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8ccb2a4990
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.
57 lines
1.8 KiB
C
57 lines
1.8 KiB
C
// Copyright 2015-2017 Espressif Systems (Shanghai) PTE LTD
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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#pragma once
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/**
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* @file esp_clk.h
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*
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* This file contains declarations of clock related functions.
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* These functions are used in ESP-IDF components, but should not be considered
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* to be part of public API.
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*/
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/**
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* @brief Initialize clock-related settings
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*
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* Called from cpu_start.c, not intended to be called from other places.
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* This function configures the CPU clock, RTC slow and fast clocks, and
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* performs RTC slow clock calibration.
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*/
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void esp_clk_init(void);
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/**
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* @brief Get the calibration value of RTC slow clock
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*
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* The value is in the same format as returned by rtc_clk_cal (microseconds,
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* in Q13.19 fixed-point format).
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*
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* @return the calibration value obtained using rtc_clk_cal, at startup time
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*/
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uint32_t esp_clk_slowclk_cal_get();
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/**
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* @brief Update the calibration value of RTC slow clock
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*
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* The value has to be in the same format as returned by rtc_clk_cal (microseconds,
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* in Q13.19 fixed-point format).
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* This value is used by timekeeping functions (such as gettimeofday) to
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* calculate current time based on RTC counter value.
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* @param value calibration value obtained using rtc_clk_cal
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*/
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void esp_clk_slowclk_cal_set(uint32_t value);
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