saa569x NXP Semiconductors, saa569x Datasheet - Page 57

• Manufacturer Part Number: saa569x
• Description: Enhanced Tv Microcontrollers With On-screen Display Osd
• Manufacturer: NXP Semiconductors

Philips Semiconductors
When bit HOLD 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.
Bits E1 and E0 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 22.1.6.3.
For a multi-page device, each packet can only be written
into one place in the Teletext RAM. Therefore, 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.
22.1.6.2
When a new page is 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 is 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 1. If it is set for the current display block, the
acquisition section writes all valid page headers that 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.
2002 May 06
Enhanced TV microcontrollers with
On-Screen Display (OSD)
Rolling headers and time
57
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 bit TXT0.X24 POSN is set. If bit TXT1.EXT PKT OFF is
set, the extension packets corresponding to the page are
also cleared.
The last eight 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.
Bit TXT0.DISABLE HEADER ROLL 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. Bit TXT1.ACQ OFF 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 the
Do Care magazine bit is set to logic 0. When a serial
magazine mode transmission is being received, all page
headers are considered to be valid.
22.1.6.3
Teletext packets are error checked before they are written
into the page memory. The error checking carried out
depends on the packet number, the byte number, the error
check mode bits in the page request data and bit
TXT1.8-BIT (see Fig.18).
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
SAA567x; SAA569x
Objective specification