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MultiWii Serial Protocol

updated 13 April

currently implemented as a prototype in dev versions.

Multiwii serial protocol was redesigned:

    to be light, as before
    to be generic: it can be used transparenlty by a GUI, OSD, telemetry or home made config tool.
    ie no more specific OSD code should be coded in multiwii
    to be bit wire efficient: only requested data are transmitted in a binary format
    to be quite secure: data are sent with a checksum, preventing corrupted configuration to be injected.
    to be header sensitive: as it is designed with a specific header, it can be mixed with other frame, like GPS frame
    ie, it will be possible to connect either a GUI or a GPS on the same serial port without changing the conf
    to be less sensitive to evolutions:
    ie in case of parameter evolution, the main protocol will remain compatible and the GUI will be much less version dependent.
    variable data length allows to consider only the beginning of a message, leaving the other octets at the end to extend transparently the message (for instance to add a new PID)


I thought first about an implementation of Mavlink, but I think it's not what I was looking for.
Even with a partial implementation, the predefined structures are not light enough for what I have in mind.
Some messages are however inspired from mavlink structure.

The main rule remains: Multiwii never sends something on its own.
A request must be done in each case to retrieve or set data.

There are 2 main messages to consider:

    request message to multiwii
    multiwii output message


request message to multiwii
messages are formated like this:
- to request simple data without parameters or to send a specific command
$M>[code]
1 octet '$'
1 octet 'M'
1 octet '<'
1 octet [code]

- to inject new parameters in multiwii
$M>[code][data][checksum]
1 octet '$'
1 octet 'M'
1 octet '<'
1 octet [data length]
1 octet [code]
several octets [data]
1 octet [checksum]

multiwii output message
messages are formated like this:
$M>[code][data][checksum]
1 octet '$'
1 octet 'M'
1 octet '>'
1 octet [data length]
1 octet [code]
several octets [data]
1 octet [checksum]


list of message codes
multiwii output message
MSP_IDENT
multitype + version
MSP_STATUS
cycletime & errors_count & sensor present & box activation
MSP_RAW_IMU
9 DOF output
MSP_SERVO
8 servos
MSP_MOTOR
8 motors
MSP_RC
8 rc chan
MSP_RAW_GPS
fix, numsat, lat, lon, alt, speed
MSP_COMP_GPS
distance to home, direction to home
MSP_ATTITUDE
angles and heading
MSP_ALTITUDE
altitude
MSP_BAT
vbat, powermetersum
MSP_RC_TUNING
rc rate, rc expo, rollpitch rate, yaw rate, dyn throttle PID
MSP_PID
8 P I D
MSP_BOX
11 checkbox
MSP_MISC
powermeter trig
MSP_DEBUG
debug1,debug2,debug3,debug4 variable

multiwii input message
MSP_SET_RAW_RC
8 rc chan: it's a way to command multiwii via a "serial RC transmitter"
MSP_SET_RAW_GPS
fix, numsat, lat, lon, alt, speed: it's a way to inject in multiwii GPS data coming for instance from an OSD with an integrated GPS
MSP_SET_PID
8 P I D
MSP_SET_BOX
11 checkbox
MSP_SET_RC_TUNING
rc rate, rc expo, rollpitch rate, yaw rate, dyn throttle PID
MSP_ACC_CALIBRATION
to calibrate ACC
MSP_MAG_CALIBRATION
to calibrate MAG
MSP_SET_MISC
powermeter trig
MSP_RESET_CONF
to reset all params to default (new feature)
MSP_EEPROM_WRITE
to write current params to EEPROM