Altum: PPS Pin Usage and Top of Frame Output

Altum’s Pin 3 is an output. By default, it is a PPS output. For advanced integrations, this pin can be configured as a Top of Frame output instead, to signal to external devices the precise moment when the Altum captures images.

In order for this pin to send a pulse when a capture is triggered, you will need to set up the Top of Frame Output settings in the web interface (Settings > Advanced Settings) as seen below.

PPSpinOUT.png

Technical Details

The Top of Frame output is a 3.3V, 100ms pulse, with the leading edge indicating the beginning of the exposures. The output should be considered an open-drain output (though it has an edge-accelerator in both directions). Large capacitive loads can cause the edge-accelerator to be triggered multiple times as the signal rises and falls which may can cause double triggering. The output is coupled to the connector pin through a 100ohm resistor and has around 25pF of capacitance to the pin.

WARNING: Damage to the camera may occur if another output pin, such as the PPS output on a GPS receiver, is connected when Top of Frame Output mode is enabled. Also, the top of frame signal should not go through the DLS.

There are two pulse polarity settings, “Pulse low, rest high” and “Pulse high, rest low”, shown below:

ppsPinOutAltum.PNG

Post Processing Required

If you use this method, you will have to update the metadata of the images with the RTK GPS information in post processing. As a backup, it is a good idea to send constant GPS information to the camera during flight using the Serial or HTTP APIs, which the camera will write into the image metadata.

The basic workflow is as follows:

  • During flight, log the top-of-frame output from the PPS out pin.
  • After the flight, use your log to match RTK GPS information with each image.
  • Write the GPS information into the metadata of each image.
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