AM79C989JCT Advanced Micro Devices, AM79C989JCT Datasheet - Page 16

• Manufacturer Part Number: AM79C989JCT
• Description: Quad Ethernet Switching Transceiver (QuEST)
• Manufacturer: Advanced Micro Devices

Either five or six signal pins are used for the AUI func-
tion: DO , DI , PCI/CI+; and, if Interrupt is disabled,
QINT/CI-.
AUI Transmitter
The AUI circuit provides a differential transmit circuit
which operates at Pseudo Emitter Coupled Logic
(PECL) levels. The DO circuit provides an internal ter-
mination resistor of 80.4 . When the AUI port is dis-
abled, the DO driver circuit will idle at zero differential
voltage with an impedance of 80.4 .
AUI Receiver
The AUI receive circuit consists of a PECL receiver cir-
cuit. It is recommended that DI inputs be terminated
differentially with two 40.2
node connected to a 0.1 to 0.01 F by-pass capacitor
to analog ground.
In order for the AUI to unsquelch, the differential re-
ceive data must exceed requirements for both negative
amplitude and time duration. Once unsquelched, the
receive data is sent to the Manchester decode unit for
clock recovery and data extraction.
Collision
The AUI collision front-end circuit is similar to the AUI
receiver circuit. The CI inputs should be differentially
terminated with two 40.2
node of the resistors connected to a 0.1 to 0.01 F by-
pass capacitor to analog ground. In order for a collision
to be detected, the differential receive data must pass
negative amplitude and time duration. Once the colli-
sion circuit is unsquelched, the collision is indicated on
the QCLSN signal during the port 0 multiplexed time
slot.
The AUI port can be configured as a full-duplex port for
10BASE-FL application. If configured as a full-duplex
port, the collision indication will not be signaled on the
QuASI Interface.
When the AUI and Interrupt modes are enabled, the
collision front end is changed to a single-ended input
with the same threshold requirements as above. The
positive signal of the collision differential pair is used as
the collision input. The CI- signal is isolated and biased
to an idle level. This frees up the external pin to be
switched in with the interrupt driver circuitry and to
function as an open drain interrupt output.
QuASI Interface
The QuASI interface provides four 10-Mbps Ethernet
channels that are serially multiplexed to a set of shared
pins. The data rate of these pins is four times faster
than a standard 10-Mbps serial interface.
The QuASI interface is composed of a clock, QRST/
STRB, and six signal pins. The purpose of this interface
is to allow time division multiplexing of the digital serial
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resistors with the middle
resistors with the middle
P R E L I M I N A R Y
Am79C989
data. The clock input, SCLK, is nominally a 40-MHz sig-
nal. This clock input should have a frequency tolerance
to 100 ppm.
The QuEST internally divides the 40-MHz SCLK input
into four clock phases or slots. (Refer to QuASI inter-
face diagram in the Switching Characteristics section.)
When the QRST/STRB signal is de-asserted, the inter-
nal divide circuit is locked into a repeatable sequence.
The first rising edge of the SCLK input after the
de-assertion of QRST/STRB results in the input sig-
nals, QTX_EN and QTX_DATA, being locked to chan-
nel 0. To transmit data for the first channel 0 slot, the
transmit data and transmit enable signal must meet the
setup and hold times associated with the first rising
edge of SCLK after QRST/STRB is driven inactive.
The repetitive channel order for transmitting data is
channel 0 to channel 3.
The second rising edge of the SCLK input after reset
de-assertion results in the output signals, QRX_DATA,
QRX_VALID, QRX_CRS, and QCLSN, being locked to
channel 3. The receive data for the first channel 3 slot
is valid during the second rising edge of SCLK. Suc-
cessive clock edges increment the channel slot number
in a repetitive fashion. The repetitive channel order is
channel 0 to channel 3. Consequently, all signal pins
are synchronous to the clock pin, SCLK.
The STRB (strobe) function of the QRST/STRB input
pin allows the option to strobe the input for a single
clock during normal operation to ensure alignment of
the QuASI interface to channel 0. The use of the strobe
option minimizes possibility of channel misalignments.
In order to transmit a packet, QTX_EN needs to be
asserted during the correct channel or slot number. If
QTX_EN is asserted, then the NRZ QTX_DATA is inter-
preted and sent to the Manchester encode unit for
transmission to the 10BASE-T or AUI interface.
QTX_EN and QTX_DATA should contain the preamble
and data portions of the frame to be sent. The End of
Transmission Delimiter will be added by the encode
unit. As an example, if channel 0 is the only transmit
channel active, then QTX_EN signal will only be as-
serted during the slot time of channel 0. As part of the
transmission process, the QTX_DATA data signal is
looped back to the QRX_CRS and QRX_VALID signals
when in half-duplex mode and the Link Pass State.
When data is received from the network, the data is first
placed in the Elasticity FIFO. There are three signals
associated with the receive stream: QRX_CRS,
QRX_VALID, and QRX_DATA. When receive data trig-
gers the squelch paths of either the 10BASE-T or AUI
receiver, the QRX_CRS signal is asserted at the earli-
est possible time. Receive Carrier Sense (QRX_CRS)
signal is used for signaling real-time network activity to
the external device connected to the QuEST device.