VV5402 データシートの表示（PDF） - Vision

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VV5402

Multi-standard Monochrome CMOS Image Sensor

Vision 

Main Features

Video Output

The VV5402 delivers a fully-formatted

composite monochrome video signal. Stand-

ards options include EIA (320 x 244) and

CCIR (384 x 287). On chip signal conditioning

allows user-selection of linear or gamma-

corrected output.

The integrated 75Ω driver eliminates the need

for additional active components to drive

standard loads, including double terminated

lines.

Automatic Exposure and Gain Control

Automatic exposure and gain control are

enabled with AEC=1 (pin 21) and AGC=1

(pin22). However, If AEC is inhibited by pin

21, AGC is also inhibited. Inhibiting AEC or

AGC by taking pin 21 or 22 low freezes the

current value(s) for these.

Automatic Exposure Control (AEC)

The VV5402 controls exposure over a range

of 99,000:1 in EIA mode and 146,000:1 in

CCIR mode, and operates at illumination

levels as low as 0.5 lux at standard clock

frequencies. (The system clock frequency can

be reduced to provide increased sensitivity.)

Automatic exposure control is achieved by

varying pixel current integration time

according to the average light level on the

sensor. This integration time can vary from

one pixel clock period to one frame period.

Pixels above a threshold white level are

counted every frame, and the number at the

end of the frame defines the image exposure.

If the image is other than correctly exposed, a

new value for integration time is calculated

and applied for the next frame. Corrections

are either ±1/8 or ±1/64, depending upon the

degree of over or under exposure.

Automatic Gain Control (AGC)

The VV5402 automatically increases the

system gain of its output stage if with the

current gain setting and maximum exposure

the image is too dark. Gain can be varied from

x1 to x16 in times-two steps, giving five

different gain settings.

If the scene is too dark and the integration

period has almost reached its maximum

value, the gain value is incremented by one

step (times two). In the same frame period the

exposure value is divided by two, halving the

integration period. The exposure controller

then increases the exposure value as neces-

sary. Similarly if the image is too bright and

the integration period is short then gain will be

reduced by one step (divide by two) and the

exposure value will be doubled. The exposure

controller can then adjust the exposure value

as necessary to provide a correctly exposed

image.

Backlit Mode

The VV5402 can be configured to operate in

two auto-exposure modes, selected by the

BKLIT pin (pin28) state. The default mode

(BKLIT = 0) provides exposure control for

normally illuminated scenes. For scenes

where a bright background can cause the

foreground subject to be severely under

exposed, the ‘Backlit’ mode (BKLIT = 1) offers

superior performance.

‘Backlit Mode’ (BKLIT=1) operates by using a

higher threshold level for the exposure control

comparator over the central area of an image,

which is therefore exposed for longer and so

enhanced. The area in which the higher

comparator threshold is used when BKLIT=1

is illustrated below:

09/04/97

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