XR16L2550IM-F Datasheet, PDF (19/44 Pages) Exar Corporation – LOW VOLTAGE DUART WITH 16-BYTE FIFO

XR16L2550
REV. 1.1.3
LOW VOLTAGE DUART WITH 16-BYTE FIFO
2.19 Internal Loopback
The L2550 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 13 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 pins 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. Also,
Auto RTS/CTS is not supported during internal loopback.
FIGURE 13. INTERNAL LOOP BACK IN CHANNEL A AND B
Transmit Shift Register
(THR/FIFO)
VCC
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#
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