STV-DCA/CIF-E01 STMICROELECTRONICS [STMicroelectronics], STV-DCA/CIF-E01 Datasheet - Page 15

• Manufacturer Part Number: STV-DCA/CIF-E01
• Description: DUAL-MODE DIGITAL CAMERA CHIPSET
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

STV0680B + VV6444/6410/6500
2.6
2.6.1
2.6.2
Anti-flicker exposure and gain control
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.
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.
Flicker setting not correct
Figure 2 : Illustration of flicker problem
Version 3.4
Flicker setting correct
Detailed features
15/63