VS6724 datasheet(23/118 Pages) STMICROELECTRONICS

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VS6724

Functional description

23/118

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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