VX6854LC datasheet(67/114 Pages) STMICROELECTRONICS

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VX6854LC

Video timing

6.4

Image compression

The objective of the image compression is to reduce the required bandwidth in transmission

between the sensor and the host.

The key features of the DPCM/PCM compression algorithm are:

•

visually lossless

•

low cost implementation (no line memories are required)

•

fixed rate compression

The 10-bit to 8-bit DPCM/PCM image compression algorithm is supported by VX6854LC.

10-bit to 8-bit compression has the additional advantage that one pixel value equals 1 byte

of data.

The level of compression is controlled via the CSI_data_format register. The same register

is also used to enable and disable compression.

The compression_mode register is used to select which compression algorithm is used.

Currently only the DPCM/PCM technique is supported. Therefore the value of this register is

always 0x01.

The compression_capability register tells the host whether a sensor module does or does

not have compression and if it has compression then what is the compression technique.

Again currently only the DPCM/PCM technique is supported.

Please also refer to section 10 of the SMIA1.0 specification document.

6.5

Exposure and gain control

VX6854LC does not contain any form of automatic exposure control. To produce a correctly

exposed image the integration period and analogue gain for the pixels must be calculated

by an exposure control algorithm implemented externally. The parameters are written to the

VX6854LC using the CCI interface.

The exposure control parameters available on VX6854LC are:

•

fine integration time

•

coarse integration time

•

analogue gain

•

digital gain

The exposure control parameter registers are defined in Chapter 4: Register map on

page 29.

Integration time and analogue gain capability registers should be used to determine the

exposure control parameter limits for a given video timing configuration. See Section 6.7 of

the SMIA 1.0 part 1 specification for more information on how to interpret the integration and

gain capability registers and how to calculate exposure and gain limits.

6.5.1

Analogue gain model

VX6854LC only supports the single global analogue gain mode. VX6854LC has a 16-bit

by the SMIA1.0 description. Two extra bits can be used for fine gain between values 8 and

16 but their description is not currently supported by SMIA1.0 specification.

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