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參數(shù)資料
| 型號： | CDK8920A |
| 廠商： | Cirrus Logic, Inc. |
| 英文描述： | CRYSTAL LAN ISA PLUG-AND-PLAY ETHERNET CONTROLLER |
| 中文描述： | 水晶藍(lán)的ISA即插即用，即插即用以太網(wǎng)控制器 |
| 文件頁數(shù)： | 31/144頁 |
| 文件大小： | 1229K |
| 代理商： | CDK8920A |
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DefDis bit (Register 13, LineCTL, Bit D). If the
2-partDefDis bit is clear, the MAC uses a two-
part deferral process defined in section 4.2.3.2.1
of the Ethernet standard (ISO/IEC 8802-3,
1993). If the 2-partDefDis bit is set, the MAC
uses a simplified deferral scheme. Both schemes
are described below:
Two-Part Deferral:
In the two-part deferral proc-
ess, the 9.6
μ
s Inter Packet Gap (IPG) timer is
started whenever the internal Carrier Sense sig-
nal is de-asserted. If activity is detected during
the first 6.4
μ
s of the IPG timer, the timer is re-
set and then restarted once the activity has
stopped. If there is no activity during the first 6.4
μ
s of the IPG timer, the IPG timer is allowed to
time out (even if network activity is detected
during the final 3.2
μ
s). The MAC then begins
transmission if a transmit packet is ready and if
it is not in Backoff (Backoff is described later in
this section). If no transmit packet is pending,
the MAC continues to monitor the network. If
activity is detected before a transmit frame is
ready, the MAC defers to the transmitting station
and resumes monitoring the network.
The two-part deferral scheme was developed to
prevent the possibility of the IPG being short-
ened due to a temporary loss of carrier. Figure
3.7 diagrams the two-part deferral process.
Simple Deferral:
In the simple deferral scheme,
the IPG timer is started whenever Carrier Sense
is de-asserted. Once the IPG timer is finished
(after 9.6
μ
s), if a transmit frame is pending and
if the MAC is not in Backoff, transmission be-
gins (even if network activity is detected during
the 9.6
μ
s IPG). If no transmit packet is pending,
the MAC continues to monitor the network. If
activity is detected before a transmit frame is
ready, the MAC defers to the transmitting station
and resumes monitoring the network. Figure 3.8
diagrams the simple deferral process.
Collision Resolution:
If a collision is detected
while the CS8920A is transmitting, the MAC re-
sponds in one of three ways depending on
whether it is a normal collision (within the first
512 bits of transmission) or a late collision (after
the first 512 bits of transmission):
Normal Collisions:
If a collision is detected be-
fore the end of the preamble and SFD, the MAC
finishes the preamble and SFD, transmits the jam
sequence (32-bit pattern of all 0’s), and then in-
itiates Backoff. If a collision is detected after the
transmission of the preamble and SFD but before
512 bit times, the MAC immediately terminates
transmission, transmits the jam sequence, and
then initiates Backoff. In either case, if the
Onecoll bit (Register 9, TxCMD, Bit 9) is clear,
the MAC will attempt to transmit a packet a total
of 16 times (the initial attempt plus 15 retrans-
missions) due to normal collisions. On the 16th
collision, it sets the 16coll bit (Register 8,
TxEvent, Bit F) and discards the packet. If the
T
x
F
r
a
m
e
R
e
ad
y
a
nd
N
o
t
i
n
Ba
ck
o
f
f
Transmit
Frame
Start Monitoring
Network Activity
Network
Active
Network
Active
Yes
No
Yes
No
No
Yes
Wait
9.6
μ
s
Figure 3.8. Simple Deferral
CS8920A
DS238PP2
31
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