am79c960 Advanced Micro Devices, am79c960 Datasheet - Page 37

• Manufacturer Part Number: am79c960
• Description: Pcnettm-isa Single-chip Ethernet Controller
• Manufacturer: Advanced Micro Devices

scriptor. In any case, lookahead will be performed to the
third buffer and the information gathered will be stored in
the chip, regardless of the state of the ownership bit. As
in the transmit flow, lookahead operations are per-
formed only once.
This activity continues until the PCnet-ISA controller
recognizes the completion of the packet (the last byte of
this receive message has been removed from the
FIFO). The PCnet-ISA controller will subsequently
update the current RDTE status with the end of packet
(ENP) indication set, write the message byte count
(MCNT) of the complete packet into RMD2 and over-
write the “current” entries in the CSRs with the
“next” entries.
Media Access Control
The Media Access Control engine incorporates the es-
sential protocol requirements for operation of a
compliant Ethernet/802.3 node, and provides the inter-
face between the FIFO sub-system and the Manchester
Encoder/Decoder (MENDEC).
The MAC engine is fully compliant to Section 4 of ISO/
IEC 8802-3 (ANSI/IEEE Standard 1990 Second Edition)
and ANSI/IEEE 802.3 (1985).
The MAC engine provides programmable enhanced
features designed to minimize host supervision and pre
or post-message processing. These features include
the ability to disable retries after a collision, dynamic
FCS generation on a packet-by-packet basis, and auto-
matic pad field insertion and deletion to enforce
minimum frame size attributes.
The two primary attributes of the MAC engine are:
Transmit And Receive Message Data
Encapsulation
The MAC engine provides minimum frame size enforce-
ment for transmit and receive packets. When
APAD_XMT = 1 (bit 11 in CSR4), transmit messages
will be padded with sufficient bytes (containing 00h) to
ensure that the receiving station will observe an infor-
mation field (destination address, source address,
length/type, data and FCS) of 64 bytes.
ASTRP_RCV = 1 (bit 10 in CSR4), the receiver will auto-
matically strip pad bytes from the received message by
observing the value in the length field, and stripping ex-
cess bytes if this value is below the minimum data size
(46 bytes). Both features can be independently over-
Transmit and receive message data encapsulation
— Framing (frame boundary delimitation, frame
— Addressing (source and destination address
— Error detection (physical medium transmission
Media access management
— Medium allocation (collision avoidance)
— Contention resolution (collision handling)
synchronization)
handling)
errors)
P R E L I M I N A R Y
When
Am79C960
ridden to allow illegally short (less than 64 bytes of
packet data) messages to be transmitted and/
or received.
Framing (Frame Boundary Delimitation, Frame
Synchronization)
The MAC engine will autonomously handle the con-
struction of the transmit frame. Once the Transmit FIFO
has been filled to the predetermined threshold (set by
XMTSP in CSR80), and providing access to the channel
is currently permitted, the MAC engine will commence
the 7-byte preamble sequence (10101010b, where first
bit transmitted is a 1). The MAC engine will subse-
quently append the Start Frame Delimiter (SFD) byte
(10101011b) followed by the serialized data from the
Transmit FIFO. Once the data has been completed, the
MAC engine will append the FCS (most significant bit
first) which was computed on the entire data portion of
the message.
Note that the user is responsible for the correct ordering
and content in each of the fields in the frame, including
the destination address, source address, length/type
and packet data.
The receive section of the MAC engine will detect an in-
coming preamble sequence and lock to the encoded
clock. The internal MENDEC will decode the serial bit
stream and present this to the MAC engine. The MAC
will discard the first 8 bits of information before search-
ing for the SFD sequence. Once the SFD is detected, all
subsequent bits are treated as part of the frame. The
MAC engine will inspect the length field to ensure mini-
mum frame size, strip unnecessary pad characters (if
enabled), and pass the remaining bytes through the Re-
ceive FIFO to the host. If pad stripping is performed, the
MAC engine will also strip the received FCS bytes, al-
though the normal FCS computation and checking will
occur. Note that apart from pad stripping, the frame will
be passed unmodified to the host. If the length field has
a value of 46 or greater, the MAC engine will not attempt
to validate the length against the number of bytes con-
tained in the message.
If the frame terminates or suffers a collision before
64 bytes of information (after SFD) have been received,
the MAC engine will automatically delete the frame from
the Receive FIFO, without host intervention.
Addressing (Source and Destination Address
Handling)
The first 6 bytes of information after SFD will be inter-
preted as the destination address field. The MAC engine
provides facilities for physical, logical, and broadcast
address reception. In addition, multiple physical ad-
dresses can be constructed (perfect address filtering)
using external logic in conjunction with the EADI
interface.
Error Detection (Physical Medium Transmission
Errors)
The MAC engine provides several facilities which report
and recover from errors on the medium. In addition, the
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