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+Protocol Definition between IHU and MPU
+=======================================
+
+
+Introduction
+------------
+
+The two controllers use SPI to communicate with each other. In
+addition to the 4 Lines needed for SPI there are 3 extra connections
+between IHU and MPU. One is for the IHU to control the power of the MPU.
+Holding this pin low prevents the MPU from running and therefore keeps
+power consumption at a minimum also the whole payload electronic connected
+to the MPU has to be in powerdown when this PIN goes low. The second line
+is for reset. After a high to low transition the MPU has to perform a 
+hardware reset. This also resets any payload hardware.
+The third line is used by the MPU to inform the IHU that it wishes
+to communicate. After this line goes high the IHU has to start the
+communication by starting the SPI clock.
+
+
+Frames
+------
+
+The following chart shows a single frame which is used for communication
+
+Field-length:  1      1      1        LEN          1
+            +------+------+------+--------------+------+
+Name        | 0xAA | MSG  | LEN  |    DATA      | CS   |
+            +------+------+------+--------------+------+
+
+Every message frame starts with 0xAA followed by a message code and
+the length of the following DATA field. The length field might be zero
+if the message doesn't need any additional data. The last byte of every
+frame contains a checksum. It is calculated by XORing every variable byte
+of the message. In a message N bytes long, where Byte 1 is the first and Byte
+N the last Byte, CS is calculated starting with Byte 1 (0xAA) and ending
+with Byte N-1 (last DATA Byte)
+The minimum frame length is 4 Bytes and the maximum is 260 Bytes.
+The bytes have to be sent LSB first. Multi byte fields have to be sent
+in Little-Endian format.
+
+
+Messages
+--------
+
+Messages are separated in request and responses. Requests use upper case
+letters and replies use lower case.
+
+ 'A' - Activate Payload receiver (IHU -> IHU)
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+ This command tells the MPU to activate the payload receiver and
+ listen for incoming commands from ground stations.
+
+ Expected response: 'a'
+
+
+ 'C' - Cancel active and all pending commands (IHU -> IHU)
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+ This command cancels all downlinḱ commands in terms of 
+ communication between the satellite and an ground station.
+
+ Expected response: 'c'
+
+
+ 'F' - Finish up (IHU -> MPU)
+ ~~~~~~~~~~~~~~~
+
+ Sent by the IHU.
+
+ This command tells the MPU that the IHU is planning to shut down
+ the MPU after ?? ms. The MPU has to write pending flash pages and put
+ payload hardware into deep sleep. After finishing everything up the MPU
+ has to go into power down.
+ This command doesn't send any data.
+
+ Expected response: 'f'
+
+
+ 'P' - Ping, Time and Power Budget (IHU -> MPU)
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+ This command is sent by the IHU every minute (??) and contains the
+ following data:
+
+  2Bytes Power Budget (see Power Management for details)
+  4Bytes Timestamp (seconds since EPOCH)
+
+ Expected response: 'p'
+
+
+ 'T' - Get Time (MPU -> IHU)
+ ~~~~~~~~~~~~~~
+
+ This command requests the current time.
+
+ Expected response: 't'
+
+
+ 'V' - Config Value update (MPU -IHU)
+ ~~~~~~~~~~~~~~~~~~~~~~~~~
+ 
+ Tell the IHU that the configuration values have been updated by a ground
+ station. 
+ Config Values:
+  - Beacon Interval in seconds (1 Byte)
+
+
+
+ 'a' - Ack to activate receiver (MPU -> IHU)
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+ Sent as an acknowledge to a received 'A' when the
+ receiver is enabled.
+
+ 'c' - Ack to cancel commands (MPU -> IHU)
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+ Sent as an acknowledge to a received 'C' when all commands
+ are canceled.
+
+
+ 'f' - Ack to finish up (MPU -> IHU)
+ ~~~~~~~~~~~~~~~~~~~~~~
+
+ Sent as an acknowledge to a received 'F'.
+
+
+ 'p' - Ack to finish up (MPU -> IHU)
+ ~~~~~~~~~~~~~~~~~~~~~~
+
+ Sent as an acknowledge to a received 'P'.
+
+
+ 't' - Get Time (IHU -> MPU)
+ ~~~~~~~~~~~~~~
+
+ Sends the current Timestamp after receiving a 'T' request.
+ This message sends the 4 Byte Timestamp (seconds since EPOCH)
+ as data.
+
+
+ 'v' - Ack to config value change (IHU -> MPU)
+ ~~~~~~~~~~~~~~~~~~~~~~~~~
+
+ Sent as an acknowledge to a received 'V' when the new configuraion
+ is in place.
+
+
+
+Example Communication
+---------------------
+
+IHU sends a Ping to MPU, telling it that the current power
+budget is 2424 units of power and the current time is
+1305501574 seconds since epoch. The MPU acknowledges the
+message.
+
+IHU->MPU:  0xAA 'P' 0x06 0x09 0x78 0x4d 0xd0 0x5f 0x86 0xc9
+MPU->IHU:  0xAA 'p' 0x00 0xda
+
+
+Power Management
+----------------
+
+FIXME
+
+
+
+Notes
+-----
+
+EPOCH is the arbitrary point in time where Timestamp is 0x00000000.
+(depending on choice of RTC, probably 1970-01-01 00:00)