The DS21348 uses a set of laser-trimmed delay lines along with a precision Digital-to-Analog Converter
(DAC) to create the waveforms that are transmitted onto the E1 or T1 line. The waveforms created by the
DS21348 meet the latest ETSI, ITU, ANSI, and AT&T specifications. The user will select which
waveform is to be generated by setting the ETS bit (CCR1.7) for E1 or T1 operation, then programming
the L2/L1/L0 bits in Common Control Register 4 for the appropriate application. See Table 9-1 and Table
9-2 for the proper L2/L1/L0 settings.
A 2.048MHz or 1.544MHz TTL clock is required at TCLK for transmitting data at TPOS and TNEG.
ITU specification G.703 requires an accuracy of ±50ppm for both T1 and E1. TR62411 and ANSI specs
require an accuracy of ±32ppm for T1 interfaces. The clock can be sourced internally by RCLK or
JACLK. See CCR1.2, CCR1.1, CCR1.0, and for details. Due to the nature of the design of the
transmitter in the DS21348, very little jitter (less than 0.005 UIpp broadband from 10Hz to 100kHz) is
added to the jitter present on TCLK. Also, the waveforms created are independent of the duty cycle of
TCLK. The transmitter in the DS21348 couples to the E1 or T1 transmit twisted pair (or coaxial cable in
some E1 applications) via a 1:2 step-up transformer. In order for the device to create the proper
waveforms, the transformer used must meet the specifications listed in Table 9-3.
The DS21348 has automatic short-circuit limiter which limits the source current to 50mA (rms) into a 1Ω
load. This feature can be disabled by setting the SCLD bit (CCR2.5) = 1. When the current limiter is
activated, TCLE (SR.2) will be set even if short circuit limiter is disabled. The TPD bit (CCR4.0) will
power-down the transmit line driver and 3-state the TTIP and TRING pins. The DS21348 can also detect
when the TTIP or TRING outputs are open circuited. When an open circuit is detected, TOCD (SR.1)
will be set.
The DS21348 contains an onboard jitter attenuator that can be set to a depth of either 32 or 128 bits via
the JABDS bit (CCR4.2). The 128-bit mode is used in applications where large excursions of wander are
expected. The 32-bit mode is used in delay sensitive applications. The characteristics of the attenuation
are shown in . The jitter attenuator can be placed in either the receive path or the transmit path by
appropriately setting or clearing the JAS bit (CCR4.3). Also, the jitter attenuator can be disabled (in
effect, removed) by setting the DJA bit (CCR4.1). In order for the jitter attenuator to operate properly, a
2.048MHz or 1.544MHz clock must be applied at MCLK. ITU specification G.703 requires an accuracy
of ±50ppm for both T1 and E1. TR62411 and ANSI specs require an accuracy of ±32ppm for T1
interfaces. There is an onboard PLL for the jitter attenuator, which will convert the 2.048MHz clock to a
1.544 MHz rate for T1 applications. Setting JAMUX (CCR1.3) to a logic 0 bypasses this PLL. Onboard
circuitry adjusts either the recovered clock from the clock/data recovery block or the clock applied at the
TCLK pin to create a smooth jitter free clock which is used to clock data out of the jitter attenuator FIFO.
It is acceptable to provide a gapped/bursty clock at the TCLK pin if the jitter attenuator is placed on the
transmit side. If the incoming jitter exceeds either 120 UIpp (buffer depth is 128 bits) or 28 UIpp (buffer
depth is 32 bits), then the DS21348 will divide the internal nominal 32.768MHz (E1) or 24.704MHz (T1)
clock by either 15 or 17 instead of the normal 16 to keep the buffer from overflowing. When the device
divides by either 15 or 17, it also sets the Jitter Attenuator Limit Trip (JALT) bit in the Receive
Information Register 1 (RIR1).
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