STV-5301-E01 Ver la hoja de datos (PDF) - Vision

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STV-5301-E01

Mono and Colour QSIF Digital Video CMOS Image Sensors

Vision 

CMOS Sensor; Customer Datasheet, Rev 3.0, 25 September 2000

VV5301 & VV6301

5. Exposure Control

5.1 Calculating Exposure Period

The exposure time, comprising coarse and fine components, for a pixel and the analogue gain are programmable via the serial

interface.

The coarse exposure value sets the number of complete lines a pixel exposes for, while the fine exposure sets the number of

additional pixel clock cycles a pixel integrates for. The sum of the two gives the overall exposure time for the pixel array.

Exposure Time = ((Coarse setting x Line Period) + (Fine setting)) x (CLKI clock period) x Clock Divider Rationote1

note1: Clock Divider Ratio = 1/(Basic Clock Division * Optional Pixel Clock Divisor)

5.2 Automatic Exposure Control

With automatic exposure control selected VV5301/VV6301 uses a complex algorithm to automatically set the exposure value for

the current scene. When combined with clock control and gain control the VV5301/VV6301 can operate over a very wide range of

illumination levels.

5.3 Updating Exposure, Gain and Clock Division Settings

Although the user can write a new exposure, gain or clock division parameter at any point within the field the sensor will only

consume these new external values at a certain point. The exception to this behaviour are when the user has selected immediate

update of gain If the user has selected the former then the new gain value will be applied as soon as the serial interface message

has completed. The fine and coarse exposure values are always written in a “timed” manner. There are two “update pending”

flags available to the user (see Status0 reg[2] for details) that allows the user to detect when the sensor has consumed one of the

timed parameters. In the next section of this document we will detail all the timed parameters and describe when they are

updated.

It is important to realise that there is a 1 frame latency between a new exposure value being applied to the sensor array and the

results of this new exposure value being read-out. The same latency does not exist for the gain value. To ensure that the effect of

the new exposure and gain values are coincident the sensor delays the application of the new gain value by approximately one

frame relative to the application of the new exposure value.

If the user is using the autoincrement option in the serial interface when writing a new series of exposure/gain and clock division

parameters then it is important to ensure that the sensor receives the complete message bunch before updating any of the

parameters. It is also important that the timed parameters are updated in the correct order, we will discuss this fully in the next

section. If an autoincrement message sequence is in progress but we have reached the point in the field timing where the gain

value would normally be updated, we actually inhibit the update. We inhibit the update to ensure that the gain change is not

passed to the sensor while a change in the exposure is still pending.

5.4 Clock Control

The system clock can be divided down internally to extend the operating range of VV5301/VV6301 by allowing longer exposure

times. The clock divisor options are as follows:

Clock divisor register

Effective clock division

2’b00

1

2’b01

2

2’b10

4

2’b11

8

Table 4 : Available clock division

If the user increases the clock divisor setting then the effective exposure period is also increased.

cd5301_6301f-3-0.fm

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