29C516E
4. Check–Bit Generation
The Check–bit Generator produces 8 check–bits
(whatever N22 value) from the incoming User Data Word
UxD[0..15] according the Table 2.
Example: to create check–bit 0, bit 13, 12, 8, 7, 6, 5, 4 and
0 of the Data Word are XORed together.
If memory devices 8–bit wide are used, 24 bits
(MD[0..15] & MC[0..7]) are stored to give error
detection. But if memory devices 1–bit or 4–bit wide are
used, 22 bits (MD[0..15] & MC[0..5]) are stored to give
error detection.
Table 2: Check Bit Generation (indicates a bit of UxD bus used in the XOR/NXOR)
MC[..]
PARITY
15
0
1
2
3
Even(XOR)
Even(XOR)
Odd(NXOR)
Odd(NXOR)
x
x
x
x
x
x
14
13
x
12
x
x
x
x
x
11
10
9
UxD[..]
8
x
x
7
x
6
x
x
x
x
5
x
4
x
x
x
x
x
x
x
x
x
x
x
3
2
1
0
x
4
5
6
7
Even(XOR)
Even(XOR)
Even(XOR)
Odd(NXOR)
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
5. Syndrome Generation
The syndrome Generator produces 8 syndrome–bits
(whatever N22 value) from the incoming Memory Data
Word MD[0..15] and the associated Check–bits MC[0..7]
(or MC[0..5]) according the Table 3.
Syndrome–bit SY[x] is the XOR of the generated
Check–bit MC[x] with the generation of Chek–bit on
MD[..].
Example: to create syndrome–bit 3, first the bit 14,
13, 10, 4, 3, 2, 1 and 0 of the Data Word
(MD[14,13,10,4,3,2,1,0]) are NXORed. Then, the result
is XORed with the associated Check–bit (MC[3]) of the
Check–byte read in the same time as Data Word is
checked.
If the memory uses x8 devices, then the bits should be
physically divided as follows: MC[0..7], MD[0..7] and
MD[8..15] . For x4 organization, the bits should be
divided MC[0..2]+MC[6], MC[3..5]+MC[7], MD[0..3],
MD[4..7], MD[8..11] and MD[12..15].
Table 3: Syndrome Bit Generation (indicates a bit of MD and MC buses used in the XOR/NXOR)
SY[..]
PARITY
15
0
1
2
3
EVEN(XOR)
EVEN(XOR)
ODD(NXOR)
ODD(NXOR)
x
x
x
x
x
x
14
13
x
12
x
x
x
x
x
11
10
9
MD[..]
8
x
x
7
x
6
x
x
x
x
5
x
4
x
x
x
x
x
x
x
x
x
x
x
x
x
3
2
1
0
x
x
7
5
4
MC[..]
3
6
2
1
0
x
4
5
EVEN(XOR)
EVEN(XOR)
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
6
7
EVEN(XOR)
ODD(NXOR)
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
Rev.
E
(03
2007)
5
ATMEL [ ATMEL CORPORATION ]
