RTC Time Module¶
|Since||Origin / Contributor||Maintainer||Source|
|2015-06-25||DiUS, Johny Mattsson, Bernd Meyer email@example.com||Johny Mattsson||rtctime.c|
The rtctime module provides advanced timekeeping support for NodeMCU, including keeping time across deep sleep cycles (provided
rtctime.dsleep() is used instead of
node.dsleep()). This can be used to significantly extend battery life on battery powered sensor nodes, as it is no longer necessary to fire up the RF module each wake-up in order to obtain an accurate timestamp.
This module is intended for use together with NTP (Network Time Protocol) for keeping highly accurate real time at all times. Timestamps are available with microsecond precision, based on the Unix Epoch (1970/01/01 00:00:00). However, the accuracy is (in practice) no better then 1ms, and often worse than that.
Time keeping on the ESP8266 is technically quite challenging. Despite being named RTC, the RTC is not really a Real Time Clock in the normal sense of the word. While it does keep a counter ticking while the module is sleeping, the accuracy with which it does so is highly dependent on the temperature of the chip. Said temperature changes significantly between when the chip is running and when it is sleeping, meaning that any calibration performed while the chip is active becomes useless mere moments after the chip has gone to sleep. As such, calibration values need to be deduced across sleep cycles in order to enable accurate time keeping. This is one of the things this module does.
Further complicating the matter of time keeping is that the ESP8266 operates on three different clock frequencies - 52MHz right at boot, 80MHz during regular operation, and 160MHz if boosted. This module goes to considerable length to take all of this into account to properly keep the time.
To enable this module, it needs to be given a reference time at least once (via
rtctime.set()). For best accuracy it is recommended to provide reference
times at regular intervals. The
sntp.sync() function has an easy way to do this. It is important that a reference time is provided at least twice, with the second time being after a deep sleep.
Note that while the rtctime module can keep time across deep sleeps, it will lose the time if the module is unexpectedly reset.
This module can compensate for the underlying clock not running at exactly the required rate. The adjustment is in steps of 1 part in 2^32 (i.e. around 0.25 ppb). This adjustment
is done automatically if the
sntp.sync() is called with the
autorepeat flag set. The rate is settable using the
set() function below. When the platform
is booted, it defaults to 0 (i.e. nominal). A sample of modules shows that the actual clock rate is temperature dependant, but is normally within 5ppm of the nominal rate. This translates to around 15 seconds per month.
In the automatic update mode it can take a couple of hours for the clock rate to settle down to the correct value. After that, how well it tracks will depend on the amount of variation in timestamps from the NTP servers. If they are close, then the time will track to within a millisecond or so. If they are further away (say 100ms round trip), then time tracking is somewhat worse, but normally within 10ms.
- Time is kept across the deep sleep. I.e.
rtctime.get()will keep working (provided time was available before the sleep).
- This call never returns. The module is put to sleep immediately. This is both to support accurate time keeping and to reduce power consumption.
- The time slept will generally be considerably more accurate than with
- A sleep time of zero does not mean indefinite sleep, it is interpreted as a zero length sleep instead.
When the sleep timer expires, the platform is rebooted and the Lua code is started with the
init.lua file. The clock is set reasonably accurately.
rtctime.dsleep(microseconds [, option])
microsecondsnumber of microseconds to sleep for. Maxmium value is 4294967295us, or ~71 minutes.
optionsleep option, see
This function does not return.
-- sleep for a minute rtctime.dsleep(60*1000000)
-- sleep for 5 seconds, do not start RF on wakeup rtctime.dsleep(5000000, 4)
For applications where it is necessary to take samples with high regularity, this function is useful. It provides an easy way to implement a "wake up on the next 5-minute boundary" scheme, without having to explicitly take into account how long the module has been active for etc before going back to sleep.
rtctime.dsleep_aligned(aligned_us, minsleep_us [, option])
aligned_usboundary interval in microseconds
minsleep_usminimum time that will be slept, if necessary skipping an interval. This is intended for sensors where a sample reading is started before putting the ESP8266 to sleep, and then fetched upon wake-up. Here
minsleep_usshould be the minimum time required for the sensor to take the sample.
optionsets the sleep option, if specified.
-- sleep at least 3 seconds, then wake up on the next 5-second boundary rtctime.dsleep_aligned(5*1000000, 3*1000000)
Converts a Unix timestamp to calendar format. Neither timezone nor DST correction is performed - the result is UTC time.
timestamp seconds since Unix epoch
A table containing the fields:
year1970 ~ 2038
monmonth 1 ~ 12 in current year
dayday 1 ~ 31 in current month
ydayday 1 ~ 366 in current year
wdayday 1 ~ 7 in current weak (Sunday is 1)
tm = rtctime.epoch2cal(rtctime.get()) print(string.format("%04d/%02d/%02d %02d:%02d:%02d", tm["year"], tm["mon"], tm["day"], tm["hour"], tm["min"], tm["sec"]))
Returns the current time. If current time is not available, zero is returned.
A three-value timestamp containing:
secseconds since the Unix epoch
usecthe microseconds part
ratethe current clock rate offset. This is an offset of
rate / 2^32(where the nominal rate is 1). For example, a value of 4295 corresponds to 1 part per million.
sec, usec, rate = rtctime.get()
Sets the rtctime to a given timestamp in the Unix epoch (i.e. seconds from midnight 1970/01/01). If the module is not already keeping time, it starts now. If the module was already keeping time, it uses this time to help adjust its internal calibration values. Care is taken that timestamps returned from
rtctime.get() never go backwards. If necessary, time is slewed and gradually allowed to catch up.
It is highly recommended that the timestamp is obtained via NTP (see SNTP module), GPS, or other highly accurate time source.
Values very close to the epoch are not supported. This is a side effect of keeping the memory requirements as low as possible. Considering that it's no longer 1970, this is not considered a problem.
Times are specified across two arguments, seconds and microseconds. If microseconds are not specified, the default is 0 (i.e., time is exactly at the boundary between two seconds).
In addition to the time, a third argument specifies the local clock's drift rate estimate, as documented in
rtctime.get(). If this argument is not given or is
nil, the current rate estimate will not be altered, and the rate estimate may be set without changing the time by leaving the seconds and microseconds arguments
nil. The rate is not (unfortunately) a fixed parameter for a particular nodemcu board. It varies with (at least) the temperature and the age of the board.
rtctime.set([seconds], [microseconds], [rate])
secondsthe seconds part, counted from the Unix epoch
microsecondsthe microseconds part
ratethe rate in the same units as for
rtctime.get(). The stored rate is not modified if not specified.
-- Set time to 2015 July 9, 18:29:49 rtctime.set(1436430589, 0)
This takes a time interval in 'system clock microseconds' based on the timestamps returned by
tmr.now and returns
an adjusted time interval in 'wall clock microseconds'. The size of the adjustment is typically pretty small as it (roughly) the error in the
crystal clock rate. This function is useful in some precision timing applications.
microsecondsa time interval measured in system clock microseconds.
The same interval but measured in wall clock microseconds
local start = tmr.now() -- do something local end = tmr.now() print ('Duration', rtctime.adjust_delta(end - start)) -- You can also go in the other direction (roughly) local one_second = 1000000 local ticks_in_one_second = one_second - (rtctime.adjust_delta(one_second) - one_second)