SAA5531 Philips Semiconductors, SAA5531 Datasheet - Page 46

• Manufacturer Part Number: SAA5531
• Description: Enhanced TV microcontrollers with On-Screen Display OSD
• Manufacturer: Philips Semiconductors

Philips Semiconductors
When the HOLD bit is set to a logic 0 the teletext decoder
will not recognise any page as having the correct page
number and no pages will be captured. In addition to
providing the user requested hold function this bit should
be used to prevent the inadvertent capture of an unwanted
page when a new page request is being made.
For example, if the previous page request was for
page 100 and this was being changed to page 234, it
would be possible to capture page 200 if this arrived after
only the requested magazine number had been changed.
The E1 and E0 bits control the error checking which should
be carried out on packets 1 to 23 when the page being
requested is captured. This is described in more detail in
Section 18.1.6.3.
For a multi-page device, each packet can only be written
into one place in the teletext RAM so if a page matches
more than one of the page requests the data is written into
the area of memory corresponding to the lowest numbered
matching page request.
At power-up each page request defaults to any page, hold
on and error check Mode 0.
18.1.6.2
When a new page has been requested it is conventional
for the decoder to turn the header row of the display green
and to display each page header as it arrives until the
correct page has been found.
When a page request is changed (i.e. when the TXT3 SFR
is written to) a flag (PBLF) is written into bit 5, column 9,
row 25 of the corresponding block of the page memory.
The state of the flag for each block is updated every TV
line, if it is set for the current display block, the acquisition
section writes all valid page headers which arrive into the
display block and automatically writes an alphanumeric
green character into column 7 of row 0 of the display block
every TV line.
2000 Feb 23
Enhanced TV microcontrollers with
On-Screen Display (OSD)
Rolling headers and time
46
When a requested page header is acquired for the first
time, rows 1 to 23 of the relevant memory block are
cleared to space, i.e. have 20H written into every column,
before the rest of the page arrives. Row 24 is also cleared
if the TXT0.X24 POSN bit is set. If the
TXT1.EXT PKT OFF bit is set the extension packets
corresponding to the page are also cleared.
The last 8 characters of the page header are used to
provide a time display and are always extracted from every
valid page header as it arrives and written into the display
block.
The TXT0.DISABLE HEADER ROLL bit prevents any data
being written into row 0 of the page memory except when
a page is acquired off air i.e. rolling headers and time are
not written into the memory. The TXT1.ACQ OFF bit
prevents any data being written into the memory by the
teletext acquisition section.
When a parallel magazine mode transmission is being
received only headers in the magazine of the page
requested are considered valid for the purposes of rolling
headers and time. Only one magazine is used even if don’t
care magazine is requested. When a serial magazine
mode transmission is being received all page headers are
considered to be valid.
18.1.6.3
Before teletext packets are written into the page memory
they are error checked. The error checking carried out
depends on the packet number, the byte number, the error
check mode bits in the page request data and the
TXT1.8-BIT bit.
If an uncorrectable error occurs in one of the Hamming
checked addressing and control bytes in the page header
or in the Hamming checked bytes in packet 8/30, bit 4 of
the byte written into the memory is set, to act as an error
flag to the software. If uncorrectable errors are detected in
any other Hamming checked data the byte is not written
into the memory.
Error checking
Preliminary specification
SAA55xx