pw_chrono_freertos

pw_chrono_freertos is a collection of pw_chrono backends that are implemented using FreeRTOS.

Warning

This module is still under construction, the API is not yet stable.

SystemClock backend

The FreeRTOS based system_clock backend implements the pw_chrono:system_clock facade by using xTaskGetTickCountFromISR() and xTaskGetTickCount() based on the current context. An InterruptSpinLock is used to manage overflows in a thread and interrupt safe manner to produce a signed 64 bit timestamp.

The SystemClock::now() must be used more than once per overflow of the native FreeRTOS xTaskGetTickCount*() overflow. Note that this duration may vary if portSUPPRESS_TICKS_AND_SLEEP(), vTaskStepTick(), and/or xTaskCatchUpTicks() are used.

SystemTimer backend

The FreeRTOS based system_timer backend implements the pw_chrono:system_timer facade by using FreeRTOS’s Software Timers API. pw::chrono::SystemTimer instances use StaticTimer_t & xTimerCreateStatic, dynamic memory allocation is never used. This means that both #define configUSE_TIMERS 1 and #define configSUPPORT_STATIC_ALLOCATION 1 must be set in the FreeRTOS configuration.

This also means that FreeRTOS’s Timer Service Daemon Task is used and must be configured appropriately by the user through configTIMER_TASK_PRIORITY, configTIMER_QUEUE_LENGTH, and configTIMER_TASK_STACK_DEPTH.

Build targets

The GN build for pw_chrono_freertos has one target: system_clock. The system_clock target provides the pw_chrono_backend/system_clock_config.h and pw_chrono_freertos/config.h headers and the backend for the pw_chrono:system_clock.