AD9920ABBCZ datasheet(61/112 Pages) AD

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AD9920A

Rev. B | Page 61 of 112

SUBCK High Precision Operation

High precision shuttering is used in the same manner as normal

shuttering, but it 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 82. Finer resolution of the exposure time is

possible using this mode. Leaving the SUBCKHP_TOG registers

set to their 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 one field. For exposure times greater

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

The AD9920A includes a field counter (primary field counter)

to regulate long exposure times. The primary field counter

(Address 0x70) must be activated 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

registers (Address 0x74), respectively. As shown in Figure 83,

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 (see Table 59). 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 AD9920A ignores the first VSG and SUBCK masks in the

subsequent fields. This is generally desired so that the exposure

time begins in the field after the exposure operation is initiated.

Figure 83 shows operation with SUBCKMASK_SKIP1 = 1. The

same functionality can also be achieved using the PRIMARY_

DELAY register along with the PRIMARY_ACTION register.

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 is 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 normal activity. In addition, a secondary

field counter is available that has no effect on the SUBCK and

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

Counters section.

SUBCKSUPPRESS Register

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

SUBCKSUPPRESS register can be programmed. This register

setting delays the start of the SUBCK pulses until the specified

number of lines following SGACTLINE1.

Read After Exposure

To read the CCD data after exposure, the SG should resume

normal activity while the SUBCK remains null. By default, the

AD9920A generates the VSG pulses in every field. When only

a single exposure and a single frame read are desired, as in the

case of preview mode, the VSG and SUBCK pulses can operate

in every field.

Other applications require that a greater number of frames be

read, in which case SUBCK must be masked until the readout is

finished. The SUBCKMASK_NUM register specifies the total

number of fields (exposure and read) to mask SUBCK. A two-field

CCD frame read mode typically requires two additional fields of

SUBCK masking (SUBCKMASK_NUM = 2). A three-field,

6-phase CCD requires three additional fields of SUBCK mask-

ing after the read begins (SUBCKMASK_NUM = 3).

Note that the SUBCKMASK_SKIP1 register setting allows

SUBCK pulses at the beginning of the field of exposure.

Table 43. SUBCK and Exposure/Read Register Parameters

Length (Bits)

Range

Description

SGMASK_NUM

0 to 8191 number of fields

Exposure duration (number of fields to suppress VSG) for LS operation.

SUBCKMASK_NUM

0 to 8191 number of fields

Exposure plus readout duration (number of fields to suppress SUBCK) for LS.

SUBCKMASK_SKIP1

On/off

Suppress SG/SUBCK masks for one field (default = 0). Typically set to 1.

SUBCKSUPPRESS

0 to 8191 lines

Number of lines to suppress the start of SUBCK pulses after SGACTLINE1.

SUBCKNUM

1 to 8191 number of pulses

Total number of SUBCK pulses per field, at one pulse per line.

SG_SUPPRESS

On/off

Suppress the SG and allow SUBCK to finish at SUBCKNUM.

SUBCK_TOG1

0 to 16383 pixel locations

SUBCK Toggle Position 1.

SUBCK_TOG2

0 to 16383 pixel locations

SUBCK Toggle Position 2.

SUBCK_POL

Low/high

SUBCK start polarity.

SUBCKHP_TOG1

0 to 16383 pixel locations

High precision SUBCK Toggle Position 1. Selectable as SG or VD updated.

SUBCKHP_TOG2

0 to 16383 pixel locations

High precision SUBCK Toggle Position 2. Selectable as SG or VD updated.

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