esp-idf/components/esp32/include/esp_clk.h
Ivan Grokhotkov 8ccb2a4990 esp32: make time monotonic across resets
Small changes to clock calibration value will cause increasing errors
the longer the device runs. Consider the case of deep sleep, assuming
that RTC counter is used for timekeeping:
- before sleep:
   time_before = rtc_counter * calibration_val
- after sleep:
   time_after = (rtc_counter + sleep_count) * (calibration_val + epsilon)
where 'epsilon' is a small estimation error of 'calibration_val'.
The apparent sleep duration thus will be:
time_after - time_before = sleep_count * (calibration_val + epsilon)
                           + rtc_counter * epsilon

Second term on the right hand side is the error in time difference
estimation, it is proportional to the total system runtime (rtc_counter).

To avoid this issue, this change makes RTC_SLOW_CLK calibration value
persistent across restarts. This allows the calibration value update to
be preformed, while keeping time after update same as before the update.
2017-06-16 12:06:04 +08:00

57 lines
1.8 KiB
C

// Copyright 2015-2017 Espressif Systems (Shanghai) PTE LTD
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
#pragma once
/**
* @file esp_clk.h
*
* This file contains declarations of clock related functions.
* These functions are used in ESP-IDF components, but should not be considered
* to be part of public API.
*/
/**
* @brief Initialize clock-related settings
*
* Called from cpu_start.c, not intended to be called from other places.
* This function configures the CPU clock, RTC slow and fast clocks, and
* performs RTC slow clock calibration.
*/
void esp_clk_init(void);
/**
* @brief Get the calibration value of RTC slow clock
*
* The value is in the same format as returned by rtc_clk_cal (microseconds,
* in Q13.19 fixed-point format).
*
* @return the calibration value obtained using rtc_clk_cal, at startup time
*/
uint32_t esp_clk_slowclk_cal_get();
/**
* @brief Update the calibration value of RTC slow clock
*
* The value has to be in the same format as returned by rtc_clk_cal (microseconds,
* in Q13.19 fixed-point format).
* This value is used by timekeeping functions (such as gettimeofday) to
* calculate current time based on RTC counter value.
* @param value calibration value obtained using rtc_clk_cal
*/
void esp_clk_slowclk_cal_set(uint32_t value);