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esp32: make time monotonic across resets

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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.
This commit is contained in:
Ivan Grokhotkov
2017-06-12 19:51:17 +08:00
parent cc8af68244
commit 8ccb2a4990
4 changed files with 52 additions and 18 deletions
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23
components/esp32/clk.c
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@@ -13,11 +13,15 @@
// limitations under the License.
#include <stdint.h>
#include <sys/cdefs.h>
#include <sys/time.h>
#include "sdkconfig.h"
#include "esp_attr.h"
#include "esp_log.h"
#include "esp_clk.h"
#include "rom/ets_sys.h"
#include "rom/uart.h"
#include "rom/rtc.h"
#include "soc/soc.h"
#include "soc/rtc.h"
#include "soc/rtc_cntl_reg.h"
@@ -82,12 +86,6 @@ void IRAM_ATTR ets_update_cpu_frequency(uint32_t ticks_per_us)
g_ticks_per_us_app = ticks_per_us;
}
/* This is a cached value of RTC slow clock period; it is updated by
* the select_rtc_slow_clk function at start up. This cached value is used in
* other places, like time syscalls and deep sleep.
*/
static uint32_t s_rtc_slow_clk_cal = 0;
static void select_rtc_slow_clk(rtc_slow_freq_t slow_clk)
{
if (slow_clk == RTC_SLOW_FREQ_32K_XTAL) {
@@ -114,19 +112,16 @@ static void select_rtc_slow_clk(rtc_slow_freq_t slow_clk)
ESP_EARLY_LOGD(TAG, "32k oscillator ready, wait=%d", wait);
}
rtc_clk_slow_freq_set(slow_clk);
uint32_t cal_val;
if (SLOW_CLK_CAL_CYCLES > 0) {
/* TODO: 32k XTAL oscillator has some frequency drift at startup.
* Improve calibration routine to wait until the frequency is stable.
*/
s_rtc_slow_clk_cal = rtc_clk_cal(RTC_CAL_RTC_MUX, SLOW_CLK_CAL_CYCLES);
cal_val = rtc_clk_cal(RTC_CAL_RTC_MUX, SLOW_CLK_CAL_CYCLES);
} else {
const uint64_t cal_dividend = (1ULL << RTC_CLK_CAL_FRACT) * 1000000ULL;
s_rtc_slow_clk_cal = (uint32_t) (cal_dividend / rtc_clk_slow_freq_get_hz());
cal_val = (uint32_t) (cal_dividend / rtc_clk_slow_freq_get_hz());
}
ESP_EARLY_LOGD(TAG, "RTC_SLOW_CLK calibration value: %d", s_rtc_slow_clk_cal);
}
uint32_t esp_clk_slowclk_cal_get()
{
return s_rtc_slow_clk_cal;
ESP_EARLY_LOGD(TAG, "RTC_SLOW_CLK calibration value: %d", cal_val);
esp_clk_slowclk_cal_set(cal_val);
}