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111 lines
3.6 KiB
Markdown
111 lines
3.6 KiB
Markdown
# Send and Receive Debug Values
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It is often necessary during software development to output individual important numbers.
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This is where the generic `NAMED_VALUE_FLOAT`, `DEBUG` and `DEBUG_VECT` packets of MAVLink come in.
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## Mapping between MAVLink Debug Messages and uORB Topics
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MAVLink debug messages are translated to/from uORB topics.
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In order to send or receive a MAVLink debug message, you have to respectively publish or subscribe to the corresponding topic.
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Here is a table that summarizes the mapping between MAVLink debug messages and uORB topics:
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| MAVLink message | uORB topic |
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| ----------------- | --------------- |
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| NAMED_VALUE_FLOAT | debug_key_value |
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| DEBUG | debug_value |
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| DEBUG_VECT | debug_vect |
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## Tutorial: Send String / Float Pairs
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This tutorial shows how to send the MAVLink message `NAMED_VALUE_FLOAT` using the associated uORB topic `debug_key_value`.
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The code for this tutorial is available here:
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- [Debug Tutorial Code](https://github.com/PX4/PX4-Autopilot/blob/main/src/examples/px4_mavlink_debug/px4_mavlink_debug.cpp)
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- [Enable the tutorial app](https://github.com/PX4/PX4-Autopilot/blob/main/boards/px4/fmu-v5/default.px4board) by ensuring the MAVLink debug app (**CONFIG_EXAMPLES_PX4_MAVLINK_DEBUG**) is in the config of your board and set to 'y'.
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All required to set up a debug publication is this code snippet.
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First add the header file:
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```C
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#include <uORB/uORB.h>
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#include <uORB/topics/debug_key_value.h>
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#include <string.h>
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```
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Then advertise the debug value topic (one advertisement for different published names is sufficient).
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Put this in front of your main loop:
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```C
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/* advertise debug value */
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struct debug_key_value_s dbg;
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strncpy(dbg.key, "velx", sizeof(dbg.key));
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dbg.value = 0.0f;
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orb_advert_t pub_dbg = orb_advertise(ORB_ID(debug_key_value), &dbg);
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```
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And sending in the main loop is even simpler:
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```C
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dbg.value = position[0];
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orb_publish(ORB_ID(debug_key_value), pub_dbg, &dbg);
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```
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:::warning
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Multiple debug messages must have enough time between their respective publishings for Mavlink to process them.
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This means that either the code must wait between publishing multiple debug messages, or alternate the messages on each function call iteration.
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:::
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The result in QGroundControl then looks like this on the real-time plot:
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## Tutorial: Receive String / Float Pairs
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The following code snippets show how to receive the `velx` debug variable that was sent in the previous tutorial.
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First, subscribe to the topic `debug_key_value`:
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```C
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#include <poll.h>
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#include <uORB/topics/debug_key_value.h>
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int debug_sub_fd = orb_subscribe(ORB_ID(debug_key_value));
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[...]
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```
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Then poll on the topic:
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```C
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[...]
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/* one could wait for multiple topics with this technique, just using one here */
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px4_pollfd_struct_t fds[] = {
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{ .fd = debug_sub_fd, .events = POLLIN },
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};
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while (true) {
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/* wait for debug_key_value for 1000 ms (1 second) */
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int poll_ret = px4_poll(fds, 1, 1000);
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[...]
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```
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When a new message is available on the `debug_key_value` topic, do not forget to filter it based on its key attribute in order to discard the messages with key different than `velx`:
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```C
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[...]
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if (fds[0].revents & POLLIN) {
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/* obtained data for the first file descriptor */
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struct debug_key_value_s dbg;
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/* copy data into local buffer */
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orb_copy(ORB_ID(debug_key_value), debug_sub_fd, &dbg);
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/* filter message based on its key attribute */
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if (strcmp(_sub_debug_vect.get().key, "velx") == 0) {
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PX4_INFO("velx:\t%8.4f", dbg.value);
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}
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}
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}
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```
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