am79c961a Advanced Micro Devices, am79c961a Datasheet - Page 70

• Manufacturer Part Number: am79c961a
• Description: Pcnet?-isa Ii Jumperless, Full Duplex Single-chip Ethernet Controller For Isa
• Manufacturer: Advanced Micro Devices

the above paragraph) of the current RDTE shows valid
ownership, then it proceeds to a poll of the next RDTE.
Following this poll, and regardless of the outcome of
this poll, transfers of receive data from the FIFO may
begin.
Regardless of ownership of the second receive
descriptor, the PCnet-ISA II controller will continue to
perform receive data DMA transfers to the first buffer,
using burst-cycle DMA transfers. If the packet length
exceeds the length of the first buffer, and the PC-
net-ISA II controller does not own the second buffer,
ownership of the current descriptor will be passed back
to the system by writing a zero to the OWN bit of RMD1
and status will be written indicating buffer (BUFF = 1)
and possibly overflow (OFLO = 1) errors.
If the packet length exceeds the length of the first (cur-
rent) buffer, and the PCnet-ISA II controller does own
the second (next) buffer, ownership will be passed
back to the system by writing a zero to the OWN bit of
RMD1 when the first buffer is full. Receive data trans-
fers to the second buffer may occur before the PC-
net-ISA II controller proceeds to look ahead to the
ownership of the third buffer. Such action will depend
upon the state of the FIFO when the status has been
updated on the first descriptor. 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, loo-
kahead operations are performed only once.
This activity continues until the PCnet-ISA II controller
recognizes the completion of the packet (the last byte of
this receive message has been removed from the
FIFO). The PCnet-ISA II controller will subsequently up-
date 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” en-
tries.
Media Access Control
The Media Access Control engine incorporates the
essential protocol requirements for operation of a com-
pliant Ethernet/802.3 node, and provides the interface
between the FIFO sub-system and the Manchester
Encoder/Decoder (MENDEC).
This section describes operation of the MAC engine
when operating in Half Duplex mode. When in Half
Duplex mode, the MAC engine is fully compliant to Sec-
tion 4 of ISO/IEC 8802-3 (ANSI/IEEE Standard 1990
Second Edition) and ANSI/IEEE 802.3 (1985). When
operating in Full Duplex mode, the MAC engine behavior
changes as described in the Full Duplex Operation sec-
tion.
The MAC engine provides programmable enhanced
features designed to minimize host supervision and pre
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Am79C961A
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 au-
tomatic 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
enforcement 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. When
ASTRP_RCV = 1 (bit 10 in CSR4), the receiver will
automatically strip pad bytes from the received mes-
sage by observing the value in the length field, and
stripping excess bytes if this value is below the mini-
mum data size (46 bytes). Both features can be inde-
pendently over-ridden to allow illegally short (less than
64 bytes of packet data) messages to be transmitted
and/or received. The use of these features reduce bus
bandwidth usage because the pad bytes are not trans-
ferred to or from host memory.
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 chan-
nel is currently permitted, the MAC engine will com-
mence the 7-byte preamble sequence (10101010b,
where first bit transmitted is a 1). The MAC engine will
subsequently 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 order-
ing and content in each of the fields in the frame,
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)