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Data Sheet
121
Rev. 1.2, 2006-01-26
QuadFALC
TM
PEF 22554 E
Functional Description E1/T1/J1
3.7.2
Transmit Clock TCLK
The transmit clock input TCLK of the QuadFALC
TM can be configured for 1.544, 3.088, 6.176, 12.352 and
24.704 MHz input frequency in T1/J1 mode and 2.048, 4.096, 8.192, 16.384 and 32.768 MHz input frequency in
E1 mode and in T1/J1 channel translation mode. If GPC6.COMP_DIS = 1
B frequency selection is done by the
register bits CMR6.STF(2:0) (CMR6_E), if GPC6.COMP_DIS = 0
B selection is done by the register bit CMR1.STF
3.7.3
Automatic Transmit Clock Switching
The transmit clock output XCLK can be derived from TCLK in two ways:
Directly. In this case the TCLK frequency must be 32.768 MHz in E1 or 24.704 MHz in T1/J1 mode. (This can
be performed only for GPC6.COMP_DIS = 1
B.) or
By using the DCO-X, were the DCO-X reference is TCLK.
If TCLK fails, then the transmit clock output XCLK will also fail. To avoid this, a so called automatic transmit clock
switching can be enabled by setting the register bit CMR6.ATCS (CMR6_E). Then SCLKX will be used instead of
TCLK if TCLK is lost. The transmit elastic buffer must be active. Automatically switching between TCLK and
SCLKX is done in the following both cases:
If the TCLK input is used directly as source for the transmit clock XCLK, the output of the DCO-X is not used.
The DCO-X reference clock is SCLKX. If loss of TCLK is detected, the transmit clock XCLK will be switched
automatically (if CMR6.ATCS = 1
B) to the DCO-X output which is synchronous to SCLKX (see multiplexer “H”
in Figure 33). If XCLK was switched to the DCO-X output and TCLK becomes active, switching of XCLK (back)
to TCLK is automatically performed if CMR6.ATCS = 1
B. All switchings of XCLK between TCLK and the DCO-
X output are shown in the interrupt status bit ISR7.XCLKSS0 which is masked by IMR7.XCLKSS0. These
kinds of switching cannot be done in general without causing phase jumps in the transmit clock XCLK.
Additionally after loss of TCLK the transmit clock XCLK is also lost during the “detection time” for loss of TCLK
and the transmit pulses are disturbed. If CMR6.ATCS is cleared, TCLK is used (again) as source for the
transmit clock XCLK, independent if TCLK is lost or not. The interrupt status bit ISR7.XCLKSS0 will be set also.
If the transmit clock XCLK is sourced by the DCO-X output and the DCO-X reference clock is TCLK, the DCO-
X reference will be switched automatically (if CMR6.ATCS = 1
after a loss of TCLK was detected. If the DCO-X reference was switched to SCLKX and TCLK becomes active,
switching of the reference (back) to TCLK is automatically performed if CMR6.ATCS = 1
B. All switching of the
reference between TCLK and SCLKX are shown in the interrupt status bit ISR7.XCLKSS1 which is masked by
IMR7.XCLKSS1. For these kinds of automatic switching, the transmit clock XCLK fulfills the jitter-, wander- and
frequency deviation- requirements as specified for E1/T1 after the clock source of the DCO-X is changed. If
CMR6.ATCS is cleared, TCLK is used (again) as reference for the DCO-X, independent of whether TCLK is
lost or not. The interrupt status bit ISR7.XCLKSS1 will be set also.
The status register bits CLKSTAT.TCLKLOS and CLKSTAT.SCLKXLOS (CLKSTAT_E, CLKSTAT_T) show if the
appropriate clock is actually lost or not, so together with ISR7.XCLKSS1 and ISR7.XCLKSS0 the complete
information regarding the current status of the transmit clock system is provided.
3.7.4
Transmit Jitter Attenuator
The transmit jitter attenuator is based on the so called DCO-X (digital clock oscillator, transmit) in the transmit path.
Jitter attenuation of the transmit data is performed in the transmit elastic buffer, see Figure 33. The DCO-X
circuitry generates a "jitter-free" transmit clock and meets the E1 requirements of ITU-T I.431, G. 736 to 739,
G.823 and ETSI TBR12/13 and the T1 requirements of AT&T PUB 62411, PUB 43802, TR-TSY 009,TR-TSY 253,
TR-TSY 499 and ITU-T I.431, G.703 and G. 824. The DCO-X circuitry works internally with the same high
frequency clock as the DCO-R. It synchronizes either to the working clock of the transmit system interface (internal
transmit clock) or the clock provided on pin TCLK or the receive clock RCLK (remote loop/loop-timed). The DCO-
X attenuates the incoming jitter starting at its corner frequency with 20 dB per decade fall-off. With the jitter
attenuated clock, which directly depends on the phase difference between the incoming clock and the jitter
attenuated clock, data is read from the transmit elastic buffer (2 frames) or from the JATT buffer (2 frames, remote
loop), see Figure 35. Wander with a jitter frequency below the corner frequency is passed transparently.