ML4658
SYSTEM DESCRIPTION (Continued)
When data is received by the chip while transmitting, a
collision condition exits. This will cause the collision
oscillator to turn on within 9 bit times. The data on the DI
AUI pair (Rx+, Rx–) changes from Tx+, Tx– to RxTP+,
RxTP–, when entering the collision state. During a
collision, if the receive data (RxTP+, RxTP–) drops out
before the transmit data (Tx+, Tx–), Rx+, Rx– will switch
back to Tx+, Tx–.
A DTE can determine that the transceiver is in Link Test
Fail one of two ways: it can monitor the LTF pin if the
transceiver is internal, or it can monitor loopback. If the
MAU is on-board the LTF pin can be sampled to
determine that the transceiver is in the link fail state. If
the MAU is external the DTE can monitor carrier sense
during transmission. A loss of carrier sense is an indication
of Link Test Fail State, since in Link Test Fail, loopback is
disabled. Note that jabber also disables loopback but with
Jabber the collision signal will be on.
SQETEST FUNCTION (SIGNAL QUALITY ERROR)
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 link fail
state, or the Jabber state.
When a packet, or two consecutive link test pulses is
received from the twisted pair input, the transceiver will
exit the Link Test Fail state upon transmit and receive
data being idle, and re-enable transmission and reception.
Link test pulses that do not occur within at most 25 to
150ms of each other are not considered consecutive.
In addition, detected pulses that occur within a time
between 2 to 7ms of a previous pulse will be considered
as noise by the link test circuitry.
For SQE to operate, the SQEN pin must be tied to V or
CC
BIAS. The SQE test can be disabled by tying the SQEN
pin to 1.2V or ground. This allows the chip to be
interfaced to a repeater.
POLARLITYCIRCUITRY
JABBERFUNCTION
The ML4658 offers automatic polarity correction. The
POLRD pin is used to report the status of the receive pair
polarity. This pin reflects the true status of the receive
polarity regardless of whether the part has autopolarity
correction or not.
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 lasts longer than 20ms the jabber logic
disables the transmitter, and turns on the collision
oscillator COL+, COL–. When Tx+ and Tx– finally go
idle, a second timer measures 0.5 seconds of idle on Tx+
and Tx– before re-enabling the transmitter and turning off
the collision oscillator. If transmission starts up again
before 0.5 seconds has expired, the timer is reset and
measures another 0.5 seconds of idle time.
Automatic Polarity Correction — ML4658 — In the link
OK state, receive polarity is updated when two
consecutive frames are received with the same Start of
Idle polarity. In the Link Test Fail state the part will use
either the Start of Idle signal or link pulses to correct the
receive polarity.
In the case where the part is powered up with the receive
polarity reversed and no frames are received, the part will
go into Link Test Fail. After two link pulses are received
with the same polarity, the part will exit Link Test Fail and
correct the receive polarity. The POLRD pin will continue
to reflect the true polarity of the receive pair.
Even though the transmitter is disabled during jabber, Link
Pulses are still transmitted if the, Link Test is enabled.
Jabber can be disabled by placing 1.2V on the SQEN/
LTD/JABD pin. This is useful for measuring jitter
performance on the transmitter.
LED DRIVERS
LINKTEST FUNCTION
The ML4658 has six LED drivers for transmit, receive,
collision, Link Test Fail, reverse polarity, and jabber. The
LEDs are normally off except for LTF which is normally on
and active high. The LEDs are tied to their respective pins
through a 510Ω resistor to 5 Volts.
Transmission — Whenever data is not being delivered to
the twisted pair link, the idle signal is applied. The idle
signal is a sequence of Link Pulses separated by a 16ms
period of silence. The idle signal starts with a period of
silence after a packet transmission ends. The link test
pulse is a single high pulse with the same amplitude
requirements as the data signal.
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 100ms. If another
transmit, receive or collision condition occurs during the
first 100ms, the LED timer will reset and begin timing
again for 100ms. The LEDs will remain on for consecutive
frames. The JAB, POLRD, and LTF LEDs do not have pulse
stretchers on them since their conditions occur long
enough for the eye to see.
Reception — The transceiver monitors the receive twisted
pair input for packet and link pulse activity. If neither a
packet nor a link test pulse is received for 50 to 150ms,
the transceiver enters the Link Test Fail state and inhibits
transmission and reception. Link pulses received with the
wrong polarity will be ignored and cause the chip to go
into link test fail.
13
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
