AD9887SLASHPCB datasheet(29/40 Pages) AD

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AD9887

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2-WIRE SERIAL CONTROL REGISTER DETAIL

CHIP IDENTIFICATION

7–0

Chip Revision

Bits 7 through 4 represent functional revisions to the

analog interface. Changes in these bits will generally

indicate that software and/or hardware changes will be

required for the chip to work properly. Bits 3 through 0

represent nonfunctional related revisions and are reset to

0000 whenever the MSBs are changed. Changes in these

bits are considered transparent to the user.

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 pixel clock from the incoming Hsync

signal. The pixel 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 221 to 4095. The higher the value loaded in this

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 VII).

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 AD9887 updates the full divide ratio only when the

LSBs are changed. Writing to this register 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 AD9887 updates the full divide ratio only when this

CLOCK GENERATOR CONTROL

7 TEST

Set to One

6–5

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 X shows the pixel rates

for each VCO range setting. The PLL output divisor is

automatically selected with the VCO range setting.

Table X. VCO Ranges

VCORNGE

Pixel Rate Range

12–35

35–70

70–110

110–140

The power-up default value is = 01.

4–2

CURRENT Charge Pump Current

Three bits that establish the current driving the loop ﬁlter

in the clock generator.

Table XI. Charge Pump Currents

CURRENT

Current ( A)

000

001

100

010

150

011

250

100

350

101

500

110

750

111

1500

See Table VII for the recommended CURRENT settings.

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 EXTCLMP = 0, a clamp signal is generated inter-

nally, 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 placement may be programmed

to any value between 1 and 255. A value of 0 is 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 EXTCLMP = 1, this register is ignored.

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