AD9888
be programmed to any value between 1 and 255. A value
of 0 is not supported.
0D 7-1 Blue Channel Offset Adjust
A 7-bit offset binary word that sets the dc offset of the
BLUE channel. See REDOFST (0B).
For the best results, the clamp duration should be set to
include the majority of the black reference signal time that
follows the Hsync signal trailing edge. Insufficient clamp-
ing time can produce brightness changes at the top of the
screen, and a slow recovery from large changes in the
Average Picture Level (APL), or brightness.
0E
7
Hsync Input Polarity Override
This register is used to override the internal circuitry that
determines the polarity of the Hsync signal going into the PLL.
Table VIII. Hsync Input Polarity Override Settings
Override Bit Result
When the external clamp control bit is set to 1, this regis-
ter is ignored.
0
1
Hsync Polarity Determined by Chip
Hsync Polarity Determined by User
HSYNC PULSEWIDTH
07
7-0 Hsync Output Pulsewidth
An 8-bit register that sets the duration of the Hsync
output pulse.
The default for Hsync polarity override is 0 (polarity
determined by chip).
0E
6
HSPOL
Hsync Input Polarity
The leading edge of the Hsync output is triggered by the
internally generated, phase adjusted PLL feedback clock.
The AD9888 then counts a number of pixel clocks equal
to the value in this register. This triggers the trailing edge
of the Hsync output, which is also phase adjusted.
A bit that must be set to indicate the polarity of the Hsync
signal that is applied to the PLL Hsync input.
Table IX. Hsync Input Polarity Settings
HSPOL
Function
INPUT GAIN
08
7-0 Red Channel Gain Adjust
0
1
Active Low
Active High
An 8-bit word that sets the gain of the RED channel.
The AD9888 can accommodate input signals with a
full-scale range of between 0.5 V and 1.0 V p-p. Setting
REDGAIN to 255 corresponds to an input range
of 1.0 V. A REDGAIN of 0 establishes an input range
of 0.5 V. Note that increasing REDGAIN results in the
picture having less contrast (the input signal uses fewer
of the available converter codes). See Figure 2.
Active Low means the leading edge of the Hsync pulse
is negative-going. All timing is based on the leading edge
of Hsync, which is the falling edge. The rising edge has
no effect.
Active High means the leading edge of the Hsync pulse
is positive-going. This means that timing will be based on
the leading edge of Hsync, which is now the rising edge.
09
7-0 Green Channel Gain Adjust
An 8-bit word that sets the gain of the GREEN channel.
See REDGAIN (08).
The device will operate if this bit is set incorrectly, but the
internally generated clamp position, as established by
Clamp Placement (Register 05H), will not be placed as
expected, which may generate clamping errors.
0A
7-0 Blue Channel Gain Adjust
An 8-bit word that sets the gain of the BLUE channel.
See REDGAIN (08).
The power-up default value is HSPOL = 1.
0E
5
Hsync Output Polarity
INPUT OFFSET
0B 7-1 Red Channel Offset Adjust
One bit that determines the polarity of the Hsync output
and the SOG output. Table X shows the effect of this
option. SYNC indicates the logic state of the sync pulse.
A 7-bit offset binary word that sets the dc offset of the
RED channel. One LSB of offset adjustment equals
approximately one LSB change in the ADC offset.
Therefore, the absolute magnitude of the offset adjustment
scales as the gain of the channel is changed. A nominal
setting of 63 results in the channel nominally clamping
the back porch (during the clamping interval) to code 00.
An offset setting of 127 results in the channel clamping to
code 64 of the ADC. An offset setting of 0 clamps to code
–63 (off the bottom of the range). Increasing the value of
Red Offset decreases the brightness of the channel.
Table X. Hsync Output Polarity Settings
Setting
SYNC
0
1
Logic 1 (Positive Polarity)
Logic 0 (Negative Polarity)
The default setting for this register is 0.
Active Hsync Override
0E
4
This bit is used to override the automatic Hsync selection.
To override, set this bit to Logic 1. When overriding, the
active Hsync is set via Bit 3 in this register.
0C
7-1 Green Channel Offset Adjust
A 7-bit offset binary word that sets the dc offset of the
GREEN channel. See REDOFST (0B).
REV. B
–21–
ROCHESTER [ Rochester Electronics ]
