CN8380 Datasheet, PDF (36/91 Pages) Conexant Systems, Inc – integrated quad line interface unit unit for both 1.544 Mbps (T1) and 2.048 Mbps (E1) applications

2.0 Circuit Description
2.4 Transmitter
CN8380
Quad T1/E1 Line Interface
In Host Mode, standard pulse templates are selected per channel by writing to
register bits PULSE(2:0) [TLIU_CR; addr n2]. If desired, custom pulse shapes
can be programmed for each channel using the SHAPEn [addr n8 – nF] registers
and the PPT [TLIU_CR; addr n2] register bit. The data written into the SHAPEn
registers is 5-bit magnitude only. The first four code values of the pulse define the
first half of the symbol, and the last four values define the last half of the symbol.
The last half symbol polarity is always forced to be opposite from the first half
polarity. Figure 2-6 illustrates the shaped transmit signals.
Figure 2-6. Transmit Pulse Shape
TCLK
8 x TCLK
Digitized
AMI Pulses
Shape Data
Magnitude
(0x00–0x1F)
8380_011
Positive Pulse
Negative Pulse
2.4.6 Driver
2.4.6.1 Transmit
Termination Options
The transmit DAC converts digitally shaped AMI pulses into analog bipolar
signals. The line driver provides a high impedance, current drive for the transmit
DAC and outputs transmit signals to the XTIP[n] and XRING[n] output pins. The
high impedance driver allows line impedance matching using external parallel
resistors to meet return loss requirements. In applications which require surge
protection, pulse amplitude compensation is provided if protection resistors are
needed in series with XTIP[n] and XRING[n]. When a shorted line is detected,
transmit monitor and protection circuits reduce the output current level to less
than 50 mA peak. The standard transmit transformer for the CN8380 has a turns
ratio of 1:2 (chip-side: line-side). To minimize power consumption, an alternate
1:1.36 turns ratio transformer can be used in an unterminated configuration.
Various transmitter termination options are available to meet almost any interface
requirement. Figure 2-7 illustrates the location of the transmit termination
components. In this figure, Ct is a smoothing capacitor across XTIP and XRING.
The recommended value for Ct is 150 pF. If other components are also connected
to XTIP/XRING, such as surge protection diodes, Ct’s value should be adjusted to
maintain a total parallel capacitance of approximately 150 pF.
Rt is a parallel termination resistor selected to provide the required transmitter
return loss, typically –18 dB. If an application does not have a return loss
requirement, Rt can be omitted in order to reduce total power consumption.
2-14
Conexant
N8380DSA
Advance Information