ST92195D datasheet(152/249 Pages) STMICROELECTRONICS

152 / 249 page

152/249

ST92195C/D - TELETEXT SLICER AND ACQUISITION UNIT

TELETEXT SLICER AND ACQUISITION UNIT (Cont’d)

Data Filtering

The Acquisition Slicer’s hardware validates the

correctness of the data to be filtered. A second fil-

ter called "programmable filtering" allows software

to authorize or disable any data storage.

Hardware Filtering

When the Acquisition Unit receives the gated

FRWND signal, it will search for the Framing code

sequence in this time window.

If no valid framing code is detected, the Acquisition

Unit waits for the next TV line (i.e. the next

FRWND signal), ignoring any serial data of the

current line.

If a valid framing code is detected, the Acquisition

Unit starts a serial to parallel data conversion syn-

chronized by the framing code detection, storing in

byte format Teletext data of the current line.

After a valid Framing code, the first two bytes are

Hamming 8/4 checked. This check allows up to

one error to be automatically corrected. In this

case the corresponding byte is assumed to be er-

ror free. These two bytes contain the magazine

and the packet numbers.

If uncorrectable Hamming errors are found, the

whole TV line data flow is rejected until the next

valid Framing code is received.

If no Hamming error is found, these 2 bytes, mag-

azine and packet numbers, are concatenated in a

single byte, the 3 MSB being the magazine value.

This concatenated byte is then filtered without any

further hardware check, leading to a total of 41

sampled bytes per TV line.

Programmable Filtering

The programmable filtering unit implements two

different acquisition paths, activated or stopped

under full software control.

The filtering decision (store or reject) is taken by

hardware in real time, by checking the first byte

value (concatenated magazine / packet). This de-

cision is then kept until the end of the 41 bytes

flow, generating a "write enable" flag which is for-

warded to the Data Storage Unit.

The content of the Data Storage Unit is then trans-

ferred to a TDSRAM memory area called "VBI Ar-

ea" (see also Section 7.7.4.3, Address Genera-

tion).

Service Packets

The first acquisition path is dedicated to the ancil-

lary packets X/29, X/30 and X/31. Three control

bits associated with each of these Service pack-

ets, enable (if set) or disabling (if reset) the storage

of the corresponding Service packet whatever the

associated magazine and the ACQPOR or ACQM-

LR value (refer to the "Packet Request" register

description).

In case a service packet storage was requested,

the whole 41 bytes TV line data flow is transferred

(i.e. written) to the data storage area. If it was not

requested, then the 41 bytes corresponding data

flow will be rejected (i.e. not transferred to the data

storage area).

A "service packet" flag, called "SPF", is available.

This flag SPF must be reset by software before the

beginning of the next VBI. This flag will be set by

hardware each time a service packet is received

and was requested. This flag is common to the

three types of Service packets. Reading it to "1" in-

dicates that at least one (or more) Service packet

was received and stored during the previous VBI..

Warning: Accessing the Packet Request Register

during the VBI when the Teletext Acquisition is

running may lead to an incorrect functioning of the

filtering unit.

Pi (i= 29, 30, 31)

SPF

results

no storage even if

Pi broadcasted

set to 1 if Pi re-

ceived

storage of Pi, if re-

ceived

ST92195D Datasheet(PDF) 152 Page - STMicroelectronics

ST92195D Datasheet(HTML) 152 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

Datasheet Download

Link URL