Cheetah Python Camera with USB3 Interface
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User Manual
December 5, 2017
Page
62
of
78
Rev 1.0
5.2 Frame Time Control
5.2.1 Internal Line and Frame Time Control
The camera speed (frame rate) depends on the CMOS read-out time (frame time). Frame
time is the time it takes to read out all of the pixels on the CMOS imager. The following
formula (1.1) calculates the frame rate:
Frame rate [fps] = 1 / read-out time [sec]
(1.1)
5.2.1.1 Pixel Clock Rate Control
The camera read-out speed exceeds the USB3 compatible interface output rate. You can
use the Pixel Clock Rate function to program the camera’s speed to match the U
SB3 image
capture rate.
Always adjust the Pixel Clock to the maximum rate possible without the USB3 interface
missing or skipping data. This minimizes the dark current generated within the pixel and
the dark current noise.
5.2.1.2 Frame Time Control
After adjusting the Pixel Clock to minimize the line read-out time, you can increase the
frame time using the programmable frame time function. When enabled, the sensor reads
out the frame, then idles and inserts a vertical blanking period at the end of the frame
readout to provide the desired frame rate.
In this way, you can match
the camera’s frame rate to application requirements.
You can
reduce the frame time to about one second with a precision of one microsecond. Using
Frame Time control, you can achieve exposure times longer than the time needed to read
out the image sensor. You might need to adjust frame time to control the amount of
throughput if connecting more than one USB device to the same host computer.
If the exposure time is greater than 50ms, keep
camera vibration to a minimum to keep a
motion induced smear from appearing on the image.
5.2.2 Camera Output Control
The USB3 compatible interface transfers data at 5.0Gbps, but uses 8b/10b encoding to
achieve DC-Balance and allow reasonable clock recovery. This means the maximum
theoretical data rate is 4Gbps. However, hardware and drivers vary considerably between
manufacturers and with firmware overhead, the practical data bit rates range between
2.6Gbps and 3.2Gbps depending upon the hardware and drivers in the receiving
computer. Note: if the camera output rate exceeds the USB3 compatible interface
bandwidth, adjust the camera line rate using the Pixel Clock control to match the camera
output rate to the USB3 transfer rate.
The table below describes tested USB3 compatible full frame rates.
