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AD9991

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Generating Line Alternation for V-Sequence and HBLK

During low resolution readout, some CCDs require a different

number of vertical clocks on alternate lines. The AD9991 can

support this by using the VPATREPO and VPATREPE regis-

ters. This allows a different number of VPAT repetitions to be

programmed on odd and even lines. Note that only the number

of repeats can be different in odd and even lines, but the VPAT

group remains the same.

Additionally, the HBLK signal can also be alternated for odd

and even lines. When the HBLKALT register is set high, the

HBLK TOG1 and TOG2 positions will be used on odd lines,

while the TOG3–TOG6 positions will be used on even lines.

This allows the HBLK interval to be adjusted on odd and even

lines if needed.

Figure 19 shows an example of VPAT repetition alternation and

HBLK alternation used together. It is also possible to use VPAT

and HBLK alternation separately.

Second V-Pattern Group during VSG Active Line

Most CCDs require additional vertical timing during the sensor

gate line. The AD9991 supports the option to output a second

V-pattern group for V1–V6 during the line when the sensor gates

VSG1–VSG5 are active. Figure 20 shows a typical VSG line,

which includes two separate sets of V-pattern groups for V1–V6.

The V-pattern group at the start of the VSG line is selected in the

same manner as the other regions, using the appropriate VSE-

QSEL register. The second V-pattern group, unique to the VSG

line, is selected using the VPATSECOND register, located with

the Field registers. The start position of the second VPAT group

uses the VPATLEN register from the selected VPAT registers.

Because the VPATLEN register is used as the start position and

not as the VPAT length, it is not possible to program multiple

repetitions for the second VPAT group.

VPATREPO = 2

VPATREPE = 5

VPATREPO = 2

NOTES

1. THE NUMBER OF REPEATS FOR THE V-PATTERN GROUP MAY BE ALTERNATED ON ODD AND EVEN LINES.

2. THE HBLK TOGGLE POSITIONS MAY BE ALTERNATED BETWEEN ODD AND EVEN LINES IN ORDER TO GENERATE DIFFERENT HBLK PATTERNS FOR ODD/EVEN LINES.

HBLK

TOG1

TOG2

TOG3

TOG4

TOG1

TOG2

Figure 19. Odd/Even Line Alternation of VPAT Repetitions and HBLK Toggle Positions

VSG

2ND VPAT GROUP

START POSITION FOR 2ND VPAT GROUP

USES VPATLEN REGISTER

Figure 20. Example of Second VPAT Group during Sensor Gate Line

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