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AD9937

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Vertical Sensor Transfer Gate Timing

The vertical transfer sensor gate (TG) pulses are used to trans-

fer the pixel charges from the light-sensitive image area into the

light-shielded vertical registers. When a mechanical shutter is

not being used, this transfer effectively ends the exposure

period during the image acquisition. From the light-shield

vertical registers, the image is then read out line by line using

the vertical transfer pulses in conjunction with the high speed

horizontal clocks.

The AD9937 provides four programmable vertical transfer gate

pulses (TG1A, TG1B, TG3A, and TG3B). Table XIII lists the

TG registers. Two unique TG pulses can be preprogrammed

using the TGTOG_x (x = 0, 1) registers. As shown in Figure 24,

these toggle registers reference the 12-bit H counter for resolu-

tion control at the pixel level. Once the toggle positions have

been programmed, the TGPATSELx (x = 0, 1) can be used to

select which of the two TG pulses will be output on the TG1A/

B and TG3A/B pins. The TG1A/B and TG3A/B outputs are

selected as a group. As a result, the TG1A and TG1B outputs

will always be the same. This also applies for the TG3A and

TG3B outputs. For example, if TGPATSEL0 = 0, TG1A and

TG1B will have the outputs provided by the TGTOG1_0 and

TGTOG2_0 registers.

The TGMASK register can be used to individually mask (disable)

any one of the TG outputs. For example, if TGMASK = 1, the

TG1A will not be output. All TG outputs can be disabled by

setting TGEN = 0.

SHUTTER TIMING CONTROL

CCD image exposure is controlled through use of the substrate

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

out accumulated charge. The AD9937 supports two types of

OFD shutter timing: normal shutter mode and high precision

shutter mode. The registers used for OFD programming are

described in Table XIV.

Normal Shutter Mode

Figure 24 shows the VD and OFD output for normal shutter

mode. Programming the OFD outputs is similar to program-

ming the TG pulse whereas two unique OFD pulses can be

preprogrammed using the OFDTOG_x (x = 0, 1) registers. The

OFDTOG_x registers reference the 12-bit HD counter as shown

in Figure 24. Once the toggle positions have been programmed,

the OFDPATSEL register is used to select which of the two

preprogrammed OFD pulses will be output. The OFD will pulse

once per line for as many lines set in the OFDNUM register.

High Precision Shutter Mode

High precision shuttering is controlled in the same way as nor-

mal shuttering but requires a second set of shutter registers. In

this mode, the OFD still pulses once per line, but the last OFD

in the field will have an additional OFD pulse whose location is

determined by the OFDHPTOG1 and OFDHPTOG2 registers.

An example of this is shown in Figure 25. Finer resolution of

the exposure time is possible using this mode. Leaving both

OFDHPTOG registers set to 4095 disables the high precision

shutter mode (default setting).

Table XIII. TG Registers

Length

(Bits)

Type

Range

Description

TGEN

Control 0x10

High/Low

TG Output Enable Control (0 = Disable, 1 = Enable)

TGTOG1_0

Shut_Reg(1)

0–4095 Pixel Location

TG0 Pulse Toggle Position 1

TGTOG2_0

Shut_Reg(1)

0–4095 Pixel Location

TG0 Pulse Toggle Position 2

TGTOG1_1

Shut_Reg(2)

0–4095 Pixel Location

TG1 Pulse Toggle Position 1

TGTOG2_1

Shut_Reg(2)

0–4095 Pixel Location

TG1 Pulse Toggle Position 2

TGACTLINE

Mode_Reg(1)

0–127 Pixel Location

Line in Field where TG Outputs are Active

TGPATSEL0

Mode_Reg(1)

High/Low

TG1 A/B Pattern Selector (0 = TG0, 1 = TG1)

TGPATSEL1

Mode_Reg(1)

High/Low

TG3 A/B Pattern Selector (0 = TG0, 1 = TG1)

TGMASK

Mode_Reg(1)

4 Individual Bits

TG Masking Control (0 = No Masking, 1 = Mask TG1A,

2 = Mask TG1B, 3 = Mask TG3A, 4 = Mask TG3B)

CLPOB

CCD REGION 1

CCD REGION 2

CCD REGION 0

Figure 23. Example with CLPOBEN1 = 0

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