avr: added docu

doc/OS

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+
+Aquamaniac Home Control System (AqHCS) uses a minimalistic operating system.
+
+That OS doesn't use devicer driver object structures due to RAM contraints, it just defines a few
+callback labels which need to be provided by drivers, apps and the main code.
+
+Main routines for drivers are:
+- Init
+- Fini (not used for now)
+- Run
+- OnEvery100ms
+- OnMessageReceived
+
+This system is event driven. There is one single main loop which uses some defined labels as callbacks.
+
+Basically the main loop works like this:
+
+--------------------------------------------------------------------------------------------------------X8
+loop:
+  - sleep (waiting to be awakended by interrupt)
+  - call RUN routines of every active driver
+  - call onEveryLoop
+  - goto loop
+--------------------------------------------------------------------------------------------------------X8
+
+
+The main RUN routine consists of a number of calls to RUN functions of used drivers and apps, e.g.:
+
+--------------------------------------------------------------------------------------------------------X8
+RunDrivers:
+    call BaseTimer_Run   ; main clock event source
+#ifdef MODULES_TTYONUART1
+    call TtyOnUart1_Run
+#endif
+#ifdef MODULES_COM2WI
+    call COM2WI_Run
+#endif
+    ret
+--------------------------------------------------------------------------------------------------------X8
+
+This means new drivers have their RUN functions registered by adding them to the RunDrivers routine with
+"#ifdef" clauses around them. That way you only need to add the line "#define MODULES_TTYONUART1" in your main
+application to have the driver automatically used.
+
+
+The main driver of the system is the BaseTimer module. It is supplied by interrupt of an AVR timer circuit.
+Its RUN routine then calls some other routines in the main app like "onEverySecond" etc.
+
+
+Required labels for the main application are:
+
+- onSystemStart:     called first after system start
+- onMessageReceived: called for every network package received
+- onEvery100ms:      called every 100ms which is the base clock for all other functions
+- onEverySecond:     called every second
+- onEveryMinute:     called every minute
+- onEveryHour:       called every hour
+- onEveryDay:        called every day
+- onEveryLoop:       called on every main loop
+
+These callbacks must all be defined in the main source file but callbacks for functionality not provided or used by the main 
+system might just execute a "ret" instruction.
+
+
+
+