AD9992BBCZ datasheet(47/92 Pages) AD

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AD9992

Rev. C | Page 47 of 92

SHUTTER TIMING CONTROL

The AD9992 supports the generation of electronic shuttering

(SUBCK) and also features flexible general-purpose outputs

(GPO) to control mechanical shuttering, CCD substrate bias

switching, and strobe circuitry. In the following documentation,

the terms sense gate (SG) and vertical sense gate (VSG) are used

interchangeably.

SUBSTRATE CLOCK OPERATION (SUBCK)

The CCD image exposure time is controlled by the substrate

clock signal (SUBCK), which pulses the CCD substrate to clear

out accumulated charge. The AD9992 supports three types of

electronic shuttering: normal, high precision, and low speed.

Along with the SUBCK pulse placement, the AD9992 can

accommodate different readout configurations to further

suppress the SUBCK pulses during multiple field readouts.

The SUBCK signal is a programmable string of pulses, each

occupying a line following the primary sense gate active line,

SGACTLINE1 (registers are shown in Table 21). The SUBCK

signal has programmable pulse width, line placement, and

number of pulses to accurately control the exposure time.

SUBCK: Normal Operation

By default, the AD9992 operates in the normal SUBCK

configuration, in which the SUBCK signal is pulsing in every

VD field (see Figure 56). The SUBCK pulse occurs once per

line, and the total number of repetitions within the field

determines the length of the exposure time. The SUBCK pulse

polarity and toggle positions within a line are programmable using

the SUBCK_POL and SUBCK_TOG1 registers (see Table 21).

The number of SUBCK pulses per field is programmed in the

SUBCKNUM register (Address 0x75).

As shown in Figure 56, the SUBCK pulses always begin in the line

following the SG-active line, which is specified in the SGACTLINE

registers for each field. The SUBCK_POL, SUBCK_TOG1,

SUBCK_TOG2, SUBCKNUM, and SUBCKSTARTLINE

registers are updated at the start of the line after the sensor gate

line, as described in the Updating New Register Values section.

SUBCK: High Precision Operation

High precision shuttering is used in the same manner as normal

shuttering but uses an additional register to control the last

SUBCK pulse. In this mode, the SUBCK still pulses once per

line, but the last SUBCK in the field has an additional SUBCK

pulse, whose location is determined by the SUBCKHP_TOG

registers, as shown in Figure 57. Finer resolution of the exposure

time is possible using this mode. Leaving the SUBCKHP_TOG

registers set to its maximum value (0xFFFFFF) disables the last

SUBCK pulse (default setting).

SUBCK: Low Speed Operation

Normal and high precision shutter operations are used when

the exposure time is less than 1 field. For exposure times greater

than 1 field, the low speed (LS) shutter features can be used.

The AD9992 includes a field counter (primary field counter) to

regulate long exposure times. The primary field counter must

be activated (Address 0x70) to serve as the trigger for the LS

operation. The durations of the LS exposure and read are

specified by the SGMASK_NUM and SUBCKMASK_NUM

this mode suppresses the SUBCK and VSG outputs for up to

8192 fields (VD periods).

To activate an LS shutter operation, trigger the start of the exposure

by writing to the PRIMARY_ACTION register bits according to

the desired effect. When the primary counter is activated, the next

VD period becomes the first active period of the exposure for

which the VSG and SUBCK masks are applied.

Optionally, if the SUBCKMASK_SKIP1 register is enabled, the

AD9992 ignores the first VSG and SUBCK masks in subsequent

fields. This is generally desired so that the exposure time begins

in the field after the exposure operation is initiated. Figure 58

shows operation with SUBCKMASK_SKIP1 = 1.

If the PRIMARY_ACTION register is used while the

SUBCKMASK_NUM and SGMASK_NUM registers are set to 0,

the behavior of the SUBCK and VSG signals are not different

from the normal shutter or high precision shutter operations.

Therefore, the primary field counter can be used for other tasks

(described in the General-Purpose Outputs (GPOs) section)

without disrupting the normal activity. In addition, there exists

a secondary field counter that has no effect on the SUBCK and

VSG signals. These counters are described in detail in the Field

Counters section.

SUBCK Start Line

By default, the SUBCK pulses begin in the line following

SGACTLINE1. For applications where the SUBCK pulse should

be suppressed for one or more lines following the VSG line, the

SUBCKSTARTLINE register can be programmed. This register

setting delays the start of the SUBCK pulses until the specified

number of lines following SGACTLINE1.

Caution

A value of 1 should not be used in the SUBCKSTARTLINE

begin in the next line after the SG line. A value of 2 is used to

specify the SUBCK pulses to begin two lines after the SG line,

and so on.

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