Synchronous and external trigger documentation.

documentation/asciidoc/computers/camera/external_trigger.adoc

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+== External Trigger on the Global Shutter Camera
+The global shutter camera can be triggered externally by pulsing the external trigger (denoted on the board as XTR) connection on the board.
+Multiple cameras can be connected to the same pulse, allowing for an alternative way to synchronise two cameras.
+
+The exposure time is equal to the low pulse-width time plus an additional 14.26us. 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.
+
+In this example, we will be using a Raspberry Pi Pico to create the pulses that will drive the camera modules.
+
+image::images/external_trigger.jpg[Image showing pulse format]
+
+=== Preparation
+
+WARNING: This modification includes removing an SMD soldered part. You should not attempt this modification unless you feel you are competent to complete it.
+When soldering to the Camera board, please remove the plastic back cover to avoid damaging it.
+
+**If your board has transistor Q2 fitted, shown in blue on the image below, you will need to remove R11 from the board, as shown in red.** 
+This connects GP0 to XTR and without removal, the camera will not operate in external trigger mode.
+The location of the components is displayed below.
+
+image::images/resistor.jpg[Image showing resistor to be removed]
+
+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 does not need 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 Micropython 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 set the camera running:
+[,bash]
+----
+libcamera-hello -t 0 --qt-preview --shutter 3000
+----
+A frame should now be generated every time that the Pico pulses the pin. Variable framerate is acceptable, and can be controlled by simply
+varying the duration between pulses. No options need to be passed to libcamera-apps to enable external trigger.
+
+=== Note
+When running libcamera apps, you will need to specify a fixed shutter duration (the value does not matter).
+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

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+== Synchronous Captures
+
+Both the HQ Camera and the Global Shutter Camera, have support for synchronous captures.
+Making use of the XVS pin (Vertical Sync) allows one camera to pulse when a frame capture is initiated.
+The other camera can then listen for this sync pulse, and capture a frame at the same time as the other camera.
+
+For correct operation, both cameras require a 1.65v pull up voltage on the XVS line, which is created by a potential divider through the 3.3v and GND pins on the Raspberry Pi.
+How to setup the potential divider is displayed below.
+
+image::images/synchronous_camera_wiring.jpg[Image showing potential divider setup]
+
+=== HQ Camera
+==== Preparation
+
+Connect a potential divider of 2 equally high impedance ( > 1kOhm) 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 sink.h264
+----
+
+Start the source running
+[,bash]
+----
+libcamera-vid --frames 300 --qt-preview -o source.h264
+----
+
+Frames should be synchronous. Use --frames to ensure the same number of frames are captured, and that the recordings are exactly the same length.
+Running the sink first ensures that no frames are missed.
+
+==== Note
+
+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.
+
+=== Global Shutter Camera
+The global shutter camera can also be operated in a synchonous mode. However, the source will record one extra frame (see xref:camera_software.adoc#note-2[note] below).
+
+A much better alternative method to ensure that both cameras capture the same amount of frames is to use the external trigger method, 
+described xref:camera_software.adoc#external-trigger-on-the-global-shutter-camera[here].
+
+To operate as source and sink together, the Global Shutter Cameras also require connection of the XHS (horizontal sync) pins together. 
+However, these do not need connection to a pullup resistor.
+
+==== 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.
+
+FFmpeg has the ability to resync these two videos. By dropping the first frame from the source, we then get two recordings of the same frame
+ length and with the same starting point.
+
+[,bash]
+----
+ffmpeg -i source.h264 -vf select="gte(n\, 1)" source.h264
+----

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[]