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SE4400 Integration Guide
Image Acquisition Procedure
Figure 5-3
summarizes the Mode 3 imaging acquisition procedure.
Table 5-1
provides a
summary of the Mode 3 input and output signal functions.
The CCD is exposed to light during the time that EXSFT is low. This exposure time must
be an integer multiple of line times "H", unless it begins after the negative edge of VREF, in
which case this time is not restricted. The rising edge of EXSFT begins the transfer of data
received during exposure. The first 13 lines are dummy, meaning they are either optically
blocked or they are used as a timing delay. VREF subsequently goes high and remains high
as long as the DSP's internal A/D converter is sampling valid vertical lines. The delay
between the negative edge of VREF and the positive edge of EXSFT can be reduced to a
minimum of 16H, but each frame must be at least 490H in length. This delay is listed as
32H in
Figure 5-3
in order to achieve a frame rate of 30fps. If the focus motor position is
altered (see
Motor Control
on page 5-16), a 20 ms settling time must pass before the
exposure period can begin, so for each frame in which the focus was changed the exposure
time cannot exceed a frame time minus 20 ms.
The negative edge of EXHT triggers the reading of the next horizontal line of the CCD. This
triggering edge must comply with the timing requirements shown in
Figure
5-3. The duty
cycle of EXHT has no significance. After the negative edge of EXHT, 45 dummy pixels are
read out. HREF subsequently goes high and remains high as long as the A/D converter is
sampling valid horizontal pixels. During the time when both HREF and VREF are high, the
8-bit pixel data, Pix_D0 through Pix_D7, should be sampled on each positive edge of
PCLK. The delay between the negative edge of HREF and the positive edge of EXHT
cannot be reduced from 78T. EXHT should pulse continuously while the SE4400 is
capturing images.
2
Use the I
C registers to set the gain. The subsequent frame is then transferred with the new
gain. Use an auto gain mode to simplify the process. For information on setting gain, see
Exposure & Gain Control
on page 5-16.
During the first frame (Frame 0), meaningless data is transferred because the CCD has not
been exposed to light since it was reset. Discard this data.
5-6
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