T1/E1 Short Haul Transceiver with Crystal-less Jitter Attenuation — LXT350
2.7.1.4
Dual Loopback (DLOOP)
See Figure 9. In Hardware mode, DLOOP is selected by setting both the RLOOP and LLOOP pins
High. In Host mode set bits CR2.ERLOOP = 1 and CR2.ELLOOP = 1. In DLOOP mode, the
transmit clock and data inputs (TCLK and TPOS/TNEG or TDATA) loop back through the Jitter
Attenuator (unless disabled) to RCLK and RPOS/RNEG or RDATA. The data and clock recovered
from the twisted-pair line loop back through the transmit circuits to TTIP and TRING without jitter
attenuation.
Figure 9. Dual Loopback
2.7.2
Internal Pattern Generation
2.7.2.1
Transmit All Ones (TAOS)
See Figure 10. When TAOS is active, the transceiver ignores the TPOS and TNEG inputs and
transmits a continuous stream of 1’s at the TCLK frequency. When TCLK is not supplied, TAOS
timing is derived from MCLK. This can be used as the Alarm Indication Signal (AIS–also called
the Blue Alarm).
Both TAOS and LLOOP can operate simultaneously as shown in Figure 5, however, RLOOP
inhibits TAOS. When both TAOS and LLOOP are active, TCLK and TPOS/TNEG loop back to
RCLK and RPOS/RNEG (through the jitter attenuator if enabled), and the all ones pattern is also
routed to TTIP/TRING.
In Host mode, TAOS is activated when bit CR2.ETAOS = 1. In Hardware mode, setting the TAOS
pin High activates TAOS.
Figure 10. TAOS Data Path
2.7.2.2
Quasi-Random Signal Source (QRSS)
See Figure 11. For T1 operation, the Quasi-Random Signal Source (QRSS) is a 220-1 pseudo-
random bit sequence (PRBS) with no more than 14 consecutive zeros. For E1 operation, QRSS is
215-1 PRBS with inverted output.
Datasheet
23
ETC [ ETC ]
