am79c970 Advanced Micro Devices, am79c970 Datasheet - Page 59

• Manufacturer Part Number: am79c970
• Description: Pcnettm-pci Single-chip Ethernet Controller For Pci Local Bus
• Manufacturer: Advanced Micro Devices

inactivity, ’Link beat pulses’ will be periodically sent over
the twisted pair medium to constantly monitor me-
dium integrity.
When the link test function is enabled (DLNKTST bit in
CSR15 is cleared), the absence of link beat pulses and
receive data on the RXD pair will cause the TMAU to go
into a link fail state. In the link fail state, data transmis-
sion, data reception, data loopback and the collision de-
tection functions are disabled, and remain disabled until
valid data or >5 consecutive link pulses appear on the
RXD pair. During link fail, the Link Status (LNKST pin)
signal is inactive. When the link is identified as func-
tional, the Link Status signal is asserted. The LNKST
pin displays the Link Status signal by default.
Transmission attempts during Link Fail state will pro-
duce no network activity and will produce LCAR and
CERR error indications.
In order to inter-operate with systems which do not im-
plement Link Test, this function can be disabled by set-
ting the DLNKTST bit in CSR15. With link test disabled,
the data driver, receiver and loopback functions as well
as collision detection remain enabled irrespective of the
presence or absence of data or link pulses on the RXD
pair. Link Test pulses continue to be sent regardless of
the state of the DLNKTST bit.
Polarity Detection and Reversal
The T-MAU receive function includes the ability to invert
the polarity of the signals appearing at the RXD pair if
the polarity of the received signal is reversed (such as in
the case of a wiring error). This feature allows data
frames received from a reverse wired RXD input pair to
be corrected in the T-MAU prior to transfer to the MEN-
DEC. The polarity detection function is activated follow-
ing H_RESET or Link Fail, and will reverse the receive
polarity based on both the polarity of any previous link
beat pulses and the polarity of subsequent frames with a
valid End Transmit Delimiter (ETD).
When in the Link Fail state, the T-MAU will recognize
link beat pulses of either positive or negative polarity.
Exit from the Link Fail state is made due to the reception
of 5 – 6 consecutive link beat pulses of identical polarity.
On entry to the Link Pass state, the polarity of the last 5
link beat pulses is used to determine the initial receive
polarity configuration and the receiver is reconfigured to
subsequently recognize only link beat pulses of the pre-
viously recognized polarity.
Positive link beat pulses are defined as received signal
with a positive amplitude greater than 585 mV (LRT =
HIGH) with a pulse width of 60 ns – 200 ns. This positive
excursion may be followed by a negative excursion.
This definition is consistent with the expected received
signal at a correctly wired receiver, when a link beat
pulse which fits the template of Figure 14-12 of the
1-926
AMD
P R E L I M I N A R Y
Am79C970
10BASE-T Standard is generated at a transmitter and
passed through 100 m of twisted pair cable.
Negative link beat pulses are defined as received sig-
nals with a negative amplitude greater than 585 mV with
a pulse width of 60 ns – 200 ns. This negative excursion
may be followed by a positive excursion. This definition
is consistent with the expected received signal at a re-
verse wired receiver, when a link beat pulse which fits
the template of Figure 14-12 in the 10BASE-T Standard
is generated at a transmitter and passed through 100 m
of twisted pair cable.
The polarity detection/correction algorithm will remain
“armed” until two consecutive frames with valid ETD of
identical polarity are detected. When “armed”, the re-
ceiver is capable of changing the initial or previous po-
larity configuration based on the ETD polarity.
On receipt of the first frame with valid ETD following
H_RESET or link fail, the T-MAU will utilize the inferred
polarity information to configure its RXD input, regard-
less of its previous state. On receipt of a second frame
with a valid ETD with correct polarity, the detection/cor-
rection algorithm will “lock-in” the received polarity. If the
second (or subsequent) frame is not detected as con-
firming the previous polarity decision, the most recently
detected ETD polarity will be used as the default. Note
that frames with invalid ETD have no effect on updating
the previous polarity decision. Once two consecutive
frames with valid ETD have been received, the T-MAU
will disable the detection/correction algorithm until
either a Link Fail condition occurs or H_RESET
is activated.
During polarity reversal, an internal POL signal will be
active. During normal polarity conditions, this internal
POL signal is inactive. The state of this signal can be
read by software and/or displayed by LED when en-
abled by the LED control bits in the Bus Configuration
Registers (BCR4–BCR7).
Twisted-Pair Interface Status
Three signals (XMT, RCV and COL) indicate whether
the T-MAU is transmitting, receiving, or in a collision
state with both functions active simultaneously. These
signals are internal signals and the behavior of the LED
outputs depends on how the LED output circuitry
is programmed.
The T-MAU will power up in the Link Fail state and nor-
mal algorithm will apply to allow it to enter the Link Pass
state. In the Link Pass state, transmit or receive activity
will be indicated by assertion of RCV signal going active.
If T-MAU is selected using the PORTSEL bits in CSR15,
then when moving from AUI to T-MAU selection the
T-MAU will be forced into the LINK Fail state.
In the Link Fail state, XMT, RCV and COL are inactive.