saa569x NXP Semiconductors, saa569x Datasheet - Page 63

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

Philips Semiconductors
22.1.6.7
One of the uses of packet 26 is to transmit characters that
are not in the basic Teletext character set. The family
automatically decodes packet 26 data and, if a character
corresponding to that being transmitted is available in the
character set, automatically writes the appropriate
character code into the correct location in the Teletext
memory.
This is not a full implementation of the packet 26
specification allowed for in level 2 Teletext, and so is often
referred to as level 1.5.
By convention, the packets 26 for a page are transmitted
before the normal packets. To prevent the default
character data overwriting the packet 26 data, there is a
mechanism which prevents packet 26 data from being
overwritten. The mechanism is disabled when the Spanish
national option is detected because the Spanish
transmission system sends even parity (i.e. incorrect)
characters in the basic page locations corresponding to
the characters sent via packet 26 and these will not
overwrite the packet 26 characters anyway. The special
treatment of the Spanish national option is disabled if bit
TXT12.SPANISH is cleared (logic 0) or if bit
TXT8.DISABLE SPANISH is set (logic 1).
Packet 26 data is processed regardless of bit
TXT1.EXT PKT OFF, but setting bit TXT1.X26 OFF
disables packet 26 processing.
2002 May 06
handbook, full pagewidth
Enhanced TV microcontrollers with
On-Screen Display (OSD)
Packet 26 processing
Bytes in the table
row n
n + 1
n + 6
n + 7
bits in each byte
column
0
Fig.22 Transmitted/subtitle page organisation.
8
7xx
bit
7
6xx
5xx
16
4xx
63
Bit TXT8.PKT26 RECEIVED is set by the hardware
whenever the packet 26 decoding hardware writes a
character into the page memory. The flag can be reset by
writing a logic 0 into the SFR bit.
22.1.6.8
The 525-line format (see Fig.23) is similar to the 625-line
format but the data rate is lower and there are fewer data
bytes per packet (32 rather than 40). There are still
40 characters per display row so extra packets are sent,
each containing the last eight characters for four rows.
These packets can be identified by the ‘tabulation bit’ (T),
which replaces one of the magazine bits in 525-line
Teletext. When an ordinary packet with T = 1 is received,
the decoder puts the data into the four rows, starting with
that corresponding to the packet number, but with the two
LSBs set to logic 0. For example, a packet 9 with T = 1
(packet X/1/9) contains data for rows 8, 9, 10 and 11.
The error checking carried out on data from packets with
T = 1 depends on the setting of bit TXT1.8-BIT and the
error checking control bits in the page request data and is
the same as that applied to the data written into the same
memory location in the 625-line format.
The rolling time display (the last eight characters in row 0)
is taken from any packets X/1/1, 2 or 3 received.
In parallel magazine mode, only packets in the correct
magazine are used for the rolling time. Packet number
X/1/0 is ignored.
3xx
24
2xx
525-line World System Teletext
1xx
8xx
0
32
SAA567x; SAA569x
Objective specification
MGD160
39