SAA7205 Philips Semiconductors, SAA7205 Datasheet - Page 31

• Manufacturer Part Number: SAA7205
• Description: MPEG-2 systems demultiplexer
• Manufacturer: Philips Semiconductors

Philips Semiconductors
7.10
The Demultiplexer contains a ITU-R System B compatible
Teletext (TXT) filter. This filter extracts relevant data from
the incoming data stream in accordance with the syntax
specified by the European Telecommunications Standards
Institute (ETSI). The TXT filter interprets the data, provides
temporary storage (2 kBytes) and outputs the data in a
TTC/TTD protocol (compatible with SAA9042 and
SAA5270), or in a TTR/TTX protocol (compatible with
SAA7183). The TTC/TTD output protocol is shown in
Fig.17 and the connection of SAA9042 to the
demultiplexer is shown in Fig.18. The SAA9042 and
SAA5270 teletext decoders are assumed to operate in
‘Normal Synchronous Mode’, applying 4 channel
acquisition. Some of the options associated with MPEG2
PES packets, such as PTS handling and CRC checking
are not implemented in the demultiplexer TXT filter.
The TXT filter does support interfacing with the
microcontroller, for use with future extensions such as
Close Caption (CC) and OSD. The TXT filter can therefore
be used to retrieve full PES packets. Various modes of
operation can be configured (address 0x0800,
see Table 13).
The PID of the TXT filter is programmable ‘txt_pid’
(address 0x0801, see Table 13). The delay between an
active horizontal sync edge and the start of TTD/TTX
output is controlled by sync_to_window_delay ‘sw_del
[6 to 0]’ (address 0x0802, see Table 13). The active
horizontal sync edge is defined by ‘sync_parity’ (address
0x0800, see Table 13), logic 0 meaning falling edge. All of
the control registers are write only. The TXT filter however
also has some readable registers which contain the
current values of PES scrambling control, PES flags
(address 0x0805, see Table 13), data_identifier,
data_unit_identifier (address 0x0806, see Table 13,) and
data_unit_flags (address 0x0807, see Table 13,).
Table 7 TXT filter modes and error codes
1997 Jan 21
MPEG-2 systems demultiplexer
CODE
Interfacing to SAA9042 and SAA5270 teletext
decoders and SAA7183 EURO-DENC
00
01
10
11
TXT INPUT MODE
close_caption
c_download
teletext
idle
TXT OUTPUT MODE
31
The status register of the TXT filter (address 0x0808,
see Table 13) contains the current error code and the
number of 16-bit words in the TXT FIFO.
The TXT interface is capable of supporting TXT insertion
into the vertical blanking interval of a CVBS signal. For this
purpose, it provides an SAA7183 (EURO-DENC)
compatible TXT output. If EURO-DENC requests data via
TTR, the demultiplexer provides it at 6.9375 Mbit/s. This
frequency is generated by dividing 27 MHz by 3 or 4 in a
specific sequence. The rhythm required by the
EURO-DENC is exactly matched. The interpretation of the
field_parity bit, in the TXT data stream, is programmable
(‘parity_sign’, address 0x0800, see Table 13). Allocation
of TXT data to odd or even fields can therefore be
configured as desired. Field allocation can be switched on
or off with ‘check_field’ (address 0x0800, see Table 13).
The TXT filter can be separately enabled by setting the
input and output modes to ‘idle’ (see Table 7) in the
txt_mode register (address 0x0800, see Table 13) and
reset (‘txt_reset’, address 0x0804, see Table 13). When
the TXT filter is used in one of the microcontroller
interaction modes close_caption or c_download, the
FIFO may generate a warning that the TXT_FIFO is almost
full. The threshold for this warning can be set to any value
between 0 and 1023, being the number of 16-bit words in
the TXT_FIFO (‘fifo_tresh [9 to 0]’, address 0x0803,
see Table 13). An interrupt is also generated at the
moment an overflow occurs. At this point the TXT_FIFO is
automatically reset to empty. If the microcontroller is
writing to the TXT_FIFO, overflow must be prevented and
the reset must be performed by the microcontroller.
TTXrq/TTX
TTC/TTD
idle
idle
TXT_FIFO ERROR CODE
threshold passed
Preliminary specification
not used
overflow
no error
SAA7205H