Omnivision CameraChip OV9650, Implementation Manual

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12 Proprietary to OmniVision Technologies Version 1.1, December 7, 2004
OV9650 Color CMOS SXGA (1.3 MegaPixel) CameraChip™ O
mni ision
3.3.2 Dummy Pixel Adjustment (Output Formatter)
By inserting dummy pixels in each row’s output, the frame rate can be changed while leaving the
pixel rate unchanged (see
Section 6.2.2 ).
3.3.3 Dummy Line Adjustment
By inserting dummy lines in each frame output, the frame rate can be changed while leaving the
pixel rate unchanged (see
Section 6.2.2 ).
3.4 Exposure Control
The OV9650 C
AMERA
C
HIP
supports both automatic and manual exposure control modes. The
exposure time is defined as the interval from the cell precharge to the end of the photo-induced
current measurement and can be controlled manually or by using the AEC function. This exposure
control uses a 'rolling' shutter as exposure time is set on a row-by-row basis rather than on a
frame-by-frame basis.
Exposure Time interval is defined as follows (15 fps in SXGA mode):
t
EXPOSURE
= 2
×
(1520
×
t
INT CLK
)
×
AEC[15:0] where AEC[15:0] is defined as:
AEC[15:0] = {MSB, LSB} = { AECHM [5:0] (0xA1), AECH [7:0] (0x10), COM1 [1:0] (0x04)}
Each bit in AEC[15:0] = t
ROW
interval = 2
×
(1520
×
t
INT CLK
)
Note that both the AEC and AGC functions are interactive so registers and functions may be
common to both. Also, in general, the AEC is the primary control and will be adjusted before the
AGC (the AGC acts to adjust and center the AEC).
The algorithm used for the electronic exposure control is based on average luminance of the full,
center-quarter, or center-half image. Each frame has digitally-generated averages (YUV for YUV
mode or RGB for RGB mode) which are used to set the exposure time. The exposure is optimized
for a "normal" scene that assumes the subject is well lit relative to the background. In situations
where the image is not well lit, the AEC white/black ratio may be adjusted to suit the needs of the
application.
3.4.1 Digital Exposure Control
3.4.1.1 Manual Exposure Control Mode
The manual exposure control mode allows for the companion backend processor to control the
OV9650 image exposure. The companion backend processor may write exposure values to
C
AMERA
C
HIP
register AECHM [5:0] (0xA1) (MSB), AECH [7:0] (0x10), and COM1 [1:0] (0x04) (LSB)
according to its corresponding Auto-Exposure Control (AEC) algorithm.