Data Sheet

CMOS VGA (640 x 480) image sensor with OmniPixel3-HS™ technology
OV7675/OV7175
proprietary to OmniVision Technologies PRODUCT SPECIFICATION version 2.0
4.4 AEC/AGC algorithms
4.4.1 exposure control
The OV7675/OV7175 supports both automatic and manual exposure control modes. The exposure time is defined as
the interval from the cell pre-charge 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, which means the exposure time is
set on a row-by-row basis rather than on a frame-by-frame basis.
4.4.2 exposure time
Exposure time unit is the interval of row, as shown below.
t
Exposure
= AEC[15:0] x t
Row
interval
where AEC[15:0] is set by register {0x07[5:0], 0x10[7:0], 0x04[1:0]}
AEC[15:0] = {0x07[5:0], 0x10[7:0], 0x04[1:0]}
The OV7675 array always output, VGA resolution, the row interval is
t
Row interval
= 2 x (784 + Dummy Pixels) x t
INT CLK,
and AEC[15:0] is limited by the number of rows of VGA resolution plus the number of dummy lines.
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 (AGC acts to adjust and center the AEC).
The algorithm used for the electronic exposure control is based on luminance of the full, center quarter, or center half
image. The exposure is optimized for a “normal” scene that assumes the subject is well lit relative to the background.
4.4.3 banding filter
The OV7675/OV7175 also provides a rolling horizontal band eliminate function in auto exposure mode. A banding filter
is deployed to filter out the banding effect caused by the 50/60 Hz lighting. To enable this function, register 0x13[5] must
be set to high. The OV7675/OV7175 only supports manual banding filter.
In 50 or 60Hz flicker light, the exposure time must be a multiple of the flicker interval to avoid band shown on the image.
For 50Hz light, the exposure time must be
t
Exposure
= N/100
and for 60Hz light, the exposure time must be
t
Exposure
= N/120
where N is a positive integer.
Simpo PDF Password Remover Unregistered Version - http://www.simpopdf.com