MT9M114_16 Datasheet, PDF (35/62 Pages) ON Semiconductor – High-Definition (HD) System-On- A-Chip (SOC) Digital Image Sensor

MT9M114: 1/6-Inch 720p High-Definition (HD) System-On-A-Chip (SOC) Dig-
ital Image Sensor
Camera Control and Auto Functions
Auto Exposure
The auto exposure algorithm performs automatic adjustments of the image brightness
by controlling exposure time and analog gains of the sensor core as well as digital gains
applied to the image.
Auto exposure is implemented by a firmware driver that analyzes image statistics
collected by the exposure measurement engine, makes a decision, and programs the
sensor core and color pipeline to achieve the desired exposure. The measurement
engine subdivides the image into 25 windows organized as a 5 x 5 grid.
Four auto exposure algorithm modes are available:
• Average brightness tracking (ABT) or Average Y (ae_rule_algo VAR= 9, 0x0004, 0x0000
or REG= 0xA404, 0x0000)
The average brightness tracking AE uses a constant average tracking algorithm where
a target brightness value is compared to a current brightness value, and the gain and
integration time are adjusted accordingly to meet the target requirement.
• Weighted Average Brightness (ae_rule_algo VAR= 9, 0x0004, 0x0001 or REG= 0xA404,
0x0001)
Each of the 25 windows can be assigned a weight relative to other window weights,
which can be changed independently of each other. For example, the weights can be
set to allow the center of the image to be weighted higher than the periphery. See
Figure 26.
Figure 26: 5 x 5 Grid
MT9M114/D Rev. 11, 2/16 EN
• Adaptive Weighted AE for highlights (ae_rule_algo VAR= 9, 0x0004, 0x0002 or REG=
0xA404, 0x0002)- The scene will be exposed based on the brightness of each window,
and will adapt to correctly expose the highlights (brighter windows). This would
correctly expose the foreground of an image when the background is dark.
• Adaptive Weighted AE for lowlights(ae_rule_algo VAR= 9, 0x0004, 0x0003 or REG=
0xA404, 0x0003)- The scene will be exposed based on the brightness of each window,
and will adapt to correctly expose the lowlights. This would correctly expose the fore-
ground of an image when the background is brighter.
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