ST92195D datasheet(153/249 Pages) STMICROELECTRONICS

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ST92195C/D - TELETEXT SLICER AND ACQUISITION UNIT

TELETEXT SLICER AND ACQUISITION UNIT (Cont’d)

Page Handler

The page handler controls all packet types other

filtered than those already mentioned (i.e. packets

X/0 to X/28).These packets are filtered magazine-

wise by the Acquisition Unit.

Two registers, called “Page Opened” Register

(ACQPOR) and “Magazine Locked” Register (AC-

QMLR), are programmed by software before each

VBI and define for each magazine which filtering

type is requested.

Note: A Teletext page is opened by receiving a

“header” (a header corresponds to a packet X/0).

A page can not be closed by hardware. Also, re-

ceiving a service packet does not close the current

page of the corresponding magazine (if it was al-

ready opened).

Assuming magazine i (counted from 0 to 7), the

associated bits PORi and MLRi act as follows:

When a magazine is closed, then no Data of this

magazine will be stored (except for the Service

packets described in the preceding paragraph).

When a “page closed” configuration is set up, then

no data of the corresponding magazine will be

stored until the next header of that magazine is re-

ceived. Starting from this new header, with a

«page open» configuration, all the following maga-

zine information will be stored till the end of the

VBI. The data storage will be always done to the

VBI area.

A status register called “New Header Received”

registers, is also available. It provides for each

magazine a flag NHRi which must be reset by soft-

ware before the beginning of the next VBI. This

flag will be set to “1” by hardware when a new

header of the corresponding magazine is received

and stored.

Warning: Accessing the “Page Opened”, the

“Magazine Locked” or the “New Header Received”

Registers during the VBI when the Teletext Acqui-

sition is running may lead to a malfunctioning of

the filtering unit.

Once the VBI is finished, the application can easily

determine what happened during the VBI by

checking the current state of the 3 registers (AC-

QPOR, ACQMLR and ACQNHR). The following

table clarifies the different possible situations that

may be produced at the end of the VBI.

Warning: The (MLRi=1 & PORi=1) combination is

reserved for non-user mode.

MLRi PORi

Comments

Magazine locked. No storage even a

header coming

Page closed, wait for new header

Page opened. Storage

MLRi PORi NHRi

Comments

Magazine locked; no storage done

No storage for that magazine dur-

ing the VBI

Storage started on new header re-

ceived

Current page not yet closed; data

received or not

Current page closed; new page

started

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