CS61884 datasheet(24/72 Pages) CIRRUS

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CS61884

DS485PP4

9. TRANSMITTER

The CS61884 contains eight identical transmitters

that each use a low power matched impedance driv-

er to eliminate the need for external load matching

resistors, while providing superior return loss. As a

result, the TTIP/TRING outputs can be connected

directly to the transformer allowing one hardware

circuit for 100

Ω (T1/J1), 120 Ω (E1), and 75 Ω

(E1) applications.

Digital transmit data is input into the CS61884

through the TPOS/TNEG input pins. These pins ac-

cept data in one of three formats: unipolar, bipolar,

or RZ. In either unipolar or bipolar mode, the

CS61884 internally generates a pulse shape com-

pliant to the ANSI T1.102 mask for T1/J1 or the

G.703 mask for E1 (Refer to Figure 6 and

Figure 7). The pulse shaping applied to the transmit

data can be selected in hardware mode or in host

mode.

In hardware mode, the pulse shape is selected for

all channels via the LEN[2:0] pins (Refer to

Table 5 on page 25). This sets the pulse shape for

all eight transmitters to one of the prestored line

lengths. The CBLSEL pin in combination with the

LEN[2:0] pins set the line impedance for all eight

channels. The CBLSEL pin also selects between

E1 120

Ω or E1 75Ω modes, when the LEN pins are

configured for E1 operation mode.

In host mode, the pulse shape for each channel can

be set independently, during NRZ operation mode,

for proper clock recovery and jitter attenuation. In

RZ Mode each channel can be set to either T1/J1 or

E1, when there is no Mclk present (Refer to RZ

Mode (See Section 9.3 on page 25).

To select the standard pulse shapes, the channels

are selected individually using the Line Length

Channel ID Register (10h) (See Section 14.17 on

page 38), then the LEN[3:0] bits in the Line

Length Data Register (11h) (See Section 14.18 on

page 39) are set for the desired line length for that

channel. The LEN bits select the line type and im-

pedance for both the receiver and the transmitter of

the addressed channel.

NOTE: In host mode the CBLSEL pin is not used.

500

1.0

0.5

-0.5

250

750

1000

Normalized

Amplitude

Output Pulse

Shape

ANSI T1.102,

AT&T CB 119

Specifications

TIME (nanoseconds)

Figure 6. Pulse Mask at T1/J1 Interface

Figure 7. Pulse Mask at E1 Interface

000000000000000000000000000000000000

269 ns

244 ns

194 ns

219 ns

488 ns

Nomina l P u ls e

100

110

120

-10

-20

Perc en t of

no m in al pe ak

volta ge

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