AD9883SLASHPCB datasheet(15/24 Pages) AD

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REV. 0

AD9883

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TWO-WIRE SERIAL CONTROL REGISTER DETAIL

CHIP IDENTIFICATION

7–0

Chip Revision

An 8-bit register which represents the silicon revision.

Revision 0 = 0000 0000, Revision 1 = 0000 0001,

Revision 2 = 0000 0010.

PLL DIVIDER CONTROL

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 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.

The 12-bit value of the PLL divider supports divide ratios

from 2 to 4095. The higher the value loaded in this regis-

ter, the higher the resulting clock frequency with respect

to a fixed Hsync frequency.

VESA has established some standard timing specifications,

which will assist in determining the value for PLLDIV as

a function of horizontal and vertical display resolution

and frame rate (Table V).

However, many computer systems do not conform pre-

cisely 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 of PLLDIV is 1693

(PLLDIVM = 69h, PLLDIVL = Dxh).

The AD9883 updates the full divide ratio only when the

LSBs are changed. Writing to the MSB by itself will not

trigger an update.

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 AD9883 updates the full divide ratio only when this

CLOCK GENERATOR CONTROL

7–6

VCO Range Select

Two bits that establish the operating range of the clock

generator.

VCORNGE must be set to correspond with the desired

operating frequency (incoming pixel rate).

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 actu-

ally operates at a higher frequency but then divides down

the clock rate afterwards. Table VII shows the pixel rates

for each VCO range setting. The PLL output divisor is

automatically selected with the VCO range setting.

Table VII. VCO Ranges

VCORNGE

Pixel Rate Range

12–36

36–72

72–110

The power-up default value is = 01.

5–3

CURRENT Charge Pump Current

Three bits that establish the current driving the loop filter

in the clock generator.

Table VIII. Charge Pump Currents

CURRENT

Current ( A)

000

001

100

010

150

011

250

100

350

101

500

110

750

111

1500

CURRENT must be set to correspond with the desired

operating frequency (incoming pixel rate).

The power-up default value is CURRENT = 001.

7–3

Clock Phase Adjust

A five-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.

CLAMP TIMING

7–0

Clamp Placement

An eight-bit register that sets the position of the internally

generated clamp.

When Clamp Function (Register 0Fh, Bit 7) = 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 between 1 and

255. Values of 0, 1, 2, 4, 8, 16, 32, 64, and 128 are not

supported.

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.

When Clamp Function = 1, this register is ignored.

7–0

Clamp Duration

An 8-bit register that sets the duration of the internally

generated clamp.

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

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