Protocol Definition between IHU and MPU
=======================================


Introduction
------------

The two controllers use RS-232 with TTL-Levels to communicate with each other.
In addition to the 2 Lines needed for RS-232 there are 2 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.


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)