Modules Reference: Communication
This module implements the MAVLink protocol, which can be used on a Serial link or UDP network connection. It communicates with the system via uORB: some messages are directly handled in the module (eg. mission protocol), others are published via uORB (eg. vehicle_command).
Streams are used to send periodic messages with a specific rate, such as the vehicle attitude.
When starting the mavlink instance, a mode can be specified, which defines the set of enabled streams with their rates.
For a running instance, streams can be configured via
mavlink stream command.
There can be multiple independent instances of the module, each connected to one serial device or network port.
The implementation uses 2 threads, a sending and a receiving thread. The sender runs at a fixed rate and dynamically
reduces the rates of the streams if the combined bandwidth is higher than the configured rate (
-r) or the
physical link becomes saturated. This can be checked with
mavlink status, see if
rate mult is less than 1.
Careful: some of the data is accessed and modified from both threads, so when changing code or extend the functionality, this needs to be take into account, in order to avoid race conditions and corrupt data.
Start mavlink on ttyS1 serial with baudrate 921600 and maximum sending rate of 80kB/s:
mavlink start -d /dev/ttyS1 -b 921600 -m onboard -r 80000
Start mavlink on UDP port 14556 and enable the HIGHRES_IMU message with 50Hz:
mavlink start -u 14556 -r 1000000 mavlink stream -u 14556 -s HIGHRES_IMU -r 50
mavlink <command> [arguments...] Commands: start Start a new instance [-d <val>] Select Serial Device values: <file:dev>, default: /dev/ttyS1 [-b <val>] Baudrate default: 57600 [-r <val>] Maximum sending data rate in B/s (if 0, use baudrate / 20) default: 0 [-u <val>] Select UDP Network Port (local) default: 14556 [-o <val>] Select UDP Network Port (remote) default: 14550 [-t <val>] Partner IP (broadcasting can be enabled via MAV_BROADCAST param) default: 127.0.0.1 [-m <val>] Mode: sets default streams and rates values: custom|camera|onboard|osd|magic|config|iridium, default: normal [-f] Enable message forwarding to other Mavlink instances [-v] Verbose output [-w] Wait to send, until first message received [-x] Enable FTP stop-all Stop all instances status Print status for all instances stream Configure the sending rate of a stream for a running instance [-u <val>] Select Mavlink instance via local Network Port default: 0 [-d <val>] Select Mavlink instance via Serial Device values: <file:dev> -s <val> Mavlink stream to configure -r <val> Rate in Hz (0 = turn off) boot_complete Enable sending of messages. (Must be) called as last step in startup script.
uORB is the internal pub-sub messaging system, used for communication between modules.
It is typically started as one of the very first modules and most other modules depend on it.
No thread or work queue is needed, the module start only makes sure to initialize the shared global state. Communication is done via shared memory. The implementation is asynchronous and lock-free, ie. a publisher does not wait for a subscriber and vice versa. This is achieved by having a separate buffer between a publisher and a subscriber.
The code is optimized to minimize the memory footprint and the latency to exchange messages.
The interface is based on file descriptors: internally it uses
ioctl. Except for the
publications, which use
orb_advert_t handles, so that they can be used from interrupts as well (on NuttX).
Messages are defined in the
/msg directory. They are converted into C/C++ code at build-time.
If compiled with ORB_USE_PUBLISHER_RULES, a file with uORB publication rules can be used to configure which modules are allowed to publish which topics. This is used for system-wide replay.
Monitor topic publication rates. Besides
top, this is an important command for general system inspection:
uorb <command> [arguments...] Commands: start status Print topic statistics top Monitor topic publication rates [-a] print all instead of only currently publishing topics [<filter1> [<filter2>]] topic(s) to match (implies -a)