Cirrus Logic CS8900A - Manual (Page 31)

DS271F5 31
CS8900A
Crystal LAN™ Ethernet Controller
CIRRUS LOGIC PRODUCT DATASHEET
tire packet has been received, the MAC vali-
dates the FCS. If an error is detected, the
CRCerror bit (Register 4, RxEvent, Bit C) is
set.
3.9.2.3 Enforcing Minimum Frame Size
The MAC provides minimum frame size en-
forcement of both transmit and receive pack-
ets. When the TxPadDis bit (Register 9,
TxCMD, Bit D) is
clear, transmit frames will be padded with ad-
ditional bits to ensure that the receiving station
receives a legal frame (64 bytes, including
CRC). When TxPadDis is set, the CS8900A
will not add pad bits and will transmit frames
less that 64 bytes. If a frame is received that is
less than 64 bytes (including CRC), the Runt
bit (Register 4, RxEvent, Bit D) will be set indi-
cating the arrival of an illegal frame.
3.9.3 Transmit Error Detection and Han-
dling
The MAC engine monitors Ethernet activity
and reports and recovers from a number of er-
ror conditions. For transmission, the MAC re-
ports the following errors in the TxEvent
register (Register 8) and BufEvent register
(Register C):
3.9.3.1 Loss of Carrier
Whenever the CS8900A is transmitting on the
AUI port, it expects to see its own transmission
“looped back” to its receiver. If it is unable to
monitor its transmission after the end of the
preamble, the MAC reports a loss-of-carrier
error by setting the Loss-of-CRS bit (Register
8, TxEvent, Bit 6). If the Loss-of-CRSiE bit
(Register 7, TxCFG, Bit 6) is set, the host will
be interrupted.
3.9.3.2 SQE Error
After the end of transmission on the AUI port,
the MAC expects to see a collision within 64 bit
times. If no collision is detected, the SQEerror
bit (Register 8, TxEvent, Bit 7) is set. If the
SQEerroriE bit is set (Register 7, TxCFG, Bit
7), the host is interrupted. An SQE error may
indicate a fault on the AUI cable or a faulty
transceiver (it is assumed that the attached
transceiver supports this function).
3.9.3.3 Out-of-Window (Late) Collision
If a collision is detected after the first 512 bits
have been transmitted, the MAC reports a late
collision by setting the Out-of-window bit (Reg-
ister 8, TxEvent, Bit 9). The MAC then forces a
bad CRC and terminates the transmission. If
the Out-of-windowiE bit (Register 7, TxCFG,
Bit 9) is set, the host is interrupted. A late col-
lision may indicate an illegal network configu-
ration.
3.9.3.4 Jabber Error
If a transmission continues longer than about
26 ms, the MAC disables the transmitter and
sets the Jabber bit (Register 8, TxEvent, Bit A).
The output of the transmitter returns to idle and
remains there until the host issues a new
Transmit Command. If the JabberiE bit (Regis-
ter 7, TxCFG, Bit A) is set, the host is interrupt-
ed. A Jabber condition indicates that there
may be something wrong with the CS8900A
transmit function. To prevent possible network
faults, the host should clear the transmit buf-
fer. Possible options include:
Reset the chip with either software or hard-
ware reset (see Section 3.3 on page 19).
Issue a Force Transmit Command by setting
the Force bit (Register 9, TxCMD, bit 8).
Issue a Transmit Command with the TxLength
field set to zero.
3.9.3.5 Transmit Collision
The MAC counts the number of times an indi-
vidual packet must be retransmitted due to