AD9925BBCZRL datasheet(38/96 Pages) AD

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AD9925

Rev. A | Page 38 of 96

SUBCK Suppression

UBC

l be

lowin

e (

ress

one

e VSG

S re

f the SUBCK pulses for lines following the VSG line.

Readout after Exposure

After the exposure, the readout of the CCD data occurs, begin-

ning with the sensor gate (VSG) operation. By default, the

AD9925 is generating the VSG pulses in every field. In the case

where only a single exposure and a single readout frame is

needed, such as the CCD’s preview mode, the VSG and SUBCK

pulses can operate in every field.

However, in many cases, during readout, the SUBCK output

needs to be further suppressed until the readout is completed.

The READOUT register specifies the number of additional

fields after the exposure to continue the suppression of SUBCK.

READOUT can be programmed for zero to seven additional

fields and should be preprogrammed at startup, not at the same

time as the exposure write. A typical interlaced CCD frame

readout mode will generally require two additional fields of

SUBCK suppression (READOUT = 2). A 3-field, 6-phase CCD

will require three additional fields of SUBCK suppression after

the readout begins (READOUT = 3).

If the SUBCK output is required to start back up during the last

field of readout, simply program the READOUT register to one

less than the total number of CCD readout fields.

Like the exposure operation, the readout operation must be

triggered using the TRIGGER register.

Using the TRIGGER Register

As described above, by default, the AD9925 will output the

SUBCK and VSG signals on every field. This works well for

continuous single-field exposure and readout operations, such

as the CCD’s live preview mode. However, if the CCD requires a

longer exposure time, or if multiple readout fields are needed,

the TRIGGER register needs to initiate specific exposure and

readout sequences.

Typically, the exposure and readout bits in the TRIGGER regis-

ter are used together. This will initiate a complete exposure-plus

readout operation. Once the exposure has been completed, the

readout will automatically occur. The values in the EXPOSURE

and READOUT registers will determine the length of each

operation.

It is possible to independently trigger the readout operation

ill cause the

e next VD, and the SUBCK output will be

T register.

er is also used to control the STROBE,

MSHUT, and VSUB signal transitions. Each of these signals is

individually controlled, although they will be dependent on the

triggering of the exposure and readout operation.

See Figure 49 for a complete example of triggering the exposure

and readout operations.

VSUB Control

The CCD readout bias (VSUB) can be programmed to accom-

modate different CCDs. Figure 48 shows two different modes

that are available. In Mode 0, VSUB goes active during the field

of the last SUBCK when the exposure begins. The on position

(rising edge in Figure 48) is programmable to any line within

the field. VSUB will remain active until the end of the image

readout. In Mode 1, the VSUB is not activated until the start of

the readout.

An additional function called VSUB keep-on is also available.

When this bit is set high, the VSUB output will remain on (active)

even after the readout has finished. To disable the VSUB, set

this bit back to low.

MSHUT and STROBE Control

MSHUT and STROBE operation is shown in Figure 49, Figure

50, and Figure 51. Table 20 shows the registers parameters for

controlling the MSHUT and STROBE outputs. The MSHUT

output is switched on with the MSHUTON registers, and it will

remain on until the location specified in the MSHUTOFF is

reached. The location of MSHUTOFF is fully programmable to

anywhere within the exposure period, using the FD (field), LN

(line), and PX (pixel) registers. The STROBE pulse is defined by

the on and off positions. STROBON_FD is the field in which

the STROBE is turned on, measured from the field containing

the last SUBCK before exposure begins. The STROBON_ LN

PX register gives the line and pixel positions with respect to

STROBON_FD. The STROBE off position is programmable to

any field, line, and pixel location with respect to the field of the

last SUBCK.

Normally, the S

Ks wil

gin to pulse on the line fol

without triggering the exposure operation. This w

the sensor gate lin

VSG).

th some CCDs, the SUBCK pu e

readout to occur at th

needs to be supp

ed for

or more lines following th

suppressed according to the value of the READOU

line. The SUBCKSUPPRES

gister allows for the suppression

The TRIGGER regist

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