CS8900A
Crystal LAN™ ISA Ethernet Controller
the final 3.2 µs). The MAC then begins transmis- mit a packet a total of 16 times (the initial attempt
sion 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 contin-
ues to monitor the network. If activity is detected
plus 15 retransmissions) due to normal collisions.
On the 16th collision, it sets the 16coll bit
(Register 8, TxEvent, Bit F) and discards the pack-
et. If the Onecoll bit is set, the MAC discards the
before a transmit frame is ready, the MAC defers to packet without attempting any retransmission.
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 dia-
Start Monitoring
Network Activity
grams the two-part deferral process.
3.9.5.3 Simple Deferral
In the simple deferral scheme, the IPG timer is
Yes
Network
Active?
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,
No
transmission begins the 9.6 µs IPG). If no transmit
packet is pending, the MAC continues to monitor
Start IPG
Timer
the network. If activity is detected before a transmit
frame is ready, the MAC defers to the transmitting
station and resumes monitoring the network. Fig-
ure 11 diagrams the simple deferral process.
IPG
Timer =
Yes
6.4 s?
µ
3.9.5.4 Collision Resolution
No
Wait
3.2
If a collision is detected while the CS8900A is
transmitting, the MAC responds 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):
µ
s
No
Network
Active?
Yes
3.9.5.5 Normal Collisions
If a collision is detected before the end of the pre-
amble and SFD, the MAC finishes the preamble
and SFD, transmits the jam sequence (32-bit pat-
tern of all 0’s), and then initiates Backoff. If a col-
lision is detected after the transmission of the
preamble and SFD but before 512 bit times, the
MAC immediately terminates transmission, trans-
mits the jam sequence, and then initiates Backoff.
In either case, if the Onecoll bit (Register 9, TxC-
MD, Bit 9) is clear, the MAC will attempt to trans-
No
Tx
Frame
Ready and Not
in Backoff?
No
Yes
Network
Active?
Yes
Transmit
Frame
Figure 10. Two-Part Deferral
CIRRUS LOGIC PRODUCT DATA SHEET
32
DS271PP3
CIRRUS [ CIRRUS LOGIC ]
