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MT9T111_DG - Rev. B 9/10 EN
91
©2007 Aptina Imaging Corporation. All rights reserved.
MT9T111: Developer Guide
Auto Focus
Preliminary
Auto Focus
The AF algorithm implemented in the MT9T111 seeks to maximize the sharpness of
vertical lines in the sensor's image output by guiding an external lens actuator to the
position of the best lens focus. The algorithm's implementation has a hardware compo-
nent called the focus measurement engine (FME) and a firmware component called AF
driver. The algorithm is lens-actuator-independent: it provides guidance by means of an
abstract 8-bit variable called the logical_lens_position, leaving the translation of its
changes into physical lens movements to a separate auto focus mechanics (AFM) driver,
as shown in Figure 51.
Figure 51:
Auto Focus Functional Block Diagram
AF Algorithm
The AF algorithm relies on the auto focus mechanics (AFM) driver and the master two-
wire serial interface (I
2
C-type driver) or VGPIO driver to generate digital output signals
needed to move different lens actuators. The MT9T111 supports various types of actua-
tors including both micro-electro mechanical systems (MEMS) and voice-coil motor
(VCM) types of actuators. The MT9T111 does not support stepping motor type actua-
tors. The AFM driver must correctly indicate at all times if the lens it controls is
stationary or moving. This is required to prevent the AF driver from using line sharpness
measurements distorted by concurrent lens motion, and from issuing new commands
to move the lens while the previous one is still being executed.
Table 34 on page 92 shows the driver ICs supported by the MT9T111. Users should
program the AFM variable offset 0x0003[2:0] to select the driver IC that is connected to
MT9T111. AFM variable offset 0x0021 stores the slave address for I
2
C type of driver IC.
In addition to those drives indicated in Table 34 on page 92, the MT9T111 supports other
types of driver interfaces through the use of customized code.
MT9T111
MCU
AF & AFM
Drivers
Driver
Master
I
2
C
IFP
Focus
Measurement
Engine
VGPIO
Ports
Sensor
Core
Image
Data
SCLK
S
DATA
Sharpness
Score
VGPIO
Actuator
Lens