AD9920ABBCZ datasheet(71/112 Pages) AD

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AD9920A

Rev. B | Page 71 of 112

SG CONTROL USING GPO

The AD9920A uses two of the GPO signals to generate the SG

signals for the three-level outputs V5 and V6. Because GPO5

and GPO6 are used as inputs to the vertical driver, they must

be properly initialized at power-up to avoid incorrect V-driver

output levels. During different CCD timing modes, the GPO

signals can be controlled in several ways to produce the proper

SG signal operation.

GPO5/GPO6 Power-Up Settings

GPO5 and GPO6 should be programmed with a polarity of

high at power-up by setting the GP5_POL and GP6_POL bits

(Address 0x7A, Bits[5:4]) equal to 11. This setting provides the

correct polarity in the V-driver, because the XSG signals should

be active low at the V-driver inputs. At power-up, the GPO5

and GPO6 outputs should also be enabled, by setting Register

Address 0x7A, Bits[21:20] and Bits[13:12] all equal to 1, so that

there is a defined state at all times.

Manual Control of GPO5

Figure 91 shows an example exposure/readout sequence of the

AD9920A used in 18-channel mode without any GPO signals

used for SG control. Figure 92 shows the 19-channel mode,

with GPO5 used to control the SG signal for the V5 output. In

this configuration, the GPO manual control method is used.

A serial write to the GP5_PROTOCOL register is used to set

the protocol of GPO5 equal to 1 (no counter association). The

manual trigger bit for GPO5 (Address 0x70, Bit 10) is then

written on the field previous to the field that requires the GPO5

(SG) signal. At the end of the readout, the GP5_PROTOCOL

Triggered Control of GPO5

Figure 93 shows the 19-channel mode, again with GPO5 used

to control the SG signal for the V5 output. In this configuration,

however, the secondary counter (scheduled toggles) method is

used. A serial write to the GP5_PROTOCOL register is used to

set the protocol of GPO5 equal to 6 (link to secondary counter).

At the start of the exposure, the field toggle location for GPO5 is

programmed to the desired field count value to trigger the GPO5

signal. Then, the secondary counter is triggered. The secondary

counter automatically increments and generates the GPO pulse

in the proper field location during readout. At the end of the

readout, the GPO5 protocol is automatically reset to idle.

The advantage of using the secondary counter is that no serial

writes are required during exposure or readout, unlike the manual

control method. The disadvantage is that more information must

be programmed before the start of exposure, such as the exact

field location where the GPO pulse is needed, taking into

account the length of the exposure and readout fields.

SUBCK

MECHANICAL

SHUTTER

OPEN

CLOSED

SERIAL

WRITES

OPEN

VSG

STILL IMAGE READOUT

CCD

OUT

DRAFT IMAGE

STILL IMAGE

FIRST FIELD

STILL IMAGE

SECOND FIELD

STILL IMAGE

THIRD FIELD

DRAFT IMAGE

tEXP

Figure 91. Exposure/Readout Operation Without Using GPO for SG Signal

AD9920ABBCZ Datasheet(PDF) 71 Page - Analog Devices

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