[docs] Document RTC memory

docs/source/Reference/RTC.rst

@@ -0,0 +1,65 @@
+RTC Reference
+*************
+
+The ESP microprocessors we use, have some special memory which will retain its information as long as the node remains powered.
+Espressif calls this _RTC_ RAM and thus we're using the same term for it.
+
+The name may be a bit strange, as RTC usually refers to _Real Time Clock_.
+
+Currently we only support RTC functions on the ESP8266 and ESP8285.
+ESP32 does also have RTC memory, but that's organised a bit different.
+
+
+RTC layout ESPEasy
+------------------
+
+On the ESP82xx the RTC memory is addressable per 32 bit.
+
+In total, there is 768 bytes (192 addressable blocks).
+
+
+* 0 .. 63  (256 bytes) Reserved for esp8266/Arduino library.
+* 64 .. 73 (40 bytes) ``RTCStruct``.
+* 74 .. 121 (200 bytes) UserVar - Task variables of 12 tasks
+* 122 .. 123 UserVar checksum:  RTC_BASE_USERVAR + (sizeof(UserVar) / 4)
+* 128 .. 131 (16 bytes) Cache Controller (C016) meta data
+* 132 .. 191 (240 bytes) Cache Controller (C016) data 6 blocks per sample => max 10 samples
+
+
+RTC Struct
+----------
+
+We store several variables in this struct that may be useful to survive a reboot.
+
+WiFi related settings for a quick reconnect:
+
+* Last connected WiFi channel
+* Last WiFi Settings Index
+* Last BSSID (48 bit)
+
+Counters and states for diagnostics, recovering or protection:
+
+* Deep Sleep State (To signal whether we just woke up from deep sleep)
+* Boot Failed Count (to start disable plugins and/or controllers to find the source of crashes)
+* Factory Reset Counter  (to protect the flash when format + clean install crashes)
+* Flash Day Counter (to limit the number of writes per day)
+* Flash Counter (32 bit)
+* Boot Counter (32 bit)
+* Last Scheduled job (32 bit)
+
+System Time:
+* Last Sys Time (32 bit) - Last known Unix time.
+
+This last value, the last known system (Unix) time, is stored in RTC to have some idea of time after a warm reboot. (e.g. deliberate reboot, or crash)
+
+Sometimes, when a node crashes, it may not be able to connect to WiFi right away or reach a NTP service.
+If this node does need to act on (local) time based rules, it would be very practical if it has some concept of time.
+
+There is an unknown period between storing the last known Unix time and restoring it after reboot.
+This does lead to some (unknown) offset in local time, for each reboot.
+
+Typical this offset is roughly between 900 and 1500 msec per (deliberate) reboot.
+If the reboot was caused by a crash (e.g. Watchdog timer crash), the offset can be quite a bit longer.
+
+The last known system time is stored on every scheduler action, so a few times a second at least.
+

docs/source/index.rst

@@ -35,6 +35,7 @@ Welcome to ESP Easy's documentation!
    Reference/Events.rst
    Reference/GPIO.rst
    Reference/SystemVariable.rst
+   Reference/RTC.rst
    Reference/ChangeLog.rst
 
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