camera: Add info about pin 7/8 usage

en/peripherals/camera.md

@@ -33,17 +33,24 @@ Mode | Description
 
 ## Trigger Hardware Configuration {#hardware_setup}
 
-You can choose which pins to use for triggering using the [TRIG_PINS](../advanced_config/parameter_reference.md#TRIG_PINS) parameter.
+The pins used to trigger image capture for GPIO, PWM or Seagull-based triggering (i.e. when not using a MAVLink camera) are set using the [TRIG_PINS](../advanced_config/parameter_reference.md#TRIG_PINS) parameter.
 The default is 56, which means that trigger is enabled on *FMU* pins 5 and 6.
 
 > **Note** On a Pixhawk flight controller that has both FMU and I/O boards these FMU pins map to `AUX5` and `AUX6` (e.g. Pixhawk 4, CUAV v5+). 
   On a controller that only has an FMU, the pins map to `MAIN5` and `MAIN6` (e.g. Pixhawk 4 mini, CUAV v5 nano).
   At time of writing triggering only works on FMU pins - you can't trigger a camera using pins on the I/O board.
 
 <span></span>
-> **Warning** With `TRIG_PINS` set to its **default** value of 56, you can use the AUX pins 1, 2, 3 and 4 as actuator outputs (for servos/ESCs).
-  Due to the way the hardware timers are handled (1234 and 56 are 2 different groups handled by 2 timers), this is the ONLY combination which allows the simultaneous usage of camera trigger and FMU actuator outputs.
-  **DO NOT CHANGE THE DEFAULT VALUE OF `TRIG_PINS` IF YOU NEED ACTUATOR OUTPUTS.**
+> **Warning** With `TRIG_PINS=56` (default) you can use the AUX pins 1 to 4 as actuator outputs (for servos/ESCs).
+  With `TRIG_PINS=78`, you can use the AUX pins 1-6 as actuator outputs.
+  Any other combination of pins can be selected, but this will disable use of the other FMU pins as outputs.
+
+Some cameras can provide image capture signalling to a flight controller camera-capture pin (this enables more accurate image timestamp than the time that the trigger command is sent).
+- Pixhawk FMUv5x boards use the board-specific camera capture pin (PI0).
+- Other board use FMU PWM pin 6 (hardcoded) for camera capture.
+
+> **Note** PX4 emits the MAVLink [CAMERA_TRIGGER](https://mavlink.io/en/messages/common.html#CAMERA_TRIGGER) message (with timestamp and image sequence number).
+  If camera capture is configured, the timestamp from the camera capture driver is used, otherwise the triggering timestamp.
 
 ## Trigger Interface Backends {#trigger_backend}
 
@@ -52,9 +59,9 @@ The camera trigger driver supports several backends - each for a specific applic
 Number | Description
 --- | ---
 1 | enables the GPIO interface. The AUX outputs are pulsed high or low (depending on the `TRIG_POLARITY` parameter) every [TRIG_INTERVAL](../advanced_config/parameter_reference.md#TRIG_INTERVAL) duration. This can be used to trigger most standard machine vision cameras directly. Note that on PX4FMU series hardware (Pixhawk, Pixracer, etc.), the signal level on the AUX pins is 3.3v.
-2  | Enables the Seagull MAP2 interface. This allows the use of the [Seagull MAP2](http://www.seagulluav.com/product/seagull-map2/) to interface to a multitude of supported cameras. Pin 1 of the MAP2 should be connected to the lower AUX pin of `TRIG_PINS` (therefore, pin 1 to AUX 5 and pin 2 to AUX 6 by default). In this mode, PX4 also supports automatic power control and keep-alive functionalities of Sony Multiport cameras like the QX-1.
+2  | Enables the Seagull MAP2 interface. This allows the use of the [Seagull MAP2](http://www.seagulluav.com/product/seagull-map2/) to interface to a multitude of supported cameras. Pin/Channel 1 (camera trigger) and Pin/Channel 2 (mode selector) of the MAP2 should be connected to the lower and higher AUX pins of `TRIG_PINS`, respectively (therefore, channel/pin 1 to AUX 5 and channel/pin 2 to AUX 6 by default). Using Seagull MAP2, PX4 also supports automatic power control and keep-alive functionalities of Sony Multiport cameras like the QX-1.
 3 | Enables the MAVLink interface. In this mode, no actual hardware output is used. Only the `CAMERA_TRIGGER` MAVLink message is sent by the autopilot (by default, if the MAVLink application is in `onboard` mode. Otherwise, a custom stream will need to be enabled).
-4 | Enables the generic PWM interface. This allows the use of [infrared triggers](https://hobbyking.com/en_us/universal-remote-control-infrared-shutter-ir-rc-1g.html) or servos to trigger your camera.
+4 | Enables the generic PWM interface. This allows the use of [infrared triggers](https://hobbyking.com/en_us/universal-remote-control-infrared-shutter-ir-rc-1g.html) or servos to trigger your camera. The trigger signal is duplicated on both pins specified using `TRIG_PINS`.
 
 ## Other Parameters 
 
@@ -115,7 +122,7 @@ In this example, we will use a Seagull MAP2 trigger cable to interface to a Sony
 
 The recommended camera settings are:
 
-* `TRIG_INTERFAC=2` (Seagull MAP2).
+* `TRIG_INTERFACE=2` (Seagull MAP2).
 * `TRIG_MODE=4` (Mission controlled).
 * Leave the remaining parameters at their defaults.
 
@@ -174,7 +181,7 @@ The following diagram illustrates the sequence of events which must happen in or
 
 ![Sequence diag](../../assets/camera/sequence_diagram.jpg)
 
-<!-- Could generate using Mermaid: https://mermaidjs.github.io/mermaid-live-edito
+<!-- Could generate using Mermaid: https://mermaidjs.github.io/mermaid-live-editor
 {/% mermaid %/}
 sequenceDiagram
   Note right of PX4 : Time sync with mavros is done automatically