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SX IrDA Virtual Peripheral Implementation
AN16
7.0
Appendix A - Frame Check Sequence (FCS)
The FCS is a 16-bit CCITT CRC covering the payload data before transparency is applied (i.e. FCS must also be trans-
parent) as described on pages 113-116 of IrLAP.
• The FCS is initialized to $FFFF.
• For transmission the complement of the FCS is sent.
• For reception the FCS of the data including the FCS bytes will result in $F0B8 for a valid frame.
7.0.1 Basic FCS Algorithm
X
=
FCS-Low xor DATA
FCS-Low
=
F(X)-Low xor FCS-High
FCS-High
=
F(X)-High
Table
7-1. Virtual Table Algorithm F(X)
X
=
X xor (X << 4)
F(X)-High =
X xor (X >> 5)
F(X)-Low
=
(F(X)-High >> 4) xor (X << 3)
<< N
=
Shift left N bits (does not wrap around)
>> N
=
Shift right N bits (does not wrap around)
7.0.2 Final FCS Algorithm
X
=
FCS-Low xor DATA
X
=
X xor (X << 4)
A
=
X xor (X >> 5)
FCS-Low
=
(A >> 4) xor (X << 3) xor FCS-High
FCS-High
=
A
The FCS-Low variable is used to hold temporary variable X resulting in:
FCS-Low
=
FCS-Low xor DATA
FCS-Low
=
FCS-Low xor (FCS-Low << 4)
A
=
FCS-Low xor (FCS-Low >>
5)
FCS-Low
=
(A >> 4) xor (FCS-Low << 3) xor FCS-High
FCS-High
=
A
Bit
Algorithm
15
X7
xor X
3
14
X6
xor X
2
13
X5
xor X
1
12
X4
xor X
0
11
X3
10
X7
xor X
3
xor
X2
9
X6
xor X
2
xor
X1
8
X5
xor X
1
xor
X0
7
X4
xor X
0
6
X3
5
X2
4
X1
3
X7 xor X3 xor X0
2
X6 xor X2
1
X5 xor X1
0
X4 xor X0
