VS6724 Datasheet, PDF (23/118 Pages) STMicroelectronics – 2 Megapixel single-chip camera module

VS6724
Functional description
2.7
Microprocessor functions
The microprocessor inside the VS6724 handles the I²C communication with the host
processor. From this communication it allows the host control over the device via the User
interface registers.
Dark calibration: The microprocessor uses information from the array to ensure that the
optimal and consistent ‘black’ level is achieved from the output.
Automatic exposure control: Using the information output by the statics gathering engine
the appropriate exposure settings for the scene is calculated and using a combination of
exposure control, analogue gain and digital gain the device will outputs a correctly exposed
image. The host can program the exposure target within a range of EV values or can control
the system manually setting a known Exposure time, analogue gain and digital gain.
Flicker cancellation: The 50/60 Hz flicker frequency present in many lighting sources is
visible when the integration time is not a integer multiple of this frequency. The VS6724 will
adjust the integration time to ensure that the flicker effect is minimized. Note that flicker is
present when using a integration time shorter than the period of the lighting frequency (less
than 8.333 ms for 60 Hz, and less than 10 ms for 50 Hz).
Automatic white balance: Using the information output by the statics gathering engine the
microprocessor adjusts the gains applied to the individual color channels in order to achieve
a correctly color balanced image. In addition to the standard white balance operation a
constrainer operates on this information which ensures that the white balance achieved fits
within the expected color temperature of real life illuminants.
Frame rate control: VS6724 contains a firmware based programmable timing generator.
This automatically designs internal video timings, PLL multipliers, clock dividers etc. to
achieve a target frame rate with a given input clock frequency.
Optionally an automatic frame rate controller can be enabled. This system examines the
current exposure status and adapts the frame rate based on this information. This function is
typically useful in low-light scenarios where reducing the frame rate extends the useful
integration period. This reduces the need for the application of analog and digital gain and
results in better quality images.
Active noise management: The microprocessor is able to modify certain video pipe
functions according to the current exposure settings determined by the automatic exposure
controller. The main purpose of this is to improve the noise level in the system under low
lighting conditions. Functions which ‘strength’ is reduced under low lighting conditions (e.g.
aperture correction) are controlled by ‘dampers’. Functions which ‘strength’ is increased
under low lighting conditions are controlled by ‘promoters’. The fade to black operation is
also controlled by the microprocessor
Fade to Black: Using programmable levels the microprocessor will fade the output signal to
black, this ensures that under the darkest conditions when the image is not of sufficient
quality the device will output black. This operation is achieved by scaling the RGB to YCbCr
matrix.
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