STV0681 datasheet(14/44 Pages) STMICROELECTRONICS

14 / 44 page

Detailed Features

STV0681

14/44

ADCS 7283313C

2.6

Anti-flicker exposure and gain control

2.6.1

General

The chipset operates automatic exposure and gain control for either 50Hz or 60Hz mains-driven

indoor lighting, using the same 12MHz crystal. This improves picture quality by selecting a set of

exposure values which minimize ‘flicker’ effects. Detection of the mains frequency is dependant on

the status of the GPIO3 pin, which can be achieved by population of a PCB link at a late stage in

production, once the country of destination is known, without the need to change the crystal

frequency.

The auto exposure and gain algorithm is always enabled during Snapshot’/self-timer/continuous

mode. When the shutter button is pressed in ‘Snapshot’ mode, the chipset captures an image if the

exposure and gain values are suitable for the current scene. If the light has suddenly changed, the

camera may emit an audible tone to indicate that more time is required to reach the correct

exposure target. In ‘Snapshot’ mode the chipset only captures the image data if sufficient light is

present in the image. In continuous capture mode, the chipset captures images regardless of

whether enough light is present.

The exposure control algorithm in STV0681 chooses exposure values which minimize “flicker”

effects from occurring under fluorescent lighting. STV0681 can only prevent flicker in lighting

possible. Hence choosing the correct anti-flicker setting is important, in order to prevent dark stripes

from appearing across the image, and this selection must be done in hardware.

2.6.2

Flashgun exposure

When the STV0681 and a flashgun module are included in the camera, and the flashgun enable

signal is high, the exposure mode operates in a different manner. The CMOS sensor progressive

scan readout requires that the sensor is set to maximum exposure so that all lines are exposed.

The flashgun operates during a few 100 µs in order to correctly expose all sensor lines, (contact ST

for more precise details), therefore the flashgun module design should have reached maximum light

output within this period after the falling edge of the flash trigger output from STV0681.

Possible flashgun implementations, regarding hardware interface, charge sensing, and flash energy

required are discussed in a separate application note AN1312 available from STMicroelectronics.

Figure 2: Illustration of flicker problem

Flicker setting not correct

Flicker setting correct

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