ST92195D datasheet(155/249 Pages) STMICROELECTRONICS

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ST92195C/D - TELETEXT SLICER AND ACQUISITION UNIT

TELETEXT SLICER AND ACQUISITION UNIT (Cont’d)

7.7.5 Hamming Decoding

The Hamming Decoding Unit is a separate block

that the application software can use to check and

decode Hamming coded data groups of three

bytes (Hamming 24/18 format) or one data byte

(Hamming 8/4 format).

The bytes must either be written to the three corre-

sponding registers ACQHD2R, ACQHD1R and

ACQHD0R or only to ACQHD0R. The bytes must

be written in the correct order, where the last sig-

nificant bit is the ACQHD0R (b0). Once the last

byte is written, in the case of Hamming 24/18

check, the result is immediately available on 18

bits, plus one bit for validating the data. In the case

of Hamming 8/4 check, the result is available on 4

bits, plus one bit for validating data.

In order to detect uncorrected remaining errors,

the ERR bit (bit 23 for Hamming 24/18 or bit 4 for

Hamming 8/4) of the bytes word acts as an “Invalid

Hamming Data” Flag upon being read (Hamming

error detected when the flag is set).

7.7.5.1 Hamming 24/18 Decoding

A1-A6

Row or column address

M1-M5

Mode description bits

D1-D6

Data bits

P1-P6

Hamming protection bits

Error Bit Address Calculation

The error bit position pos is calculated as follows:

pos = !P5*24+!P4*23+!P3*22+!P2*21+!P1*20

!Pi is Pi complemented.

If pos > 23 and not equal to 0, then there is more

than one error in the 3-byte group and the data

group is to be rejected. To correct a single-bit error

the specified bit is complemented. It is not neces-

sary to correct a protection bit that is in error.

byte

bit

b23 b22 b21 b20 b19 b18 b17 b16 b15 b14 b13 b12 b11 b10

name

byte

bit

name

Equation Number

Equation

(⊕ = xor)

=b0 ⊕ b2 ⊕ b4⊕ b6 ⊕ b8 ⊕ b10 ⊕ b12 ⊕ b14 ⊕ b16 ⊕ b18 ⊕ b20 ⊕ b22

=b1 ⊕ b2 ⊕ b5 ⊕ b6 ⊕ b9 ⊕ b10 ⊕ b13 ⊕ b14 ⊕ b17 ⊕ b18 ⊕ b21 ⊕ b22

=b3 ⊕ b4 ⊕ b5 ⊕ b6 ⊕ b11 ⊕ b12 ⊕ b13 ⊕ b14 ⊕ b19 ⊕ b20 ⊕ b21 ⊕ b22

=b7 ⊕ b8 ⊕ b9 ⊕ b10 ⊕ b11 ⊕ b12 ⊕ b13 ⊕ b14

=b15 ⊕ b16 ⊕ b17 ⊕ b18 ⊕ b19 ⊕ b20 ⊕ b21 ⊕ b22

=b0 ⊕ b1 ⊕ b2 ⊕ b3 ⊕ b4 ⊕ b5 ⊕ b6 ⊕ b7 ⊕ b8 ⊕ b9 ⊕ b10 ⊕ b11 ⊕ b12 ⊕ b13 ⊕ b14

⊕ b15 ⊕ b16 ⊕ b17 ⊕ b18 ⊕ b19 ⊕ b20 ⊕ b21 ⊕ b22 ⊕ b23

P1-P5

Conclusion

Handling

ERR

all 1

No errors in

data bits, only

in b23

data bits

all 1

No error in

data bits

don’t

care 0

& 1

single bit error

error bit ad-

dress cal-

culation

see below

0 if

pos is

from 1 to 23,

1 if

pos= 0 or

>23

don’t

care 0

& 1

double bit error

rejected

data bits

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