參數(shù)資料
型號(hào)： LM9617CCEA
廠商： National Semiconductor Corporation
英文描述： Monochrome CMOS Image Sensor VGA 30 FPS
中文描述： 單色CMOS圖像傳感器顯卡30醫(yī)科
文件頁(yè)數(shù)： 18/37頁(yè)
文件大?。?/td> 366K
代理商： LM9617CCEA
Confidential
18
www.national.com
Functional Description
(continued)
11.0 OFFSET ADJUSTMENT
For maximum image quality over a wide range of light conditions
it is necessary to set an appropriate offset voltage before using
the sensor to capture images. This offset voltage must be
applied to the offset pin (38) of the sensor, and is used to adjust
the analogue video signal being fed to the internal A/D.
The level of the offset voltage determines the black level of the
image and has a direct impact on the image quality. Too high an
offset results in a white washed or hazy looking image, while too
low of an offset results in a dark image with low contrast even
though the light conditions are good.
A fine offset adjustment should be applied to each part by pro-
gramming the offset voltage via the I
2
C compatible serial inter-
face. To program an offset voltage the following procedure
should be followed:
The sensor’s
offset, fine_i
&
fine_ctrl
pins should be connected
as shown in figure 2.
The following procedure should be followed to calibrate the off-
set
Disable the black level compensation block by writing a logic
1 to bit 4 of the
Main Configuration Register 0
(MCFG0:
address 02Hex).
The offset can be adjusted by writing to the
Offset Compen-
sation Registers
(OCR: addresses 1F, 22 & 25 hex). Writing
00hex will give the largest voltage, while writing FF hex will
give the smallest value.
Run the following binary search algorithm
For n=7 to 0 step -1
{
Set bit n in the OCR registers (addresses 1F, 22 & 25
Hex) to a logic one by writing over the I
2
C compatible
interface.
Read a full frame and calculate the average black level
(BL
average
) of the first and last 5 black pixels in the every
row of the array
If (BL
average
< 100) then
Reset bit n in the OCR registers (addresses 1F, 22 &
25 Hex) to 0
else
Keep bit n set to one.
}
Enable the black level compensation block (if desired) by writ-
ing a logic 0 to bit 4 of the
Main Configuration Register 0
(MCFG0: address 02Hex).
12.0 OFFSET & GAIN
The fine offset adjustment and calibration method described in
section 11.0 will ensure that the sensor’s black level is optimized
for a fixed analog gain setting. However, when the analog gain is
changed substantially, the black level of the sensor will shift
resulting in a white washed image.
To stop this effect from occurring, the black level needs to be re-
calibrated. This can be done as part of the contrast adjustment
which is carried out by most digital image processors. If this is
not possible then the following method can be used.
The relationship between the gain and the offset can be
described with the following equation.
where:
Offset(G)
is the offset that needs to be programmed in
the OCR1, OCR2 & OCR3 registers to ensure
the correct black level setting for an analog
gain setting of
G
.
is the offset that needs to be programmed in
the OCR1, OCR2 & OCR3 registers to ensure
the correct black level setting for unity analog
gain, (
G
=0).
is a constant and will vary from sensor to sen-
sor
is the value programmed in the VGAIN regis-
ter of the sensor which determines the sen-
sor’s analog gain.
The following procedure should be used to calculate the value of
C
:
Use the calibration procedure described in section 11.0 to deter-
mine the offset at unity gain,
offset(0)
. Note the VGAIN register
should be set to 0.
Set the sensor’s analog gain register (VGAIN) to its max setting,
31, and repeat the calibration procedure described in section
11.0
.
This will allow the offset at full gain, 31, that needs to be
programmed in the OCR1, OCR2 & OCR3 registers to ensure
the correct black level setting to be determined.
The value of C for a particular sensor can be calculated using
the following formula:
Offset(0)
C
G
Once the value of C has been calculated, offset values for differ-
ent gain settings can be calculated using equation 1. It is recom-
mended that a two decimal point accuracy for C is maintained.
Offset(G) = Offset(
0
) + C * G
0.4
Offset(
31
) - Offset(
0
)
3.95
C
=
L
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