ML4665
JABBER FUNCTION REQUIREMENTS
V
CC
51Ω
RTSET = 560Ω
I
OUT
= 15.9mA
ECL
51Ω
51Ω
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 as shown in 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.
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 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.
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.
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
Since the amplifier has a differential input, two capacitors
of equal value are required. If the signal driving the input
is single ended, one of the coupling capacitors can be tied
to AV
CC
as shown in figure 1.
The internal amplifier has a lowpass filter built-in to band
limit the input signal which in turn will improve the signal
to noise ratio.
Although the input is AC coupled, the offset voltage
within
the amplifier will be present at the amplifier’s output. This
is represented by V
OS
in figure 4. Inorder to reduce this
error a DC feedback loop is incorporated. This negative
feedback loop nulls the offset voltage, forcing V
OS
to be
zero. Although the capacitor on V
DC
is non-critical, the
pole it creates can effect the stability of the feedback loop.
To avoid stability problems, the value of this capacitor
should be at least 10 times larger than the input coupling
capacitors.
V
OUT
+
V
OS
V
OUT
–
Figure 4.
8
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
