AD9925_15 Datasheet, PDF (36/96 Pages) Analog Devices – CCD Signal Processor with Vertical Driver and Precision Timing Generator

AD9925
SHUTTER TIMING CONTROL
The CCD image exposure time is controlled by the substrate
clock signal (SUBCK), which pulses the CCD substrate to clear
out accumulated charge. The AD9925 supports three types of
electronic shuttering: normal, high precision, and low speed.
Along with the SUBCK pulse placement, the AD9925 can ac-
commodate different readout configurations to further suppress
the SUBCK pulses during multiple field readouts. The AD9925
also provides programmable outputs to control an external me-
chanical shutter (MSHUT), a strobe/flash (STROBE), and the
CCD bias select signal (VSUB).
Normal Shutter Operation
By default, the AD9925 always operates in the normal shutter
configuration, in which the SUBCK signal pulses in every VD
field (see Figure 45). The SUBCK pulse occurs once per line,
and the total number of repetitions within the field will deter-
mine the length of the exposure time. The SUBCK pulse polar-
ity and toggle positions within a line are programmable using
the SUBCKPOL and SUBCK1TOG registers (see Table 19). The
number of SUBCK pulses per field is programmed in the
SUBCKNUM register (Addr 0x63).
As shown in Figure 45, the SUBCK pulses will always begin
in the line following the SG active line, which is specified
in the SGACTLINE registers for each field. The SUBCKPOL,
SUBCK1TOG, SUBCK2TOG, SUBCKNUM, and SUBCKSUP-
PRESS registers are updated at the start of the line after the sensor
gate line, as described in the Updating New Register Values section.
High Precision Shutter 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 will have an additional
SUBCK pulse, the location of which is determined by the
SUBCK2TOG register, as shown in Figure 46. Finer resolution
of the exposure time is possible using this mode. Leaving the
SUBCK2TOG register set to its maximum value (0xFF FFFF)
will disable the last SUBCK pulse (default setting).
Low Speed Shutter Operation
Normal and high precision shutter operations are used when
the exposure time is less than one field long. For exposure times
longer than one field interval, low speed shutter operation is
used. The AD9925 uses a separate exposure counter to achieve
long exposure times. The number of fields for the low speed
shutter operation is specified in the EXPOSURE register
(Addr 0x62). As shown in Figure 47, this shutter mode will
suppress the SUBCK and VSG outputs for up to 4095 fields (VD
periods). The VD and HD outputs may be suppressed during the
exposure period by programming the VDHDOFF register to 1.
To generate a low speed shutter operation, it is necessary to
trigger the start of the long exposure by writing to the TRIGGER
Register Bit D3. When this bit is set high, at the next VD edge,
the AD9925 will begin an exposure operation. If a value greater
than 0 is specified in the EXPOSURE register, AD9925 will
suppress the SUBCK output on subsequent fields.
If the exposure is generated using the TRIGGER register and
the EXPOSURE register is set to 0, then the behavior of the
SUBCK will not be any different than that of normal shutter or
high precision shutter operations, in which the TRIGGER regis-
ter is not used.
VD
HD
XSG
t_EXP
t_EXP
SUBCK
SUBCK PROGRAMMABLE SETTINGS:
1. PULSE POLARITY USING THE SUBCKPOL REGISTER.
2. NUMBER OF PULSES WITHIN THE FIELD USING THE SUBCKNUM REGISTER (SUBNUM = 3 IN THE ABOVE EXAMPLE).
3. PIXEL LOCATION OF PULSE WITHIN THE LINE AND PULSE WIDTH PROGRAMMED USING SUBCK1 TOGGLE POSITION REGISTER.
Figure 45. Normal Shutter Mode
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