VG6640 datasheet(79/126 Pages) STMICROELECTRONICS

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DocID023597 Rev 11

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VG6640, VD6640

Video system

125

7.3

Exposure and gain control

The image sensor does not contain any form of automatic exposure control. To produce a

correctly exposed image the integration periods, analog gain and digital gain for the pixels

must be calculated by an exposure control algorithm implemented externally. The

parameters are then written to the image sensor using the control interface.

The exposure control parameters available on the image sensor are:

•

long integration time

•

medium integration time

•

short integration time

•

analog gain

•

digital gain

7.3.1

Coarse integration time

The integration time is the amount of time between a row of pixels being reset and when this

row is read. The longer the time then the more light that is captured.This sensor supports

only Coarse Integration in which the Integration period is measured in integer line times

which is derived from the pixel clock.

The sensor provides HDR output by integrating each pixel three times per frame and then

merging the data. These exposures comprise a long, medium and short integration time. At

the end of the long integration time, the long information is read in a given row, then stored

in memory. The read out pixel row is reset, then the medium integration time starts in that

row. After the end of the medium integration time, the medium information is read and then

also stored in memory. The read out pixel row is then reset again, the short integration time

then starts. Finally, the short information is read out of the pixel array. The three data values

are then digitally merged within the sensor to give a single output frame.

Figure 23 shows an example of applying a long integration of 600 lines, followed by a

medium integration of 96 lines, then a short integration of 13 lines. This diagram shows the

point at which pixels are reset, before starting integration, the integration period, then read

out. There is a latency between integration starting for the long exposure and the data being

output by the device. This latency is dependent on the integration times, fixed offsets and

the delay through ISP processing.

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