NOIV1SN1300A, NOIV2SN1300A
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The automatic exposure control loop is enabled by
asserting the aec_enable configuration in register 160.
NOTE: Dual and Triple slope integration is not
supported in conjunction with the AEC.
AEC Statistics Block
The statistics block calculates the average illumination of
the current image. Based on the difference between the
calculated illumination and the target illumination the
statistics block requests a relative gain change.
Statistics Subsampling and Windowing
For average calculation, the statistics block will
sub-sample the current image or windows by taking every
fourth sample into account. Note that only the pixels read out
through the active windows are visible for the AEC. In the
case where multiple windows are active, the samples will be
selected from the total samples. Samples contained in a
region covered by multiple (overlapping) window will be
taking into account only once.
It is possible to define an AEC specific sub-window on
which the AEC will calculate it’s average. For instance, the
sensor can be configured to read out a larger frame, while the
illumination is measured on a smaller region of interest, e.g.
center weighted.
Table 24. AEC SAMPLE SELECTION
Register Name Description
192[10] roi_aec_en-
able
When 0x0, all active windows are se-
lected for statistics calculation.
When 0x1, the AEC samples are
selected from the active pixels con-
tained in the region of interest defined
by roi_aec
253-255 roi_aec These registers define a window from
which the AEC samples will be selec-
ted when roi_aec_enable is asserted.
Configuration is similar to the regular
region of interests.
The intersection of this window with
the active windows define the selec-
ted pixels. It is important that this win-
dow at least overlaps with one or
more active windows.
Important note for rolling shutter operation: a minimum
of 4 dummy lines is required when using the automatic
exposure controller.
Target Illumination
The target illumination value is configured by means of
register desired_intensity.
Table 25. AEC TARGET ILLUMINATION
CONFIGURATION
Register Name Description
161[9:0] desired_in
tensity
Target intensity value, on 10bit scale.
For 8bit mode, target value is con
figured on desired_intensity[9:2]
Color Sensor
The weight of each color can be configured for color
sensors by means of scale factors. Note these scale factor are
only used to calculate the statistics in order to compensate
for (off-chip) white balancing and/or color matrices. The
pixel values itself are not modified.
The scale factors are configured as 3.7 unsigned numbers
(0x80 = unity).
Table 26. COLOR SCALE FACTORS
Register Name Description
162[9:0] red_scale_factor Red scale factor for AEC statist-
ics
163[9:0] green1_scale_fa
ctor
Green1 scale factor for AEC
statistics
164[9:0] green2_scale_fa
ctor
Green2 scale factor for AEC
statistics
165[9:0] blue_scale_factor Blue scale factor for AEC stat-
istics
Configure these factors to their default value for
monochrome sensors.
AEC Filter Block
The filter block low-pass filters the gain change requests
received from the statistics block.
The filter can be restarted by asserting the restart_filter
configuration of register 160.
AEC Enforcer Block
The enforcer block calculates the four different gain
parameters, based on the required total gain, thereby
respecting a specific hierarchy in those configurations.
Some (digital) hysteresis is added so that the (analog) sensor
settings don’t need to change too often.
Exposure Control Parameters
The several gain parameters are described below, in the
order in which these are controlled by the AEC for large
adjustments. Small adjustments are regulated by digital gain
only.
•Exposure Time
In rolling shutter mode, the exposure time is the time
elapsed between resetting a particular line and reading it out.
This time is constant for all lines in a frame, lest the image
be non-uniformly exposed. The exposure time is always an
integer multiple of the line time.
In a snapshot shutter mode, the exposure is the time
between the global image array reset de-assertion and the
pixel charge transfer. The granularity of the integration time
steps is configured by the mult_timer register.
NOTE: The exposure_time register is ignored when the
AEC is enabled. The register fr_length defines
the frame time and needs to be configured
accordingly.