Datasheet

ADV7188
Rev. A | Page 64 of 112
I
2
C READBACK REGISTERS
Teletext
Because teletext is a high data rate standard, the decoded bytes
are available only as ancillary data. However, a TTX_AVL bit is
provided in I
2
C so that the user can check whether the VDP has
detected teletext. Note that the TTXT_AVL bit is a plain status bit
and does not use the protocol identified in the
I C Interface
2
section.
TTXT_AVL, Teletext Detected Status Bit,
Address 0x78 [7], User Sub Map, Read Only
0—Teletext was not detected.
1—Teletext was detected.
WST Packet Decoding
For WST only, the VDP decodes the magazine and row address
of WST teletext packets and further decodes the packet’s 8 × 4
hamming coded words. This feature can be disabled by using
the WST_PKT_ DECOD_ DISABLE
bit (Bit 3, Register 0x60,
user sub map). This feature is only valid for WST.
WST_PKT_DECOD_DISABLE, Disable Hamming
Decoding of Bytes in WST, Address 0x60 [3], User Sub Map
0—Enables hamming decoding of WST packets.
1 (default)—Disables hamming decoding of WST packets.
For hamming coded bytes, the dehammed nibbles, along with error
information from the hamming decoder, are output as follows:
• Input hamming coded byte: {D3, P3, D2, P2, D1, P1, D0, P0}
(bits in decoded order)
• Output dehammed byte: {E1, E0, 0, 0, D3', D2', D1', D0'}
(where Di' is the corrected bit, and Ei represents the error
information).
Table 77. Explanation of Error Bits
in the Dehammed Output Byte
E [1:0] Error Information
Output Data Bits
in Nibble
00 No errors detected Okay
01 Error in P4 Okay
10 Double error Not okay
11 Single error found and corrected Okay
The types of WST packets that are decoded are described in
Table 78.
Table 78. WST Packet Description
Packet Byte Description
1
st
byte Magazine number—dehammed Byte 4
2
nd
byte Row number—dehammed Byte 5
3
rd
byte Page number—dehammed Byte 6
4
th
byte Page number—dehammed Byte 7
5
th
to 10
th
byte Control Bytes—dehammed Byte 8 to Byte 13
Header Packet
(X/00)
11
th
to 42
nd
byte Raw data bytes
1
st
byte Magazine number—dehammed Byte 4
2
nd
byte Row number—dehammed Byte 5
Text Packets
(X/01 to X/25)
3
rd
to 42
nd
byte Raw data bytes
1
st
byte Magazine number—dehammed Byte 4
2
nd
byte Row number—dehammed Byte 5
3
rd
byte Design code—dehammed Byte 6
4
th
to 10
th
byte Dehammed Initial teletext page, Byte 7 to Byte 12
11
th
to 23
rd
byte UTC bytes—dehammed Byte 13 to Byte 25
8/30 (Format 1) packet
Design Code = 0000 or 0001
UTC
24
th
to 42
nd
byte Raw status bytes
1
st
byte Magazine number—dehammed Byte 4
2
nd
byte Row number—dehammed Byte 5
3
rd
byte Design code—dehammed Byte 6
4
th
to 10
th
byte Dehammed Initial teletext page, Byte 7 to Byte 12
11
th
to 23
rd
byte PDC bytes—dehammed Byte 13 to Byte 25
8/30 (Format 2) packet
Design Code = 0010 or 0011
PDC
24
th
to 42
nd
byte Raw status bytes
1
st
byte Magazine number—dehammed Byte 4
2
nd
byte Row number—dehammed Byte 5
3
rd
byte Design code—dehammed Byte 6
X/26, X/27, X/28, X/29, X/30, X/31
1
4
th
to 42
nd
byte Raw data bytes
1
For X/26, X/28, and X/29, further decoding needs 24 × 18 hamming decoding. Not supported at present.