REV. 0

AD723

–19–

HSYNC

VSYNC

CSYNC

Figure 14. Sync Logic Levels (Equalization and Serration Pulses Not Shown)

SYNCHRONIZING SIGNALS

The AD723 requires explicit horizontal and vertical synchroniz-

ing signals for proper operation. This information cannot and

should not be incorporated in any of the RGB signals. However,

the synchronizing information can be provided as either separate

horizontal (HSYNC) and vertical (VSYNC) signals or as a single

composite sync (CSYNC) signal.

Internally the AD723 requires a composite sync logic signal that

is mostly high and goes low during horizontal sync time. The

vertical interval will have an inverted duty cycle from this. This

signal should occur at the output of an on-chip XNOR gate on

the AD723 whose two inputs are HSYNC (Pin 15) and VSYNC

(Pin 16). There are several options for meeting these conditions.

The first is to have separate signals for HSYNC and VSYNC.

Each should be mostly low and then high-going during their

respective time of assertion. This is the convention used by RGB

monitors for most PCs. The proper composite sync signal will

be produced by the on-chip XNOR gate when using these inputs.

If a composite sync signal is already available, it can be input

into HSYNC (Pin 15), while VSYNC (Pin 16) can be used to

change the polarity. (In actuality, HSYNC and VSYNC are

interchangeable since they are symmetric inputs to a two-

input gate).

If the composite sync input is mostly high and then low going

for active HSYNC time (and inverted duty cycle during VSYNC),

then it is already of the proper polarity. Pulling VSYNC high,

while inputting the composite sync signal to HSYNC will pass

this signal though the XNOR gate without inversion.

On the other hand, if the composite sync signal is the opposite

polarity as described above, pulling VSYNC low will cause the

XNOR gate to invert the signal. This will make it the proper

polarity for use inside the AD723. These logic conditions are

illustrated in Figure 14.