Point Grey Flea3 USB 3.0 Technical Reference 6 Image Acquisition
Figure 6.1: External trigger timing characteristics
It is possible for users to measure this themselves by configuring one of the camera’s GPIO pins to output a strobe
pulse (see Programmable Strobe Output on page 42) and connecting an oscilliscope up to the input trigger pin and the
output strobe pin. The camera will strobe each time an image acquisition is triggered; the start of the strobe pulse
represents the start of exposure.
6.4.2 Minimum Trigger Pulse Length
A digital signal debouncer helps to ensure that the camera does not respond to spurious electrical signals that are
shorter than 16 ticks of the current pixel clock setting. This safeguard results in a minimum 16-tick delay before the
camera responds to a trigger signal. The pixel clock frequency can be read from the floating point PIXEL_CLOCK_FREQ
register 0x1AF0 (page 32).
6.4.3 Maximum Frame Rate in External Trigger Mode
This section only applies to Rolling Shutter models (FL3-U3-13S2 and FL3-U3-32S2).
When image capture on a rolling shutter camera is triggered by an external source, achievable frame rate is half the
rate achievable in free-running mode, regardless of the rate that is specified. This difference is caused by a change in
the way rolling shutter cameras operate between free-running and trigger modes.
In free-running mode, integration can occur as quickly as the camera's pixel clock allows, because by the time the
bottom row of the image sensor has integrated, the top row is already read out, and is free to integrate the next
image without delay.
In trigger mode, however, rolling shutter cameras begin read-out only after the entire image is integrated. The
camera is not ready to receive another trigger until read-out is complete. Essentially, one frame is required for reset,
and one frame for read-out. As a result, the frame rate achieved in trigger mode will be half the rate specified for
free-running mode.
Revised 9/27/2012
Copyright ©2011-2012 Point Grey Research Inc.
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