MC68194(2006) Ver la hoja de datos (PDF) - ON Semiconductor

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componentes Descripción

Fabricante

MC68194
(Rev.:2006)

Carrier Band Modem (CBM)

ON Semiconductor 

MC68194

802.4 exposed DTE−DCE interface. Management and

transmission symbol requests are accepted via the CBM

physical data request channel (TXSYM0−TXSYM2,

SMREQ*, and TXCLK). The physical data indication

channel (RXSYM0−RXSYM2, SMIND*, and RXCLK) is

used to send received symbols and management responses

to the MAC.

The periphery circuitry is primarily associated with

interface to the LAN coaxial cable and data recovery. An

external crystal or clock source is required (20 MHz for 5

Mbps data rate or 40 MHz for 10 Mbps data rate) for onboard

timing and transmit clock. Also, an RC timing network sets

the jabber timeout period.

The coaxial cable interface combines the transmit and

receive signal functions. For transmission, the CBM provides

differential drive signals (TXOUT and TXOUT*) whose

signaling is ECL levels referenced to VCC (logic high [

+4.1 V, logic low [ + 3.3 V) and a gate signal called TXDIS.

The IEEE 802.4 standard puts specific requirements on the

signal transmitted to the cable:

Between +63 dB and +66 dB (1.0 mV, 75 Ω) [dBmV]

output voltage level.

Transmitter−off leakage not to exceed −20 dB

(1.0 mV, 75 Ω) [dBmV].

Signal transition time window (eye pattern)

dependent on data rate.

Because of this, an external amplifier with waveshaping

is required. The CBM TXOUT/TXOUT* outputs provide

complementary signals with virtually no slew, and the

TXDIS is an enable signal helpful for turning the external

amp off “hard” to meet the low level leakage.

On the reception side, the CBM requires a pre−amplifier

to receive the low level signal from the cable. The signal

available at the “F”−connector can range from +10 dB to +66

dB (1.0 mV, 75 Ω) [dBmV]. The signal required at the CBM

is about 12 dB above this (net gain through the transformer,

pre−amp, and any filtering). The receiver can be used in full

differential or single−ended mode.

A second part of the receiver function is the signal detect

or carrier detect function. The IEEE 802.4 requires that the

receiver detect a signal of +10 dBmV or above (i.e., be

turned “on”) and report Silence for a signal of +4.0 dBmV

or below (i.e., be turned “off”). Therefore, a 6.0 dB (2:1

voltage ratio) range or window is defined in which the signal

detect must switch. The CBM is optimized for this range

(including the pre−amp gain), although it is trimmed via an

external THRESHOLD.

The remaining external components are associated with

clock recovery. A capacitor and resistor (internal R also

provided) set one−shot timing, and an active filter for a PLL

used in clock and data recovery is required. The active filter

can be implemented via an op amp, or if 5.0 volt operation

is required, an alternate charge pump design can be used.

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