AD9920ABBCZ Datasheet, PDF (61/112 Pages) Analog Devices – 12-Bit CCD Signal Processor with V-Driver and Precision Timing Generator

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.
AD9920A
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
Register
Length (Bits) Range
SGMASK_NUM
13
0 to 8191 number of fields
SUBCKMASK_NUM 13
0 to 8191 number of fields
SUBCKMASK_SKIP1 1
On/off
SUBCKSUPPRESS 13
0 to 8191 lines
SUBCKNUM
13
1 to 8191 number of pulses
SG_SUPPRESS
1
On/off
SUBCK_TOG1
14
0 to 16383 pixel locations
SUBCK_TOG2
14
0 to 16383 pixel locations
SUBCK_POL
1
Low/high
SUBCKHP_TOG1 14
0 to 16383 pixel locations
SUBCKHP_TOG2 14
0 to 16383 pixel locations
Description
Exposure duration (number of fields to suppress VSG) for LS operation.
Exposure plus readout duration (number of fields to suppress SUBCK) for LS.
Suppress SG/SUBCK masks for one field (default = 0). Typically set to 1.
Number of lines to suppress the start of SUBCK pulses after SGACTLINE1.
Total number of SUBCK pulses per field, at one pulse per line.
Suppress the SG and allow SUBCK to finish at SUBCKNUM.
SUBCK Toggle Position 1.
SUBCK Toggle Position 2.
SUBCK start polarity.
High precision SUBCK Toggle Position 1. Selectable as SG or VD updated.
High precision SUBCK Toggle Position 2. Selectable as SG or VD updated.
Rev. B | Page 61 of 112