The rtctime module provides advanced timekeeping support for NodeMCU, including keeping time across deep sleep cycles (provided [`rtctime.dsleep()`](#rtctimedsleep) is used instead of [`node.dsleep()`](node.md#nodedsleep)). 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](https://en.wikipedia.org/wiki/Network_Time_Protocol) (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](https://en.wikipedia.org/wiki/Real-time_clock), 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()`](#rtctimeset)). For best accuracy it is recommended to provide reference
times at regular intervals. The [`sntp.sync()`](sntp.md#sntpsync) 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.
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()`](sntp.md#sntpsync) is called with the `autorepeat` flag set. The rate is settable using the [`set()`](#rtctimeset) 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.
This module uses RTC memory slots 0-9, inclusive. As soon as [`rtctime.set()`](#rtctimeset) (or [`sntp.sync()`](sntp.md#sntpsync)) has been called these RTC memory slots will be used.
Puts the ESP8266 into deep sleep mode, like [`node.dsleep()`](node.md#nodedsleep). It differs from [`node.dsleep()`](node.md#nodedsleep) in the following ways:
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.
-`minsleep_us` minimum 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_us` should be the minimum time required for the sensor to take the sample.
-`option` as with `dsleep()`, the `option` sets the sleep option, if specified.
-`rate` the 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.
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()`](#rtctimeget) *never go backwards*. If necessary, time is slewed and gradually allowed to catch up.
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.
