am79c974 Advanced Micro Devices, am79c974 Datasheet - Page 57

• Manufacturer Part Number: am79c974
• Description: Pcnettm-scsi Combination Ethernet And Scsi Controller For Pci Systems
• Manufacturer: Advanced Micro Devices

Collision Detect Function
Activity on both twisted pair signals RXD and TXD
constitutes a collision, thereby causing the COL signal
to be activated. (COL is used by the LED control circuits)
COL will remain active until one of the two colliding sig-
nals changes from active to idle. However, transmission
attempt in Link Fail state results in LCAR and CERR in-
dication. COL stays active for 2 bit times at the end of
a collision.
Signal Quality Error (SQE) Test (Heartbeat)
Function
The SQE function is disabled when the 10BASE-T port
is selected.
Jabber Function
The Jabber function inhibits the twisted pair transmit
function of the T-MAU TXD is active for an excessive
period (20 ms – 150 ms). This prevents any one node
from disrupting the network due to a ‘stuck-on’ or faulty
transmitter. If this maximum transmit time is exceeded,
the T-MAU transmitter circuitry is disabled, the JAB bit is
set (CSR4, bit 1) the COL signal is asserted. Once the
transmit data stream to the T-MAU is removed, an “un-
jab” time of 250 ms – 750 ms will elapse before the
T-MAU COL and re-enables the transmit circuitry.
Power Down
The T-MAU circuitry can be made to go into power sav-
ings mode. This feature is useful in battery powered or
low duty cycle systems. The T-MAU will go into power
down mode when H_RESET is active, coma mode is ac-
tive, or the T-MAU is not selected. Refer to the Power
Am79C974
TXD+
TXP+
TXD-
TXP-
RXD+
RXD-
Figure 21. 10BASE-T Interface Connection
61.9
61.9
422
422
P R E L I M I N A R Y
Am79C974
1.21 K
100
Savings Modes section for descriptions of the various
power down modes.
Any of the three conditions listed above resets the inter-
nal logic of the T-MAU and places the device into power
down mode. In this mode, the Twisted-Pair driver pins
(TXD , TXP ) are driven LOW, and the internal T-MAU
status signals (LNKST, RCVPOL, XMT, RCV and COL)
signals are inactive.
Once H_RESET ends, coma mode is disabled, and the
T-MAU is selected. The T-MAU will remain in the reset
state for up to 10 s. Immediately after the reset condi-
tion is removed, the T-MAU will be forced into the Link
Fail state. The T-MAU will move to the Link Pass state
only after 5 – 6 link beat pulses and/or a single received
message is detected on the RD pair.
In snooze mode, the T-MAU receive circuitry will remain
enabled even while the SLEEP pin is driven LOW.
The T-MAU circuitry will always go into power down
mode if H_RESET is asserted, coma mode is enabled,
or the T-MAU is not selected.
10BASE-T Interface Connection
Figure 21 shows the proper 10BASE-T network inter-
face design.
#18738A) for more design details, and refer to
Appendix B for a list of compatible 10BASE-T filter/
transformer modules.
Note that the recommended resistor values and filter
and transformer modules are the same as those used by
the IMR (Am79C980) and the IMR+ (Am79C981).
Filter
Filter
XMT
RCV
Transformer
Module
Filter &
Refer to the Technical Manual (PID
1:1
1:1
Connector
18681A-25
TD+
TD-
RD+
RD-
RJ45
1
2
3
6
AMD
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