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ST92R195C - TELETEXT SLICER AND ACQUISITION UNIT
TELETEXT SLICER AND ACQUISITION UNIT
(Cont’d)
8.7.4.2 Data Storage
The data issued by the filtering unit is then sent to
the data storage area, which forwards these bytes
to the TDSRAM controller for storing in TDSRAM
memory. As the TDSRAM interface is synchro-
nized with the chassis sync signals, not necessar-
ily in phase with the CVBS signal, a data stack is
implemented in the data storage area. This data
stack is called the “Acquisition Buffer”.
The Acquisition Buffer is tailored to stack on a TV
line base, all the Teletext data that may come
while no RAM storage is possible. This leads to a 3
byte buffer (for more details, refer to the TDSRAM
Interface Specification).
The data bytes issued by the data filtering unit are
stored into the Acquisition Buffer only if they
passed successfully the different checks (ex-
plained in the preceding chapters; i.e. if the “Write
Enable” flag received from the filtering unit is set).
If the “Write Enable” flag is “0” then the current
byte will be rejected (not written into the Acquisi-
tion Buffer).
Acquisition Buffer Management
After being written to by the filtering unit, the Ac-
quisition Buffer is read by the TDSRAM interface
to transfer data to the internal TDSRAM. As the
two accesses are not “in phase”, the Acquisition
Buffer is built as a FIFO controlled by two pointers:
write and read. Each time a data transfer to the
TDSRAM interface is performed, a control line
called “ACQWEN” will tell the TDSRAM interface if
a write to the TDSRAM must be performed.
The ACQWEN line is set each time data is availa-
ble in the buffer (read and write pointers compari-
son). This allows a TDSRAM write. If the buffer is
empty read and write pointers comparison), the
ACQWEN line is reset and no TDSRAM write is al-
lowed. The TDSRAM interface will perform a read
operation to the TDSRAM; for more details, refer
to the TDSRAM interface specification.
8.7.4.3 Address Generation
The address generation unit provides to the
TDSRAM interface the real time address where
the byte must be stored into the internal TDSRAM.
The address is generated on a 14 bits word length,
allowing to access the TDSRAM memory as a
contiguous space of up to 16 K-bytes.
The TDSRAM memory Address, referred as “AD-
DREG”, is software programmable through 2 reg-
isters ACQAD1R, ACQAD0R (refer to the regis-
ters description for more details).
The ADDREG must be initialized by software. Af-
terwards during each VBI, it will be incremented
each time a byte is stored in the corresponding
memory area. The ADDREG follows a post-incre-
ment scheme, i.e. it is always pointing to the next
free byte location.
A second register, called the "Backup Address"
register, is implemented for storing the TV line
packet (under reception) address's first byte. An
HCVBS (Horizontal Sync Pulse) could be detected
before the end of the TV line Teletext data is re-
ceived because a sync pulse arrived during the
time when Teletext data was sliced, so the TV line
looks too short. The Backup Address value is re-
loaded into the ADDREG, providing the erasure of
the already stored data of the "too short" TV line.
At the same time, the read and write pointers of
the Buffer are re-initialized to the first Buffer loca-
tion (empty Buffer status).
Note
: A TV line which is longer than 41 bytes is
considered "too long". Only the first 41
bytes of data are stored. The next acquisition stor-
age starts after a valid Framing code detection.
Warning
: Accessing the ADDREG Registers dur-
ing the Vertical Blank Interval when the Teletext
Acquisition is running may lead to a malfunctioning
of the TDSRAM interface addressing.