AD9925BBCZRL datasheet(31/96 Pages) AD

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AD9925

Rev. A | Page 31 of 96

Sweep Mode Operation

The AD9925 contains an additional mode of vertical timing

operation called sweep mode. This mode is used to generate a

large number of repetitive pulses that span across multiple HD

lines. One example of where this mode is needed is at the start

of the CCD readout operation. At the end of the image exposure,

but before the image is transferred by the sensor gate pulses, the

vertical interline CCD registers should be free of all charge. This

can be accomplished by quickly shifting out any charge using a

long series of pulses from the XV outputs. Depending on the

vertical resolution of the CCD, up to two or three thousand clock

cycles will be needed to shift the charge out of each vertical CCD

line. This operation will span across multiple HD line lengths.

Normally, the AD9925 vertical timing must be contained within

one HD line length, but when sweep mode is enabled, the HD

boundaries will be ignored until the region is finished. To enable

sweep mode within any region, program

gister to high.

operation. The

e vertical sequence registers using the VPATREP

vent the

eep operation from overlapping the next vertical sequence.

Multiplier Mode

To generate very wide vertical timing pulses, a vertical region

may be configured into a multiplier region. This mode uses the

vertical pattern registers in a slightly different manner. Multiplier

mode can be used to support unusual CCD timing requirements,

such as vertical pulses that are wider than a single HD line length.

The start polarity and toggle positions are still used in the same

manner as the standard VPAT group programming, but the

VPATLEN is used differently. Instead of using the pixel counter

(HD counter) to specify the toggle position locations (VTOG1,

2, 3) of the VPAT group, the VPATLEN is multiplied with the

VTOG position to allow very long pulses to be generated. To

calculate the exact toggle position, counted in pixels after the

start position, use the following equation:

the appropriate SWEEP

Figure 40 shows an example of the sweep mode

number of vertical pulses needed depends on the vertical reso-

lution of the CCD. The XV output signals are generated using

the vertical pattern registers (shown in Table 15). A single pulse

is created using the polarity and toggle position registers. The

number of repetitions is then programmed to match the num-

ber of vertical shifts required by the CCD. Repetitions are pro-

grammed in th

registers. This produces a pulse train of the appropriate length.

Normally, the pulse train is truncated at the end of the HD line

length, but with sweep mode enabled for this region, the HD

boundaries are ignored. In Figure 40, the sweep region occupies

23 HD lines. After the sweep mode region is completed in the

next region, normal sequence operation will resume. When usi

sweep mode, be sure to set the region boundaries to the appro-

priate lines (using the sequence change positions) to pre

VPATLEN

VTOG

Position

Toggle

Mode

Multiplier

Because the VTOG register is multiplied by VPATLEN, the

resolution of the toggle position placement is reduced. If

peration. The

first toggle position is two, and the second toggle position is nine.

. How-

Table 16. Multiplier MODE Register Parameters

Length

Range

Description

VPATLEN = 4, then the toggle position accuracy will be reduc

to a 4-pixel step size, instead of a single pixel step size. Table 1

summarizes how the VPAT group registers are used in multi-

plier mode operation. In multiplier mode, the VPATREPO and

VPATREPE registers should always be programmed to the sam

value as the highest toggle position.

The example shown in Figure 41 illustrates this o

In nonmultiplier mode, this would cause the vertical sequence

to toggle at pixel 2 and then pixel 9 within a single HD line

ever, now toggle positions are multiplied by the VTPLEN = 4, so

the first toggle occurs at pixel count = 8, and the second toggle

occurs at pixel count = 36. Sweep mode has also been enable

to allow the toggle positions to cross the HD line boundaries.

MULTI

1 b

High/Low

High Enables Multiplier Mode.

XVPOL

1 b

High/Low

Starting Polarity of XV Signal in Each VPAT Group.

XVTOG1

12 b

0 to 4095 Pixel Location

First Toggle Position for XV Signal in Each VPAT Group.

XVTOG2

12 b

0 to 4095 Pixel Location

Second Toggle Position for XV Signal in Each VPAT Group.

XVTOG3

12 b

0 to 4095 Pixel Location

Third Toggle Position for XV Signal in Each VPAT Group.

VPATLEN

10 b

0 to 1023 Pixels

Used as Multiplier Factor for Toggle Position Counter.

VPATREP

12 b

0 to 4095

VPATREPE/VPATREPO Should Be Set to the Same Value as TOG2 or TOG3.

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