AM79C976 datasheet(69/309 Pages) AMD

69 / 309 page

8/01/00

Am79C976

P R E L I M INARY

If a collision is detected after 512-bit times have been

transmitted, the collision is termed a late collision. The

MAC engine will abort the transmission, append the

jam sequence, and increment the XmtLateCollision

counter. If RTRY_LCOL (CMD3, bit 16) is set to 1, the

retry logic treats late collisions just like normal colli-

sions. However, if the RTRY_LCOL bit is cleared to 0,

no retry attempt will be scheduled on detection of a late

collision. In this case, the transmit message will be

flushed from the FIFO.

The ISO 8802-3 (IEEE/ANSI 802.3) Standard requires

use of a “truncated binary exponential backoff” algo-

rithm, which provides a controlled pseudo random

mechanism to enforce the collision backoff interval, be-

fore retransmission is attempted.

See ANSI/IEEE Std 802.3-1990 Edition, 4.2.3.2.5:

“At the end of enforcing a collision (jamming), the

CSMA/CD sublayer delays before attempting to re-

transmit the frame. The delay is an integer multiple

of slot time. The number of slot times to delay be-

fore the nth retransmission attempt is chosen as a

uniformly distributed random integer r in the range:

0 £ r < 2k where k = min (n,10).”

The Am79C976 controller provides an alternative algo-

rithm, which suspends the counting of the slot time/IPG

during the time that receive carrier sense is detected.

This aids in networks where large numbers of nodes

are present, and numerous nodes can be in collision. It

effectively accelerates the increase in the backoff time

in busy networks and allows nodes not involved in the

collision to access the channel, while the colliding

nodes await a reduction in channel activity. Once chan-

nel activity is reduced, the nodes resolving the collision

time-out their slot time counters as normal.

This modified backoff algorithm is enabled when EMBA

(CSR3, bit 3) is set to 1.

Transmit Operation

The transmit operation and features of the Am79C976

controller are controlled by programmable options. The

Am79C976 controller provides a large transmit FIFO to

provide frame buffering for increased system latency,

automatic retransmission with no FIFO reload, and au-

tomatic transmit padding.

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Automatic transmit features such as retry on collision,

FCS generation/transmission, and pad field insertion

can all be programmed to provide flexibility in the (re-)

transmission of messages.

Disable retry on collision (DRTY) is controlled by the

DRTY bit of the Mode register (CSR15) in the initializa-

tion block.

Automatic pad field insertion is controlled by the

APAD_XMT bit in CSR4.

The disable FCS generation/transmission feature can

be programmed as a static feature or dynamically on a

frame-by-frame basis.

REX_RTRY (CMD3, bit 18) and REX_UFLO (CMD3,

bit 17) can be programmed to cause the transmitter to

automatically restart the transmission process instead

of discarding a frame that experiences an excessive

collisions or underflow error. In this case the retrans-

mission will not begin until the entire frame has been

loaded into the transmit FIFO. The RTRY_LCOL bit

(CMD3, bit 16) can be programmed either to drop a

frame after a late collision or to treat late collisions just

like normal collisions.

Transmit FIFO Watermark (XMTFW) in CSR80 sets the

point at which the controller requests more data from

the transmit buffers for the FIFO. A minimum of

XMTFW empty spaces must be available in the trans-

mit FIFO before the controller will request the system

bus in order to transfer transmit frame data into the

transmit FIFO.

Transmit Start Point (XMTSP) in CSR80 sets the point

when the transmitter actually attempts to transmit a

frame onto the media. A minimum of XMTSP bytes

must be written to the transmit FIFO for the current

frame before transmission of the current frame will be-

gin. (When automatically padded packets are being

sent, it is conceivable that the XMTSP is not reached

when all of the data has been transferred to the FIFO.

In this case, the transmission will begin when all of the

frame data has been placed into the transmit FIFO.)

The default value of XMTSP is 01b, meaning there has

to be 64 bytes in the transmit FIFO to start a transmis-

sion.

In order to ensure that collisions occurring within 512-

bit times from the start of transmission (including pre-

amble) will be automatically retried with no host inter-

vention, the transmit FIFO ensures that data contained

within the FIFO will not be overwritten until at least 64

bytes (512 bits) of preamble plus address, length, and

data fields have been transmitted onto the network

without encountering a collision. If the REX_RTRY bit

or the REX_UFLO bit is set, the transmit data will not

be overwritten until the frame has been either transmit-

ted or discarded.

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Transmit frames can be automatically padded to extend

them to 64 data bytes (excluding preamble). This al-

lows the minimum frame size of 64 bytes (512 bits) for

IEEE 802.3/Ethernet to be guaranteed with no software

intervention from the host/controlling process. Setting

the APAD_XMT bit in CSR4 enables the automatic

padding feature. The pad is placed between the LLC

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