ST92195D datasheet(149/249 Pages) STMICROELECTRONICS

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ST92195C/D - TELETEXT SLICER AND ACQUISITION UNIT

TELETEXT SLICER AND ACQUISITION UNIT (Cont’d)

7.7.3.2 Signal Windows

TXTEN (TXSCR.7) is the Teletext Enable bit. Set-

ting this bit allows the slicer to check for the pres-

ence of Teletext data. The search for data is from

line 6 to line 22 for field 1 and from line 318 to line

335 for field 2. The search window's timing is de-

fined by the signal generated by the Sync Extrac-

tor. Information from this signal is found on the bit

TXTSW (Teletext Search Window, TXSCR.3). An

extended search can be done by enabling TXTES

(Teletext Extended Search, TXSCR.6). When this

bit is enabled, the search in field 1 will start on line

2 and in field 2 on line 315.

During each search interval’s TV line the slicer

looks for the presence of the Run-In-Clock. When

the Run-In-Clock is found, data extraction is per-

formed and the Teletext Data (TTD) and Clock

(TTC) are sent to the Acquisition Unit. Enabling

the TTC clock is done using the SLCCR.7 bit. Re-

fer to the Timing and Clock Controller Chapter.

If the Run-In-Clock is not found within 12 µsecs.

from the horizontal synchro leading edge, all data

processing is stopped and no FRWND (Framing

Code Window) is generated. See Figure 88.

The Teletext Slicer generates the FRWND control

signal. This signal defines when the Acquisition

Unit must check for the presence and validity of

the framing code.

The FRWND is activated at the end of the running

clock and it lasts 24 teletext data bits.

The Slicer uses the Teletext slicer clock generated

by the Timing Controller. This clock must be ena-

bled before enabling the Slicer to guarantee cor-

rect behaviour. To synchronize all the control sig-

nals on a TV line base, the Horizontal Sync

(Hcvbs) from the Sync Extractor is used.

7.7.4 Acquisition Unit

When Teletext data are available, they are broad-

cast during the Video blank interval (VBI).

During the Teletext window, the Teletext Slicer

sends to the Acquisition Unit:

– TTC clock (at 6.9375 MHz)

– TTD serial data flow

– FRWND signal which enables the framing code

search within a precise window for each TV line

This FRWND signal is provided to the Acquisition

Unit only during the TV lines that may contain

Teletext data.

Figure 89. Framing Window Timing (FRWND)

Run-In

Clock

Teletext data

12 µs

( -1.0µs / +0.4 µs)

24 Teletext Data bits

Color Burst

FRWND

CVBS

Line Sync Pulse

Framing

Code

VR02109E

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