Transmitter – Cirrus Logic CS61884 User Manual

Page 24

CS61884

DS485F3

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

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.