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HFBR-4663

Single Chip 10BASE-FL Transceiver

HP => Agilent Technologies 

SQE Test Function Signal

Quality Error

The SQE test function allows the

DTE to determine whether the

collision detect circuitry is

functional. After each trans-

mission, during the inter packet

gap time, the collision oscillator

will be activated for typically 1 µs.

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 VCC. 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 lasts longer than

20 ms 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 1 MHz idle

signal is still transmitted.

LED Drivers

The HFBR-4663 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 500 Ω

resistor to 5 volts.

The XMT, RCV and CLSN pins

have pulse stretchers on them

which enable the LEDs to be

visible. When transmission or

reception occurs, the LED XMT,

RCV or CLSN status pins will

activate low for several milli-

seconds. If another transmit,

receive or collision condition

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 VIN+, VIN- less

than 3 µs apart. If either one of

these conditions do not exist,

LMON will go high.

Input Amplifier

The VIN+, VIN- input signal is fed

into a limiting amplifier with a

gain of about 100 and input

resistance of 1.3 kΩ. Maximum

sensitivity is achieved through the

use of a DC restoration feedback

loop and AC coupling the input.

When AC coupled, the input DC

VOUT+

bias voltage is set by an on-chip

network at about 1.7 V. These

coupling capacitors, in

conjunction with the input

impedance of the amplifier,

establish a high pass filter with 3

dB corner frequency, fL, at

1

fL = ––––––––

2π1300C

Since the amplifier has a differ-

ential 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 AVCC 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 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 VOS in Figure 4. In

order to reduce this error a DC

feedback loop is incorporated.

This negative feedback loop nulls

the offset voltage, forcing VOS to

be zero. Although the capacitor

on VDC is non-critical, the pole it

creates can affect 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.

VOS

VOUT-

Figure 4.

86

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