RTOS using ARM SysTick. However, thread stack sizing, TCB overhead, and tickless implementation details vary—always consult your RTOS documentation before finalizing configurations.

The tick period defines the smallest resolvable delay: a 1000 Hz tick gives ±1 ms resolution; 100 Hz gives ±10 ms. Delays like vTaskDelay(3) will block for *at least* 3 ticks—and up to nearly 4 ticks—depending on when the call occurs relative to the tick interrupt. The calculator highlights this quantization effect.

Tickless mode doesn’t alter tick period or stack sizing, but it modifies power estimation and requires accurate low-power timer calibration. The calculator adjusts its power impact assessment and warns if tick intervals exceed the low-power timer’s maximum timeout range—critical for maintaining timekeeping accuracy during extended sleeps.