added some documentation on MCUs

git-svn-id: https://svn.spreadspace.org/mur.sat@859 7de4ea59-55d0-425e-a1af-a3118ea81d4c

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+List of all MCUs and their purpose
+===================================
+
+  MPU - Main Processing Unit
+ ----------------------------
+    This is the most powerful processor aboard and runs the satellites main
+    operating system which is based on ChibiOS. The MPU runs the payload
+    applications and maintains the high level communication channel to the
+    groundstation network.
+
+  Tasks:
+   - store collected data -> SDCard (min. 4 GB)
+   - monitor SOL (rotations, camera) -> SDCard
+   - takes pictures using camera -> SDCard
+      check SOL if picture is worth a try
+      evaluate pictures if it is worth transferring
+   - record audio (using IDM) -> SDCard
+      encode raw audio (from IDM) to FLAC
+      evaluate audio if it is worth transferring
+   - control LIS (LED in space)
+   - monitor ANC for pressed button -> Beacon!!
+   - communicate with ground stations (using COMM)
+
+
+  IDM - Impact detection module
+ -------------------------------
+    This is basically a very low power soundcard which watches the output of
+    a small piezo microphone and starts recording when a signal is detected.
+    It then wakes the MPU for further processing.
+    Optionally the IDM may also 'play' sounds from MPU using yet another piezo.
+    This can be used as a payload application or to test the signal detection.
+
+  Tasks:
+   - wait for 'impacts' aka signals form microphone
+   - ater 'noise' start recording and wake up MPU
+   - when internal ring buffer is full stop recording
+   - MPU may start downloading the audio data which prevents
+     ring buffer from overfilling -> endless recording
+   - recording may be triggered by MPU as well
+   - MPU may send audio data which should be played using a
+     small piezo (optional)
+
+
+  IHU - internal houskeeping unit
+ ---------------------------------
+    This processor runs the low level operating system which maintains power
+    levels, reboots MPU and other subsystems on error and controls the system
+    clock (RTC).
+
+  Tasks:
+   - maintain vital-power (turn of main power to prevent power outages)
+   - monitor solar energy levels
+   - battery management (using BAT)
+   - power path control (using BAT)
+   - temperature management (sensors and heating)
+   - MPU overwatch (reset on error)
+   - CTR overwatch (reset on error)
+   - report power states and environment to MPU and CTR
+   - control RTC and report system time to MPU and CTR
+   - enforce radio silence
+
+
+  BAT - battery management
+ --------------------------
+    A simple battery monitoring and control device which reports all measured
+    values to IHU and serves as a persistent power path controller (states
+    should be maintained even after total power loss)
+
+  Tasks:
+    - periodically measure currents, voltates and tempertatures of all battery
+      packs and report them to IHU
+    - mitigate short circuit conditions by disabling the charge or discharge
+      channels for all or some battery packs
+    - report fault conditions (short circuit, defective batteries, etc... to IHU
+
+
+  CTR - command transmit/receive
+ --------------------------------
+    This MCU maintains the low level communication (TTX and CRX).
+    TTX is a simple CW beacon containing telemetry information and a sequence number
+    CRX is the emergency command receiver which may be used to invoke emergency
+    commands such as 'radio silence' or 'reset'.
+
+  Tasks:
+    - IHU overwatch (reset on error??)
+    - listen for incoming CRX commands
+    - send out telemetry beacons (TTX)
+
+
+  COMM - communications
+ -----------------------
+    This processor controls the 2m and 70cm transceivers. There are two radios.
+    Each consist of one COMM processor, one 2m transceiver and on 70cm transcevier.
+    A static network of gates takes care of assigning radios to MPU and CRX.
+
+  Tasks:
+    - initialize and control the 2m and 70cm transceivers
+    - send out data frames when requeseted by MPU or CTR
+    - wait for incoming data frames when requested by MPU or CTR
+
+
+
+Flash/Update Mechanism
+=======================
+    All MCUs inside mur.sat should be updateable through a single
+    USB Port (connected to MPU).
+
+    MCUs to update:
+     - MPU (STM32F103/STM32F405)  USB -> PC (Entry Point)
+     - IDM (PIC 16F1847)          SPI -> MPU
+     - IHU (PIC 16F887)           RS232 -> MPU
+     - BAT (PIC 16F887)           SPI -> IHU
+     - CTR (PIC 16F887)           SPI -> IHU
+     - COMM (AtTiny??) x2         RS232 -> MPU