Datasheet

PDF: 09005aef8202ec2e/Source: 09005aef8202ebf7 Micron Technology, Inc., reserves the right to change products or specifications without notice.
MT9D111__2_REV5.fm - Rev. B 2/06 EN
21 ©2004 Micron Technology, Inc. All rights reserved.
MT9D111 - 1/3.2-Inch 2-Megapixel SOC Digital Image Sensor
Architecture Overview
Micron Confidential and Proprietary
ima, if any, are sorted by position and used to build a weight histogram of the scanned
positions. The histogram is build by assigning to each position the sum of weights of all
AF windows whose normalized sharpness scores peaked at that position. The position
with the highest weight in the histogram is then selected as the best lens position. This
method of selecting the best position may be compared to voting. The voting entities are
the AF windows, i.e. different image zones. Depending on the imaged scene, they may all
look sharp at the same lens position or at different ones. If all the zones have equal
weight, the lens position at which a simple majority of them looks sharp is voted the
best. If the weights of the zones are unequal, it means that making some zones look
sharp is more important than maximizing the entire sharp-looking area in the image. If
there are no valid votes, because sharpness scores from all the AF windows vary too little
with the lens position, the AF algorithm arbitrarily chooses the start position as the best.
What happens after the first scan is user-programmable—the AF algorithm gives the
user a number of ways to proceed with final lens positioning. The user should select a
way that best fits the magnitude of lens actuator hysteresis and desired lens proximity to
the truly optimal position. Actuators with large, unknown or variable hysteresis should
do a second flyback and either jump or retrace the steps of the first scan to the best
scanned position. Actuators with constant hysteresis (like gear backlash) can be moved
to that position directly from the end position of the scan—the AF algorithm offers an
option to automatically increase the length of this move by a preprogrammed backlash-
compensating step. Finally, if the first scan is coarse relative to the positioning precision
of the lens actuator and depth of field of the lens, an optional second fine scan can be
performed around the lens position voted best after the first scan. This second scan is
done in the same way as the first, except that the positions it covers are not pre-selected.
Instead, the AF algorithm user must set step size and number of steps for the second
scan. The second scan must be followed by the same hysteresis-matching motion
sequence as the first scan, e.g. a third flyback and jump to the best position.
Modes
There are four AF camera modes that the MT9D111 can fully support if it controls the
position of the camera lens.
1. Snapshot mode
In this mode, a camera performs auto focusing upon a user command to do so. When
the auto focusing is finished, a snapshot is normally taken and there is no further AF
activity until next appropriate user command. The MT9D111 can do the auto focus-
ing using its own AF algorithm described above or a substitute algorithm loaded into
its RAM. It can then wait or automatically proceed with other operations required to
take a snapshot.
2. Locked mode
The MT9D111 can be commanded to lock the lens in its current position. Between the
command to lock the lens and another to release it, the lens does not respond to other
commands or scene changes.
3. Focus-free mode
In many situations, e.g. under low light or during video recording, it may be impossi-
ble or undesirable to focus the lens prior to every image capture. Instead, the lens can
be locked in a position most likely to produce satisfactory images, e.g. the hyperfocal
position. This position can be programmed into the MT9D111, and it can move and
hold the lens there on command.
4. Manual mode
In this mode there is no AF activity—focusing the camera is left to its user. The user
typically can move the camera lens in steps, by manually issuing commands to the
lens actuator, and observe the effect of his actions on a preview display. The MT9D111