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ST92195 ST92T195 ST92E195 - 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”
register (ACQNHR) associated to these control
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
10
Magazine locked. No storage even a
header coming
0
Page closed, wait for new header
0
1
Page opened. Storage
MLRi PORi NHRi
Comments
1
0
-
Magazine locked; no storage done
00
0
No storage for that magazine dur-
ing the VBI
00
1
Storage started on new header re-
ceived
01
0
Current page not yet closed; data
received or not
01
1
Current page closed; new page
started