AD9887APCB datasheet(19/44 Pages) AD

19 / 44 page

REV. 0

AD9887A

–19–

SCAN Function

The SCAN function is intended as a pseudo JTAG function for

manufacturing test of the board. The ordinary operation of the

AD9887A is disabled during SCAN.

To enable the SCAN function, set Register 14h, Bit 2 to 1. To

SCAN in data to all 48 digital outputs, apply 48 serial bits of

data and 48 clocks (typically 5 MHz, max of 20 MHz) to the

SCANIN and SCANCLK pins, respectively. The data is shifted in

on the rising edge of SCANCLK. The first serial bit shifted in

will appear at the RED A<7> output after one clock cycle. After

48 clocks, the first bit is shifted all the way to the BLUE B<0>.

The 48th bit will now be at the RED A<7> output. If SCANCLK

continues after 48 cycles, the data will continue to be shifted

from RED A<7> to BLUE B<0> and will come out of the

SCANOUT pin as serial data on the falling edge of SCANCLK.

This is illustrated in Figure 10. A setup time (tSU) of 3 ns should

be plenty and no hold time (tHOLD) is required (

≥ 0 ns). This is

illustrated in Figure 11.

tSU = 3ns

tHOLD = 0ns

SCANCLK

SCANIN

Figure 11. SCAN Setup and Hold

Alternate Pixel Sampling Mode

A Logic 1 input on Clock Invert (CKINV, Pin 94) inverts the

nominal ADC clock. CKINV can be switched between frames

to implement the alternate pixel sampling mode. This allows

higher effective image resolution to be achieved at lower pixel

rates but with lower frame rates.

On one frame, only even pixels are digitized. On the subsequent

frame, odd pixels are sampled. By reconstructing the entire

frame in the graphics controller, a complete image can be recon-

structed. This is very similar to the interlacing process that is

employed in broadcast television systems, but the interlacing is

vertical instead of horizontal. The frame data is still presented

to the display at the full desired refresh rate (usually 60 Hz) so

no flicker artifacts are added.

OEOEOEOEOEOE

Figure 12. Odd and Even Pixels in a Frame

O 1

Figure 13. Odd Pixels from Frame 1

Figure 14. Even Pixels from Frame 2

O1E2O1E2O1E2O1E2O1E2O1E2

Figure 15. Combine Frame Output from Graphics Controller

BIT 1

BIT 2

BIT 1

BIT 2

BIT 47

BIT 48

BIT 46

BIT 47

BIT 48

SCANCLK

RED A<7>

BLUE B<0>

SCANOUT

SCANIN

BIT 1

BIT 2

BIT 3

BIT 1

BIT 2

BIT 3

Figure 10. SCAN Timing

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