AM79C90PC datasheet(59/62 Pages) AMD

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Am79C90

MAU must generate the collision signal within

0.6

µs to 1.6 µs after the end of the transmit packet,

which is typically early enough for the LANCE de-

vice to detect it, even with its non-compliant 2-

µs

window. However, to comply with IEEE standards,

the C-LANCE device implements an SQE test win-

dow of 4

µs.

d. IEEE speciﬁcations require that receive be blinded

following transmit for 4

µs to prevent the controller

from responding to any trash that may be generated

by the MAU when it generates the SQE test signal.

However, IEEE speciﬁcations do not state that the

receiver should be blinded following a receive. The

LANCE device implements a 4.1-

µs blinding time

following receive, violating IEEE speciﬁcations. This

was erroneously implemented in the LANCE device,

since it was thought to be a moot issue under the

assumption that there should be no valid data on

the wire within 4.1

µs of the end of a receive anyway.

However, since two-part deferral after transmit and

receive are both optional, as mentioned in 7a), there

are rare situations where legal packets may arrive

with an IFS of less than 4.1

µs. To better handle this

situation, the C-LANCE device reduces the blind

time following a receive to less than 500 ns. The

blind time allows time to store and then clear the

status that was generated by the ending reception.

“Heartbeat OK” (No CERR) Deﬁnition

The heartbeat test or Signal Quality Error (SQE) test is

performed to verify the ability of the AUI to pass the col-

lision (SQE) indication to the DTE. The LANCE and

C-LANCE devices indicate a heartbeat test failure by

setting the CERR bit in CSR0 (bit 13).

At the conclusion of each transmission, the DTE opens

a time window during which it expects to see a collision

indication. In the LANCE device, this window begins

immediately when TENA deasserts and ends 2.0

µs

after RENA deasserts. The heartbeat signal is ex-

pected by the LANCE device even if the packet being

transmitted suffers a collision. This implementation vio-

lates IEEE requirements in three ways:

1. IEEE 802.3 speciﬁcations state that the heartbeat

window should begin when the input becomes idle

(RENA deasserts), not when the output becomes

idle (TENA deasserts).

2. If a collision occurs, the IEEE 802.3 speciﬁcations

indicate that the DTE should not look for the SQE

test signal.

3. As mentioned in 7c), the window should end no

earlier than 4.0

µs after RENA deasserts.

The C-LANCE device implements the heartbeat test in

full compliance with IEEE speciﬁcations. In the

C-LANCE device, the heartbeat window begins when

RENA deasserts and ends 4

µs later. In addition, the

C-LANCE device does not look for the heartbeat signal

whenever the packet being transmitted suffers a

collision.

The PCnet-ISA and MACE devices use the same

heartbeat OK deﬁnition as the C-LANCE device.

Details on the LANCE device’s violations of IEEE spec-

iﬁcations: The consequences of the violations of the

standard by the LANCE device are insigniﬁcant in prac-

tice. Item 1 (window begins when TENA deasserts, not

RENA) actually prevents the LANCE from being penal-

ized by Item 2 (heartbeat expected following a colli-

sion). That is, if the LANCE device did not violate Item

1 and started its window when RENA deasserted in-

stead of TENA, then the LANCE device could get false

CERR indications when a packet it is transmitting suf-

fers a collision. This can happen as follows. In the event

of a collision, the network may remain active for a while

after one node stops transmitting its JAM sequence

(other nodes involved in the collision may still have their

JAM on the wire). At a node that ends its JAM se-

quence relatively early, the heartbeat signal can over-

lap with the collision or the end of the collision frag-

ment, since the MAU times the heartbeat signal

generation from when the controller stops transmitting.

If this node uses a LANCE device as its controller, the

LANCE device will see this heartbeat signal only be-

cause of the violation given in Item 1. If the LANCE

device started its window when RENA deasserted in-

stead of TENA, it would miss the heartbeat signal,

since the heartbeat passes by while the collision is still

on the wire. This would give false CERR indications.

Hence, the violation of Item 1 in the LANCE device is

not a problem. In fact, it makes the violation of Item 2

generally a non-issue.

Although the violation of Item 1 masks the violation of

Item 2 as just described, the violation of Item 2 (heart-

beat still expected by the LANCE device when collision

occurs) can still lead to false CERR indications when

the LANCE device is used with a non-802.3-compliant

MAU. The IEEE 802.3 speciﬁcations state that the MAU

is to generate the SQE test signal after every transmit,

even when the transmit suffers a collision. However,

some MAUs on the market have been found not to

comply with this requirement. When operating with a

non-compliant MAU that does not generate the heart-

beat signal after a collided transmission, the LANCE

device can give false CERR indications.

As mentioned in 7c), Item 3 is generally a non-issue.

Receive Lockup

The LANCE device has an erratum in which the re-

ceiver locks up when the system bus latency is very

high. This erratum is ﬁxed in the C-LANCE device.

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