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PEB 20321
PEF 20321
Detailed Protocol Description
Data Sheet
88
2001-02-14
Receive Direction
General Features
In receive direction:
1. The starting and ending flag (7E
H
before and after a frame) is recognized
and extracted.
2. A change of the interframe time-fill is recognized and reported by an interrupt.
3. The zero insertions (a
‘
0
’
-bit after five
‘
1
’
s within a frame) are extracted.
4. The FCS at the end of a frame is checked, it is (optionally) transferred to the shared
memory together with the data.
5. The number of the bits within a frame (without zero insertions) is checked to be
divisible by 8.
6. The number of bytes within a frame is checked to be smaller than MFL + 1 (after
extraction of
‘
0
’
insertions). The check is maskable by setting the bit field MFLD in
MODE1 register.
7. The number of bits within a frame after extraction of
‘
0
’
insertions is checked
to be greater than check a) 16 for CRC = 0
32 for CRC = 1
(only for CS = 0)
check b) 32 for CRC = 0
48 for CRC = 1.
8. The occurrence of an abort flag (7F
H
) ending a frame is checked.
More detailed description of the individual features:
1. a. A frame is supposed to have started if after a sequence of 0111 1110 in the receive
data stream neither FC
H
nor FD
H
nor 7E
H
has occurred. The frame is supposed to
have started with the first bit after the closing
‘
0
’
of the sequence.
b. A frame is supposed to have stopped if a sequence of 0111 1110 or 0111 1111 is
found in the data stream after the frame has started. The last bit of the frame is
supposed to be the bit preceding the
‘
0
’
in the above sequences. The cases of
sequences 0111 1110 1111 111 and 0111 1110 0111 1111 are also supposed to
be frames of bit length
–
1 and 0 respectively.
A frame is also supposed to have stopped if more than MFL bytes were received
since the start of the frame.
c. The ending flag of a frame may be the starting flag of the next frame (shared flags
supported).
2. The receiver always remains in one of two possible interframe time-fill states:
‘
F
’
and
‘
O
’
.
Figure 32
illustrates them.
Note that a change from
‘
F
’
to
‘
O
’
and vice versa is reported by an IFC interrupt.