Synchronous and external trigger documentation.

Added files to documentation about how to externally trigger IMX296
and information about synchronising cameras for IMX477 & IMX296

documentation/asciidoc/computers/camera/external_trigger.adoc

@@ -0,0 +1,56 @@
+== External Trigger on the Global Shutter Camera
+The global shutter camera can be triggered externally by pulsing the XTR pin on the board.
+Multiple cameras can be connected to the same pulse leading to a better way to synchronise two cameras.
+
+The low puslewidth plus an extra 14.26us is equal to the exposure time. i.e. a low pulse of 10000us leads to an exposure time of 10014.26us.
+Framerate is directly controlled by how often you pulse the pin. A PWM frequency of 30Hz will lead to a framerate of 30 frames per second. 
+
+image::images/external_trigger.jpg[Image showing pulse format]
+
+=== Preparation
+
+First, remove/desolder resistor 22 on the GS Camera board. This connects GP0 to XTR and without removal, the camera will not operate
+in external trigger mode.
+
+Next solder a wire to the touchpoint of XTR on the GS Camera board. 
+Connect these to the pico - XTR to any pin. Pin 28 is used in this example. Ground is not needed to be connected.
+==== Boot up the Raspberry Pi with the camera connected.
+
+Enable external triggering through superuser mode:
+[,bash]
+----
+sudo su
+echo 1 > /sys/module/imx296/parameters/trigger_mode
+exit
+----
+
+==== Raspberry Pi Pico Code
+[,python]
+----
+from machine import Pin, PWM
+
+from time import sleep
+
+pwm = PWM(Pin(28))
+
+framerate = 30
+shutter = 6000  # In microseconds
+
+frame_length = 1000000 / framerate
+pwm.freq(framerate)
+
+pwm.duty_u16(int((1 - (shutter - 14) / frame_length) * 65535))
+----
+The low pulsewidth is equal to the shutter time, and the frequency of the PWM equals the framerate.
+
+=== Operation
+
+Run the code on the pico, and run the code for the camera:
+[,bash]
+----
+libcamera-hello -t 0 --qt-preview --shutter 3000
+----
+
+=== Note
+When running libcamera apps, you will need to specify a fixed shutter duration.
+This will ensure the AGC does not try adjusting camera's shutter speed, which is controlled by the external trigger pulse.

documentation/asciidoc/computers/camera/synchronous_cameras.adoc

@@ -0,0 +1,83 @@
+== Synchronous Captures
+=== IMX477
+==== Preparation
+
+Connect a potential divider of 2x 1.5KOhm Resistors to 3v3 and ground, creating 1.65V. This can be connected to either Pi.
+
+Solder the GND and XVS touchpoints of each HQ Camera board to each other.
+
+Connect the XVS wires to the 1.65V potential divider pull up.
+
+==== Boot up both Raspberry Pis
+
+The file `/sys/module/imx477/parameters/trigger_mode` determines which board outputs pulses, or waits to recieve pulses (source and sink).
+This parameter can only be altered in superuser mode.
+
+On the sink, run:
+[,bash]
+----
+sudo su
+echo 2 > /sys/module/imx477/parameters/trigger_mode
+exit
+----
+
+On the source, run:
+[,bash]
+----
+sudo su
+echo 1 > /sys/module/imx477/parameters/trigger_mode
+exit
+----
+
+Start the sink running:
+[,bash]
+----
+libcamera-vid --frames 300 --qt-preview -o sync.h264
+----
+
+Start the source running
+[,bash]
+----
+libcamera-vid --frames 300 --qt-preview -o sync.h264
+----
+
+Frames should be synchronous. Use --frames to ensure the same amount of frames are captured, and that the recordings are exactly the same length.
+Running the sink first ensures that no frames are missed.
+
+==== Notes
+
+The potential divider is needed to pull up the XVS pin to high whilst the source is in an idle state.
+This ensures that no frames are created or lost upon startup. The source whilst initialising goes from LOW to HIGH which can trigger a false frame.
+
+=== IMX296
+==== Preparation
+
+Create a potential divider made of 2x 1.5KOhm Resistors to 3v3 and ground, creating 1.65V. This can be connected to either pi.
+
+Solder 2 wires to the XVS touchpoint on each board and connect both of these wires together to the 1.65V potential divider.
+
+Solder the GND of each Camera board to each other. Also solder 2 wires to the XHS touchpoints on each board and connect these. No pullup is needed for XHS pin.
+
+On the boards that you wish to act as sinks, solder the two halves of the MAS pad together. This enters the sensor into sink mode.
+
+==== Boot up both Raspberry Pis
+
+Start the sink running:
+[,bash]
+----
+libcamera-vid --frames 300 -o sync.h264
+----
+Allow a delay before you start the source running (see note below). Needs to be roughly > 2 seconds.
+
+Start the source running:
+[,bash]
+----
+libcamera-vid --frames 299 -o sync.h264
+----
+==== Note
+Due to limitations of the IMX296 sensor, we are unable to get the sink to record exactly the same amount of frames as the source.
+**The source will record one extra frame before the sink starts recording.** This will need to be accounted for later in the application.
+Because of this, you need to specify that the sink records one less frame in the '--frames' option.
+
+A potential workaround for this is to use the external trigger modes for the IMX296, as described xref:camera_software.adoc#external-trigger-on-the-global-shutter-camera[here].This ensures that both cameras
+act in an identical manner.

documentation/asciidoc/computers/camera_software.adoc

@@ -61,3 +61,7 @@ include::camera/v4l2.adoc[]
 include::camera/csi-2-usage.adoc[]
 
 include::camera/raspicam.adoc[]
+
+include::camera/synchronous_cameras.adoc[]
+
+include::camera/external_trigger.adoc[]