AD9888
2-WIRE SERIAL CONTROL REGISTER DETAIL CHIP
IDENTIFICATION
Table VI. VCO Ranges
VCORNGE Pixel Rate Range
00
7-0 Chip Revision
An 8-bit register that represents the silicon revision.
Revision 0 = 0000 0000, Revision 1 = 0000 0001.
00
01
10
11
10–41
41–82
82–150
150+
PLL DIVIDER CONTROL
01
7-0 PLL Divide Ratio MSBs
The eight most significant bits of the 12-bit PLL
divide ratio PLLDIV. (The operational divide ratio is
PLLDIV + 1.)
The power-up default value is 01.
03
5-3 Charge Pump Current
Three bits that establish the current driving the loop filter
in the clock generator.
The PLL derives a master clock from an incoming Hsync
signal. The master clock frequency is then divided by an
integer value, such that the output is phase-locked to
Hsync. This PLLDIV value determines the number of pixel
times (pixels plus horizontal blanking overhead) per line.
This is typically 20% to 30% more than the number of
active pixels in the display.
Table VII. Charge Pump Currents
CURRENT
Current (mA)
000
001
010
011
100
101
110
111
50
100
150
250
350
500
750
1500
The 12-bit value of the PLL divider supports divide ratios
from 2 to 4095. The higher the value loaded in this
register, the higher the resulting clock frequency with
respect to a fixed Hsync frequency.
VESA has established standard timing specifications that
will assist in determining the value for PLLDIV as a
function of horizontal and vertical display resolution
and frame rate (Table IV).
CURRENT must be set to correspond with the desired
operating frequency (incoming pixel rate).
However, many computer systems do not conform precisely
to the recommendations, and these numbers should be
used only as a guide. The display system manufacturer
should provide automatic or manual means for optimizing
PLLDIV. An incorrectly set PLLDIV will usually produce
one or more vertical noise bars on the display. The greater
the error, the greater the number of bars produced.
The power-up default value is CURRENT = 001.
04
7-3 Clock Phase Adjust
A 5-bit value that adjusts the sampling phase in 32 steps
across one pixel time. Each step represents an 11.25° shift
in sampling phase.
The power-up default value is 16.
The power-up default value of PLLDIV is 1693
(PLLDIVM = 69H, PLLDIVL = DxH).
CLAMP TIMING
05 7-0 Clamp Placement
The AD9888 updates the full divide ratio only when the
LSBs are changed. Writing to this register by itself will
not trigger an update.
An 8-bit register that sets the position of the internally
generated clamp.
When the external clamp control bit is set to 0, a clamp
signal is generated internally, at a position established by
the clamp placement and for a duration set by the clamp
duration. Clamping is started (Clamp Placement) pixel
periods after the trailing edge of Hsync. The clamp place
ment may be programmed to any value up to 255, except 0.
02
7-4 PLL Divide Ratio LSBs
The four least significant bits of the 12-bit PLL divide
ratio PLLDIV. The operational divide ratio is PLLDIV + 1.
The power-up default value of PLLDIV is 1693
(PLLDIVM = 69H, PLLDIVL = DxH).
The clamp should be placed during a time that the input
signal presents a stable black-level reference, usually the
back porch period between Hsync and the image.
The AD9888 updates the full divide ratio only when this
register is written to.
CLOCK GENERATOR CONTROL
When the external clamp control bit is set to 1, this register
is ignored.
03
7-6 VCO Range Select
Two bits that establish the operating range of the clock
generator.
06
7-0 Clamp Duration
An 8-bit register that sets the duration of the internally
generated clamp.
VCORNGE must be set to correspond with the desired
operating frequency (incoming pixel rate).
When the external clamp control bit is set to 0, a clamp
signal is generated internally, at a position established by
the clamp placement and for a duration set by the clamp
duration. Clamping is started (Clamp Placement) pixel
periods after the trailing edge of Hsync, and continues for
(Clamp Duration) pixel periods. The clamp duration may
The PLL gives the best jitter performance at high fre-
quencies. For this reason, in order to output low pixel
rates and still get good jitter performance, the PLL actually
operates at a higher frequency but then divides down the
clock rate afterwards. Table VI shows the pixel rates for
each VCO range setting. The PLL output divisor is
automatically selected with the VCO range setting.
–20–
REV. B
ROCHESTER [ Rochester Electronics ]
