ML4663
SQE TEST FUNCTION (SIGNAL QUALITY ERROR)
V
CC
51Ω
51Ω
RTSET = 560Ω
I
OUT
= 15.9mA
ECL
51Ω
The SQE test function allows the DTE to determine
whether the collision detect circuitry is functional. After
each transmission, during the inter packet gap time, the
collision oscillator will be activated for typically 1µs. The
SQE test will not be activated if the chip is in the low light
state, or the jabber on state.
For SQE to operate, the SQEN pin must be tied to V
CC
.
This allows the MAU to be interfaced to a DTE. The SQE
test can be disabled by tying the SQEN pin to ground, for
a repeater interface.
JABBER FUNCTION REQUIREMENTS
The Jabber function prevents a babbling transmitter from
bringing down the network. Within the transceiver is a
Jabber timer that starts at the beginning of each
transmission and resets at the end of each transmission. If
the transmission last longer than 20ms the Jabber logic
disables the transmitter and turns on the collision signal
COL+, COL–. When Tx+ and Tx– finally go idle, a second
timer measures 0.5 seconds of idle time before the
transmitter is enabled and collision is turned off. Even
though the transmitter is disabled during Jabber, the 1MHz
idle signal is still transmitted.
LED DRIVERS
The ML4663 has five LED drivers. The LED driver pins are
active low, and the LEDs are normally off (except for
LMON).
The LEDs are tied to their respective pins through
a 500Ω resistor to 5V.
The
XMT, RCV
and
CLSN
pins have pulse stretchers on
them which enables the LEDs to be visible. When
transmission or reception occurs, the LED
XMT, RCV
or
CLSN
status pins will activate low for several
milliseconds. If another transmit, receive or collision
conditions occurs before the timer expires, the LED timer
will reset and restart the timing. Therefore rapid events
will leave the LEDs continuously on. The
JAB
and
LMON
LEDs do not have pulse stretchers on them since their
conditions occur long enough for the eye to see.
LOW LIGHT CONDITION
The
LMON
LED output is used to indicate a low light
condition.
LMON
is activated low when both the receive
power exceeds the Link Monitor threshold and there are
transitions on V
IN
+, V
IN
– less than 3µs apart. If either one
of these conditions do not exist,
LMON
will go high.
INPUT AMPLIFIER
The V
IN
+, V
IN
– input signal is fed into a limiting amplifier
with a gain of about 100 and input resistance of 1.3kΩ.
Maximum sensitivity is achieved through the use of a DC
restoration feedback loop and AC coupling the input.
When AC coupled, the input DC bias voltage is set by an
on-chip network at about 1.7V. These coupling capacitors,
in conjunction with the input impedance of the amplifier,
establish a high pass filter with 3dB corner frequency, f
L
,
at
1
f
L
=
(1)
2
π1
300C
V
CC
Tx
TxOUT
Figure 3. Converting Optical LED Driver Output to
Differential ECL.
RECEPTION
The input to the transceiver comes from a fiber optic
receiver (Figure 1). At the start of packet reception no
more than 2.7 bits are received from the fiber cable, and
are not transmitted onto the DI circuit. The receive
squelch will reject frequencies lower than 2.51MHz.
While in the unsquelch state, the receive squelch circuit
looks for the start of idle signal at the end of the packet.
Start of idle occurs when the input signal remains idle for
more than 160ns. When start of idle is detected, the
receive squelch circuit returns to the squelch state and the
start of idle signal is output on the DI circuit (Rx+, Rx–).
COLLISION
Whenever the receiver and the transmitter are active at
the same time the chip will activate the collision output,
except when loopback is disabled (LBDIS = V
CC
). The
collision output is a differential square wave matching the
AUI specifications and capable of driving a 78Ω load. The
frequency of the square wave is 10MHz
±
15% with a 60/
40 to 40/60 duty cycle. The collision oscillator also is
activated during SQE Test and Jabber.
LOOPBACK
The loopback function emulates a 10BASE-T transceiver
whereby the transmit data sent by the DTE is looped back
over the AUI receive pair. Some LAN controllers use this
loopback information to determine whether a MAU is
connected by monitoring the carrier sense while
transmitting. The software can use this loopback
information to determine whether a MAU is connected to
the DTE by checking the status of carrier sense after each
packet transmission.
When data is received by the chip while transmitting, a
collision condition exits. This will cause the collision
oscillator to turn on and the data on the DI pair will
follow V
IN
+, V
IN
–. After a collision is detected, the
collision oscillator will remain on until either DO or
V
IN
+, V
IN
– go idle.
Loopback can be disabled by strapping LBDIS to V
CC
.
In this mode the chip operates as a full duplex transmitter
and receiver, and collision detection is disabled. A
loopback through the transceiver can be accomplished by
tying the fiber transmitter to the receiver.
9
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
