CS8900-CQ3 Cirrus Logic, CS8900-CQ3 Datasheet - Page 32

• Manufacturer Part Number: CS8900-CQ3
• Description: Crystal LAN ISA Ethernet Controller
• Manufacturer: Cirrus Logic

the final 3.2 µs). The MAC then begins transmis-
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
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 dia-
grams the two-part deferral process.
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 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.
3.9.5.4 Collision Resolution
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):
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-
32
CIRRUS LOGIC PRODUCT DATA SHEET
mit a packet a total of 16 times (the initial attempt
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
packet without attempting any retransmission.
Ready and Not
Crystal LAN™ ISA Ethernet Controller
in Backoff?
Yes
3.2 s
Frame
Wait
Tx
Figure 10. Two-Part Deferral
No
Yes
Yes
Start Monitoring
Network Activity
Yes
Start IPG
No
Transmit
Network
Network
Network
Timer =
Active?
6.4 s?
Active?
Active?
No
No
Frame
Timer
IPG
No
Yes
CS8900A
DS271PP3