Cirrus Logic CS8900A - Manual (Page 33)

DS271F5 33
CS8900A
Crystal LAN™ Ethernet Controller
CIRRUS LOGIC PRODUCT DATASHEET
attempting transmission. The CS8900A sup-
ports two schemes for determining when to ini-
tiate transmission: Two-Part Deferral, and
Simple Deferral. Selection of the deferral
scheme is determined by the 2-partDefDis bit
(Register 13, LineCTL, Bit D). If the 2-partDef-
Dis bit is clear, the MAC uses a two-part defer-
ral 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:
3.9.5.2 Two-Part Deferral
In the two-part deferral process, the 9.6 µs In-
ter Packet Gap (IPG) timer is started whenev-
er the internal Carrier Sense signal is
deasserted. If activity is detected during the
first 6.4 µs of the IPG timer, the timer is reset
and then restarted once the activity has
stopped. If there is no activity during the first
6.4 µs of the IPG timer, the IPG timer is al-
lowed to time out (even if network activity is
detected during the final 3.2 µs). The MAC
then begins transmission if a transmit packet is
ready and if it is not in Backoff (Backoff is de-
scribed later in this section). If no transmit
packet is pending, the MAC continues to mon-
itor 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
shortened due to a temporary loss of carrier.
Figure 10 diagrams the two-part deferral pro-
cess.
3.9.5.3 Simple Deferral
In the simple deferral scheme, the IPG timer is
started whenever Carrier Sense is deasserted.
Once the IPG timer is finished (after 9.6 µs), if
a transmit frame is pending and if the MAC is
not in Backoff, transmission begins the 9.6 µs
IPG). If no transmit packet is pending, the
MAC continues to monitor the network. If activ-
ity is detected before a transmit frame is ready,
the MAC defers to the transmitting station and
resumes monitoring the network. Figure 11 di-
agrams the simple deferral process.
Transmit
Frame
Start Monitoring
Network Activity
IPG
Timer =
6.4 μs?
Network
Active?
Network
Active?
Start IPG
Timer
Network
Active?
Yes
No
Yes
Yes
Yes
No
No
No
No
Wait
3.2 μs
Yes
Tx
Frame
Ready and Not
in Backoff?
Figure 10. Two-Part Deferral