CS61581 datasheet(10/37 Pages) CIRRUS

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CS61581

DS211PP8

2. THEORY OF OPERATION

The CS61581 Universal Line Interface supports T1

and E1 data rates for both short haul and long haul

applications. The transmitter complies with all

standard T1 and E1 applications without changing

transformers. Transmitter power consumption is

minimized using the impedance matching feature,

which eliminates external resistors for standard line

impedances. When configured for long haul opera-

tion, the receiver uses gain and equalization to pro-

vide 40 dB of sensitivity. The receiver reconfigures

for short haul operation, limiting the receive sensi-

tivity and increasing the noise immunity.

2.1

Operating Mode Selection

The CS61581 can be operated in stand-alone hard-

ware interface mode (MODE pin is low), or by a

microcontroller in serial host mode (MODE pin is

high). Additional functionality is available in the

host mode including both short haul and long haul

operational modes. The CS61581 defaults to the

Long Haul configuration (LXT310/318 compati-

ble). The T1 (DSX-1 and Network interface) and

E1 (ITU-T G.703) are selectable via the serial port

by writing to a control register.

Tying TNEG high for more than 16 TCLK cycles

enables the unipolar mode, changing TPOS to

TDATA, RPOS to RDATA, and RNEG to BPV.

When configured for unipolar mode, the MODE

pin can be tied to RCLK enabling the B8ZS encod-

ers and decoders. Coder mode does not support bi-

polar data.

2.2

Master Clocks

The CS61581 requires a reference clock for the re-

ceiver and the jitter attenuator. Either a 1.544 MHz

(or 2.048 MHz) external clock can be input to

MCLK, or a 4X crystal can be connected to the on-

chip oscillator. This frequency reference should be

within 100 ppm of the nominal operating frequen-

cy. Jitter and wander on the reference clock will de-

grade jitter attenuation and receiver jitter tolerance.

If MCLK is provided, the crystal oscillator is ig-

nored.

2.3

Transmitter

The transmitter accepts digital T1 or E1 input data

and drives appropriately shaped AMI (Alternate

Mark Inversion) pulses onto a transmission line

through a transformer. The transmit data (TPOS &

TNEG or TDATA) is sampled on the falling edge

of the input clock, TCLK.

Upon power up, the CS61581 defaults to Long

Haul Mode with low-impedance drive. In this

mode, a 1:2 transformer is required (See Table 1).

The CS61581 will support both T1 and E1 opera-

tion as determined by the master clock frequency.

In host mode, T1 (DSX-1 or Network Interface),

E1 (ITU-T or G.703) or T1 long-haul pulse shapes

may be selected. Long-haul or short-haul operation

is determined by the SH/LH bit (CR2.0). The

SH/LH bit also establishes functionality of Control

Registers 1 and 2.

In the matched impedance configuration, the line

driver internally matches the impedance of the line

load; 75

. or 120 . for E1, and 100 . for T1 using

a 1:1.5 turns ratio transformer. Internal impedance

matching reduces current consumption by about a

factor of two compared to return loss achieved by

external resistors.

The T1 long-haul pulse shapes comply with FCC

Part 68 Option A (0 dB). Option B (-7.5 dB), Op-

tion C (-15 dB) or (-22.5 dB) (see Table 1). If de-

sired, the T1 pre-equalization settings can be

selected for E1 operation as well. In long-haul

mode, pulse shaping and signal level are controlled

by LBO1 and LBO2 pins or register bits.

Custom transmit pulse shapes may be implemented

by writing pulse shape coefficients to the registers.

Custom pulses may be used to correct for pulse

shape degradation or distortion caused by improper

termination, suboptimal interconnect wiring, or

CS61581

DS211F1

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