Datasheet

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MT9D111__7_REV5.fm - Rev. B 2/06 EN
157 ©2004 Micron Technology, Inc. All rights reserved.
MT9D111 - 1/3.2-Inch 2-Megapixel SOC Digital Image Sensor
Start-Up and Usage
Micron Confidential and Proprietary
Auto Exposure
Two types of auto exposure are available—preview and evaluative.
Preview
In preview AE, the driver calculates image brightness based on average luma values
received from 16 programmable equal-size rectangular windows forming a 4 x 4 grid. In
preview mode, 16 windows are combined in 2 segments: central and peripheral. Central
segment includes four central windows. All remaining windows belong to peripheral
segment. Scene brightness is calculated as average luma in each segment taken with cer-
tain weights. Variable ae.weights[3:0] specifies central zone weight, ae.weights[7:4] -
peripheral zone weight.
The driver changes AE parameters (IT, Gains, and so on) to drive brightness (ae.Cur-
rentY) to programmable target (ae.Target). Value of one step approach to target is
defined by ae.JumpDivisor variable. Expected brightness is
Ynew = ae.CurrentY+(ae.Target-ae.CurrentY)/ ae.JumpDivisor.
To avoid unwanted reaction of AE on small fluctuations of scene brightness or momen-
tary scene changes, the AE driver uses temporal filter for luma and gate around AE luma
target. The driver changes AE parameters only if buffered luma outsteps AE target gates.
Variable ae.lumaBufferSpeed defines buffering level.
32*Ybuf1=Ybuf0*(32-ae.lumaBufferSpeed)+Ycurr* ae.lumaBufferSpeed;
Values ae.lumaBufferSpeed=32 and ae.JumpDivisor=1 specify maximal AE speed.
Evaluative
A scene evaluative AE algorithm is available for use in snapshot mode. The algorithm
performs scene analysis and classification with respect to its brightness, contrast, and
composure and then decides to increase, decrease, or keep original exposure target. It
makes most difference for backlight and bright outdoor conditions.
Exposure Control
To achieve the required amount of exposure, the AE driver adjusts the sensor integration
time R9:0, R12:0, gains, ADC reference, and IFP digital gains. To reject flicker, integration
time is typically adjusted in increments of ae.R9_step. ae.R9_step specifies duration in
row times equal to one flicker period. Thus, flicker is rejected if integration time is kept a
natural factor of the flicker period.
Exposure is adjusted differently depending on illumination situation.
• In extremely bright conditions, the exposure is set using R12:0, R9:0 and analog gains.
R12:0 is used to achieve very short integration times. In this situation,
R9:0<ae.R9_step and flicker are not rejected.
• In bright conditions where R9:0>=ae., R9_step R9:0 is set as a natural factor of
ae.R9_step. Analog gains are also used, but the green gain, also called virtual gain,
does not exceed 2x. ae.minVirtGain limits minimal integration time and is expressed
in flicker periods. ae.Index indicates the current integration time expressed in the
same form.
• Under medium-intensity illumination, the integration time can increase further. For
any given exposure, the best signal-to-noise ratio can be typically obtained by using
the longest exposure and the smallest gain setting. However, a long exposure time can
slow down the output frame rate if the former exceeds the default frame rate, R9:0 >
R3:0 + R6:0 + 1. Integration ae.IndexTH23 specifies the breakpoint where AE scheme,
giving preference to increasing the shutter width, is replaced with another scheme