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STANDARD PRODUCT
PM4351 COMET
DATA SHEET
PMC-1970624
ISSUE 10
COMBINED E1/T1 TRANSCEIVER
PROPRIETARY AND CONFIDENTIAL
418
from the output of the CDRC block. In digital mode (RUNI is logic 1), the data
sent to the TJAT is a sampled version of the RDAT digital input. Note that when
line loopback is enabled, the contents of the TJAT Reference Clock Divisor and
Output Clock Divisor registers should be programmed to 2FH in T1 or FFH in E1
to correctly attenuate the jitter on the receive clock. Conceptually, the data flow
through the COMET in this loopback mode is illustrated in Figure 39. In this
figure, the dashed lines represent connections in digital mode (RUNI is logic 1).
Figure 39
- Line Loopback
BTIF
BRIF
TX
ELST
RX
ELST
Transmitter
T1-XBAS /
E1-TRAN
Framer
T1-FRMR /
E1-FRMR
TJAT
RJAT
CDRC
XLPG
RLPS
TDAT
TCLKO
TXTIP
TXRING
RXTIP
RXRING
RDAT
RCLKI
BTPCM
BTSIG
BTCLK
BRPCM
BRSIG
BRCLK
Line Loopback
14.8.2 Payload Loopback
When PAYLOAD loopback (PAYLB) is initiated by setting the PAYLB bit in the
Master Diagnostics Register (00AH) to logic 1, the COMET is configured to
internally connect the output of its RX-ELST to the PCM input of its transmitter
block. The data read out of RX-ELST is timed to the transmitter clock, and the
transmit frame alignment indication is used to synchronize the output frame
alignment of RX-ELST. The transmit frame alignment is either arbitrary (when
the TX-ELST is used) or is specified by the BTFP input (when the TX-ELST is
bypassed). Conceptually, the data flow through the COMET in this loopback
mode is illustrated in Figure 40. Note that because the transmit and receive
streams are not superframe aligned, any robbed-bit signaling in the receive
stream will not fall in the correct frame once looped back and that transmit
robbed-bit signaling will overwrite the looped back data if signaling insertion is
enabled.