LXT331
—
Dual T1/E1 Line Interface Unit
18
Datasheet
2.4
Diagnostic Mode Operation
The LXT331 offers two diagnostic modes. Analog Loopback (ALOOP) and Transmit All Ones
(TAOS) are available under both Host and Hardware control modes.
In Host mode, diagnostic modes are selected by writing the appropriate SIO bits. In Hardware
mode, diagnostic modes are selected by a combination of pin settings. The pins must be held at the
specified levels for a minimum of 20 ns (typically).
Table 5
lists Hardware Mode control settings
for the various diagnostic modes.
Transmit All Ones.
See
Figure 8
. Transmit All Ones
(TAOS) is selected when TAOS = 1. In
TAOS mode the TPOS and TNEG inputs are ignored, but the transmitter remains locked to the
TCLK input. When TAOS is selected, the transceiver transmits a continuous stream of 1s at the
TCLK frequency. If TCLK is not supplied, MCLK is used as the transmit reference. TAOS and
Analog Loopback can be selected simultaneously as shown in
Figure 9
.
Analog Loopback.
See
Figure 10
. Analog Loopback (ALOOP) is selected when ALOOP = 1. In
ALOOP mode the receive line input (RTIP/RRING) is blocked. The transmit outputs (TTIP and
TRING) are looped back through the receiver input and output at PMRK and NMRK. The
transmitter circuits are unaffected by ALOOP. Transmitting onto an improperly terminated line
may produce unexpected pulse widths at PMRK and NMRK.
Reset / Tri-State
. By holding the TRSTE pin High for at least 200 ns, all output drivers (both
digital and analog) go to the high Z state and the chip logic is reset. The reset/high Z state is
maintained for 6
μ
s after TRSTE returns Low.
Table 5. Hardware Mode Diagnostic Selection
Mode
LXT331 Pin
TRSTE
ALOOP
TAOS
Analog Loopback
L
H
L
Transmit All Ones
L
X
H
Reset/High Z
H
X
X
Figure 8. Transmit All Ones Data Path
CTTIP
Timing &
TAOS
TCLK
TPOS
TNEG
TTIP
TRING
RTIP
RRING
Transmit All Ones =
ALOOP
0
TAOS
1
PMRK
NMRK