am79c961 Advanced Micro Devices, am79c961 Datasheet - Page 59

• Manufacturer Part Number: am79c961
• Description: Pcnettm-isa Jumperless Single-chip Ethernet Controller For Isa
• Manufacturer: Advanced Micro Devices

Twisted Pair Transmit Function
The differential driver circuitry in the TXD and TXP
pins provides the necessary electrical driving capability
and the pre-distortion control for transmitting signals
over maximum length Twisted Pair cable, as specified
by the 10BASE-T supplement to the IEEE 802.3 Stan-
dard. The transmit function for data output meets the
propagation delays and jitter specified by the standard.
Twisted Pair Receive Function
The receiver complies with the receiver specifications of
the IEEE 802.3 10BASE-T Standard, including noise
immunity and received signal rejection criteria (‘Smart
Squelch’). Signals meeting these criteria appearing at
the RXD differential input pair are routed to the MEN-
DEC. The receiver function meets the propagation
delays and jitter requirements specified by the standard.
The receiver squelch level drops to half its threshold
value after unsquelch to allow reception of minimum
amplitude signals and to offset carrier fade in the event
of worst case signal attenuation conditions.
Note that the 10BASE-T Standard defines the receive
input amplitude at the external Media Dependent Inter-
face (MDI). Filter and transformer loss are not specified.
The T-MAU receiver squelch levels are designed to ac-
count for a 1 dB insertion loss at 10 MHz for the type of
receive filters and transformers usually used.
Normal 10BASE-T compatible receive thresholds are
invoked when the LRT bit (CSR15, bit 9) is LOW. When
the LRT bit is set, the Low Receive Threshold option is
invoked, and the sensitivity of the T-MAU receiver is in-
creased. Increasing T-MAU sensitivity allows the use of
lines longer than the 100 m target distance of standard
10BASE-T (assuming typical 24 AWG cable). Increased
receiver sensitivity compensates for the increased sig-
nal attenuation caused by the additional cable distance.
However, making the receiver more sensitive means
that it is also more susceptible to extraneous noise, pri-
marily caused by coupling from co-resident services
(crosstalk). For this reason, end users may wish to in-
voke the Low Receive Threshold option on 4-pair cable
only. Multi-pair cables within the same outer sheath
have lower crosstalk attenuation, and may allow noise
emitted from adjacent pairs to couple into the receive
pair, and be of sufficient amplitude to falsely unsquelch
the T-MAU.
Link Test Function
The link test function is implemented as specified by
10BASE-T standard. During periods of transmit pair in-
activity, ’Link beat pulses’ will be periodically sent over
the twisted pair medium to constantly monitor medium
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 the Link Fail state. In the Link Fail state, data trans-
mission, data reception, data loopback and the collision
detection functions are disabled and remain disabled
P R E L I M I N A R Y
Am79C961
until valid data or greater than 5 consecutive link pulses
appear on the RXD pair. During Link Fail, the Link
Status (LNKST indicated by LED0) signal is inactive.
When the link is identified as functional, the LNKST sig-
nal is asserted, and LED0 output will be activated.
In order to inter-operate with systems which do not im-
plement Link Test, this function can be disabled by
setting the DLNKTST bit. 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 regard-
less 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 pack-
ets received from a reverse wired RXD input pair to be
corrected in the T-MAU prior to transfer to the
MENDEC. The polarity detection function is activated
following 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 packets 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 occurs at 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 polar-
ity 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 transmitted sig-
nal with a positive amplitude greater than 585 mV 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 cor-
rectly wired receiver, when a link beat pulse, which fits
the template of Figure 14-12 of the 10BASE-T Standard,
is generated at a transmitter and passed through 100 m
of twisted pair cable.
Negative link beat pulses are defined as transmitted 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 packets with valid ETD of
identical polarity are detected. When “armed,” the re-
ceiver is capable of changing the initial or previous
polarity configuration according to the detected ETD
polarity.
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