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VV6411

DUAL-MODE DIGITAL CAMERA CHIPSET

STMicroelectronics 

STV0680B + VV6444/6410/6500

Detailed features

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 minimise ‘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 value has reached a suitable value for the current scene. If the light detected 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 STV0680B chooses exposure values which minimise “flicker”

effects from occurring under fluorescent lighting. STV0680B can only prevent flicker in lighting

powered by 50Hz or 60Hz electricity supply, but automatic detection of the flicker frequency is not

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

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

Flicker setting not correct

Flicker setting correct

Figure 2 : Illustration of flicker problem

2.6.2

Flashgun exposure

Where STV0680B-003 and a flashgun module are included in the camera, and the flashgun enable

signal is HIGH, the exposure mode will operate in a different manner. The CMOS sensor

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

being exposed at a certain point in time.

The “window” within which the flashgun needs to fire in order to correctly expose all sensor lines is

of the order of a few 100µsec (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 STV0680B-003.

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

energy required are discussed in a separate application note AN1312 regarding flashgun

implementation, available from ST.

Version 3.4

15/63

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