ADV7180
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 TTXT_AVL bit
has been provided in I
2
C so that the user can check whether the
VDP has detected teletext or not. Note that the TTXT_AVL bit
is a plain status bit and does not use the protocol identified in
the I
WST_PKT_DECODE_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 are output
along with some error information from the hamming decoder
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'}
(Di' – corrected bits, Ei error information).
TTXT_AVL, Teletext Detected Status, 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 teletext packets and further decodes the packet’s 8 × 4
hamming coded words. This feature can be disabled using the
WST_PKT_ DECODE_ DISABLE bit (Bit 3, Register 0x60, User
Sub Map). This feature is valid for WST only.
Table 75. Error Bits in the Dehammed Output Byte
E[1:0]
00
01
10
11
Error Information
No errors detected
Error in P4
Double error
Single error found and corrected
Output Data Bits
in Nibble
Okay
Okay
Bad
Okay
Table 76. WST Packet Description
Packet
Header Packet
(X/00)
Byte
1
st
2
nd
3
rd
4
th
5
th
to 10
th
11
th
to 42
nd
1
st
2
nd
3
rd
to 42
nd
1
st
2
nd
3
rd
4
th
to 10
th
11
th
to 23
rd
24
th
to 42
nd
1
st
2
nd
3
rd
4
th
to 10
th
11
th
to 23
rd
24
th
to 42
nd
1
st
2
nd
3
rd
4
th
to 42
nd
Description
Magazine number—Dehammed Byte 4
Row number—Dehammed Byte 5
Page number—Dehammed Byte 6
Page number—Dehammed Byte 7
Control Bytes—Dehammed Byte 8 to Byte 13
Raw data bytes
Magazine number—Dehammed Byte 4
Row number—Dehammed Byte 5
Raw data bytes
Magazine number—Dehammed Byte 4
Row number—Dehammed Byte 5
Design Code—Dehammed Byte 6
Dehammed initial teletext page, Byte 7 to Byte 12
UTC bytes—Dehammed Bytes 13 to Byte 25
Raw status bytes
Magazine number—Dehammed Byte 4
Row number—Dehammed Byte 5
Design Code—Dehammed Byte 6
Dehammed initial teletext page, Byte 7 to Byte 12
PDC bytes—Dehammed Byte 13 to Byte 25
Raw status bytes
Magazine number—Dehammed Byte 4
Row number—Dehammed Byte 5
Design Code—Dehammed Byte 6
Raw data bytes
Text Packets
(X/01 to X/25)
8/30 (Format 1) Packet
Design Code = 0000 or 0001
UTC
8/30 (Format 2) Packet
Design Code = 0010 or 0011
PDC
X/26, X/27, X/28, X/29, X/30, X/31
1
For X/26, X/28, and X/29 further decoding needs 24 × 18 hamming decoding. Not supported at present.
Rev. A | Page 59 of 112
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