FEATURES & FUNCTIONALITY
Neo
SECTION 2
Page 52
2.7.2 - Global Shutter triggering modes
Global Shutter mode, which can also be thought of as a ‘snapshot’ exposure mode, means that all pixels of the
array are exposed simultaneously. Before the exposure begins, all pixels in the array are cleared of charge
using the Global Clear. At the start of the exposure each pixel simultaneously begins to collect charge and is
allowed to do so for the duration of the exposure time. At the end of exposure, each pixel transfers the
accumulated charge simultaneously to its read out node. Global Shutter requires a reference frame to be read
out of the sensor in addition to the signal frame, therefore effectively halving the Cycle Time that would have
been achieved in Rolling Shutter mode.
The Global Shutter signals shown in the diagrams are as follows:
•
Acquisition Start
: This is an internal pulse purely for illustrative purposes and indicates when the
camera receives a command from software to start the pre-programmed acquisition sequence.
•
FIRE
: In Global Shutter mode, the FIRE output indicates the exposure period, which is identical for all
pixels. This pulse is available to the user via the FIRE output pin
•
ARM
: The ARM output from the camera is used for external and software triggering modes to indicate
when the camera is ready to accept another incoming trigger.
•
Global Clear
: Global Shutter uses Global Clear to begin a new exposure. When this pulse is HIGH,
charge is drained from every pixel thus preventing the accumulation of charge on the sensor. When the
pulse is LOW, any photo-electrons generated within a pixel are accumulated for subsequent read out.
The falling edge indicates the start of an exposure.
•
Charge Transfer
: This signal indicates when charge in the pixel is transferred to the measurement
node and effectively ends the exposure. The charge is transferred while the pulse is HIGH.
•
Frame Read Out Phase
: This signal indicates when reference and signal frames are read out of the
sensor.
The timing tables accompanying each of the triggering diagrams that follow indicate the exposure and cycle
times achievable in each triggering mode. These are based on Frame and Row Periods as shown below:
Table 15: Timing Parameters based on Sensor Clock Speed for Global Shutter
Parameter
Sensor Read out Speed
200 MHz
400 MHz
560 MHz
1 Row (2624 clock cycles)
26.24 µs
13.12 µs
9.37 µs
1 Frame ( 2160 rows)
28.34 ms
14.16 ms
10.1 ms
Charge Transfer Time
5.62 µs
2.81 µs
2 µs
Interframe (9 Rows)
236 µs
118.8 µs
84.3 µs
1 Row is the time taken to read out 2592 pixels. This is currently 2624 clock cycles. The sensor is split into 2
halves with each having an independent data output from the sensor. This means the Frame Period is 1080
rows x 2624 clock cycles. The Interframe defines the overall time taken to transfer change and select whether
the subsequent read out is a reference or signal frame.
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