Time Acquisition and Calibration

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Introduction

This section has been systematically narrated in the System Time of the ESP-IDF Programming Guide, and this document is only a supplementary summary.

Common Time Acquisition Interfaces

  • esp_timer_get_time

    • Get the time since esp_timer initialization, in μs

    • Use RTC for timing when the ESP chip is in sleep mode, and compensate for time through RTC after exiting sleep mode

  • esp_rtc_get_time_us

    • Get the time of the RTC counter (RTC_SLOW_CLK), in μs

  • esp_cpu_get_cycle_count

    • First get the CPU cycle count, then divide by the result corresponding to esp_rom_get_cpu_ticks_per_us() to get the corresponding time, in μs

  • gettimeofday

    • Use esp_timer as the clock source, in μs

    • Use RTC for timing when the ESP chip is in sleep mode, and compensate for time through RTC after exiting sleep mode

  • xTaskGetTickCount

    • Get the FreeRTOS Tick count, in Ticks

Here are some experience summaries:

  • To get the current time, it is recommended to use the POSIX function gettimeofday, but this function has a slightly higher overhead

  • Using esp_timer_get_time can generate timestamps with microsecond precision, but each call to the timing function will generate a certain amount of overhead

  • When measuring small code segments that run for less than 1-2 ms, there may be large differences in timing measurements due to the function being in flash. This problem can be solved by moving the code to IRAM

  • By using GPIO to quickly flip, very small code execution times can be observed through a logic analyzer

RC Clock Calibration

There are many sources of RTC clock, but only the external 32K clock can provide a higher precision clock, while other RC clocks have lower precision and will accumulate more errors after running for a long time. There is no significant difference in the precision of different clock sources inside the RTC.