XR16L2450 Datasheet, PDF (11/30 Pages) Exar Corporation – 2.25V TO 5.5V DUART

xr
REV. 1.1.0
XR16L2450
2.25V TO 5.5V DUART
2.12 Internal Loopback
The L2450 UART provides an internal loopback capability for system diagnostic purposes. The internal
loopback mode is enabled by setting MCR register bit-4 to logic 1. All regular UART functions operate normally.
Figure 7 shows how the modem port signals are re-configured. Transmit data from the transmit shift register
output is internally routed to the receive shift register input allowing the system to receive the same data that it
was sending. The TX pin is held at logic 1 or mark condition while RTS# and DTR# are de-asserted, and
CTS#, DSR# CD# and RI# inputs are ignored. Caution: the RX input must be held to a logic 1 during loopback
test else upon exiting the loopback test the UART may detect and report a false “break” signal.
FIGURE 7. INTERNAL LOOP BACK IN CHANNELS A AND B
VCC
Transmit Shift Register
(THR/FIFO)
MCR bit-4=1
Receive Shift Register
(RHR/FIFO)
VCC
RTS#
CTS#
VCC
DTR#
DSR#
RI#
OP1#
VCC
OP2#
CD#
TXA/TXB
RXA/RXB
RTSA#/RTSB#
CTSA#/CTSB
DTRA#/DTRB#
DSRA#/DSRB#
RIA#/RIB#
OP2A#/OP2B#
CDA#/CDB#
11