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3.5.3.1 BUFFER DESCRIPTOR FUNCTION CODE. This command sets a single function code used
for all buffer descriptors. The host must write the desired function code (FCO-FC3) into bits 0-3 of
CPA VALO before loading the command register with this command. This command is normally
given only as part of an initialization sequence or during testing. The coding is 81 and the format
is shown below:
7 6 5 4 3 2 1 0
1 1 0 1 0 1 0 1 0 1 0 1 0 1 1
3.5.3.2 FRAME DESCRIPTOR FUNCTION CODE. This command sets a single function code used
for all frame descriptors. The host must write the desired function code (FCO-FC3) into bits 0-3 of
CPA VALO before loading the command register with this command. This command is normally
given only as part of an initialization sequence or in testing. The coding is 82 and the format is
shown below:
7 6 5 4 3 2 1 0
1 1 0 1 0 1 0 1 0 1 0 1 1 1 0
3.5.3.3 DATA BUFFERS FUNCTION CODE. This command sets a single function code used for all
receive and transmit data buffers. The host must write the desired function code (FCO-FC3) into
bits 0-3 of CPA VALO before loading the command register with this command. This command
is normally given only as part of an initialization sequence or during testing. The coding is 87
and the format is shown below:
7
6
4
1
1
0
o
1
1
1
1
3.5.3.4 SET PAD TIMER PRESET REGISTER (PTP). The pad timer preset register is used by the
TBC to set the length and pattern of the preamble, and the minimum number of preamble octets
transmitted between frames. The initial value of this register is loaded by the host at offset 78 of
the initialization table. After initialization this command must be used to modify the PTP. The host
must write the new PTP value into bits 0-15 of CPA VALO before loading the command register
with this command. This register contains three parameters which include:
1. The preamble pattern, in bits 0-7 where bit 0 is the first bit of each preamble octet trans-
mitted and bit 7 is the last.
2. The minimum number of preamble octets transmitted after silence is loaded in bits 8-10.
This is equivalent to the 802.4 parameter min_posLsilence_preamble length.
3. The minimum number of preamble octets transmitted between frames resides in bits 11-
15.
The coding for the SET PTP command is 83 and its format is:
7 6 5 4 3 2 1
1 1 0 1 0 1 0 1 0 1 0 1 1 1 1
The format of the pad timer preset (PTP) register is:
15
14
13
12
11
10
9
8
7
Where:
m
=
Minimum number (minus 1) of preamble octets transmitted between frames.
b
=
Minimum number (minus 2) of preamble octets transmitted before frame after silence.
p
=
Pattern of preamble octet.
MC68824 USER'S MANUAL
MOTOROLA
3-17
II
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