CS8900A
Crystal LAN™ Ethernet Controller
collision detection, preamble generation and 3.9.2.1 Transmission
detection, and CRC generation and test. Pro-
grammable MAC features include automatic
retransmission on collision, and padding of
transmitted frames.
Once the proper number of bytes have been
transferred to the CS8900A’s memory (either
5, 381, 1021 bytes, or full frame), and provid-
ing that access to the network is permitted, the
MAC automatically transmits the 7-byte pre-
amble (1010101b...), followed by the Start-of-
Frame Delimiter (SFD, 10101011b), and then
the serialized frame data. It then transmits the
Frame Check Sequence (FCS). The data after
the SFD and before the FCS (Destination Ad-
dress, Source Address, Length, and data field)
is supplied by the host. FCS generation by the
CS8900A may be disabled by setting the In-
hibitCRC bit (Register 9, TxCMD, bit C).
Figure 8 shows how the MAC engine interfac-
es to other CS8900A functions. On the host
side, it interfaces to the CS8900A’s internal
data/address/control bus. On the network
side, it interfaces to the internal Manchester
encoder/decoder (ENDEC). The primary func-
tions of the MAC are: frame encapsulation and
decapsulation; error detection and handling;
and, media access management.
LED
Logic
Figure 9 shows the Ethernet frame format.
3.9.2.2 Reception
The MAC receives the incoming packet as a
serial stream of NRZ data from the Manches-
ter encoder/decoder. It begins by checking for
the SFD. Once the SFD is detected, the MAC
assumes all subsequent bits are frame data. It
reads the DA and compares it to the criteria
programmed into the address filter (see
Section 5.2.10 on page 87 for a description of
Address Filtering). If the DA passes the ad-
dress filter, the frame is loaded into the
CS8900A’s memory. If the BufferCRC bit
(Register 3, RxCFG, bit B) is set, the received
FCS is also loaded into memory. Once the en-
Encoder/
802.3
Decoder
CS8900A
Internal Bus
10BASE-T
& AUI
MAC
Engine
&
PLL
Figure 8. MAC Interface
3.9.2 Frame Encapsulation and Decapsu-
lation
The CS8900A’s MAC engine automatically as-
sembles transmit packets and disassembles
receive packets. It also determines if transmit
and receive frames are of legal minimum size.
Packet
Fram e
up to
7
bytes
1
byte
6
bytes
6
bytes
2
bytes
4 bytes
alternating 1s / 0s
SFD
D A
SA
Length Field
LLC data
Pad
FCS
preamble
frame length
min 64 bytes
m ax 1518 bytes
Direction of Transm ission
SFD = Start of Frame Delim iter
DA = Destination Address
SA = Source Address
LLC = Logical Link Control
FCS = Frame Check Sequence (also
called C yclic Redundancy Check, or C RC)
Figure 9. Ethernet Frame Format
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