XR19L220_07 Datasheet, PDF (17/43 Pages) Exar Corporation – SINGLE CHANNEL INTEGRATED UART AND RS-232 TRANSCEIVER

XR19L220
REV. 1.0.1
SINGLE CHANNEL INTEGRATED UART AND RS-232 TRANSCEIVER
2.18 Internal Loopback
The L220 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
including automatic hardware and software flow control. Figure 11 below shows how the internal UART 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, RTS# and DTR# pins
are held HIGH while CTS#, DSR#, CD# and RI# inputs are ignored. Caution: the RX input pin must be held at
inactive during loopback test else upon exiting the loopback test the UART may detect and report a false
“break” signal.
FIGURE 11. INTERNAL LOOP BACK
Transmit Shift Register
(THR/FIFO)
VCC
MCR bit-4=1
Receive Shift Register
(RHR/FIFO)
VCC
RTS#
CTS#
VCC
DTR#
DSR#
RI#
OP1#
CD#
OP2#
TX
RX
RTS#
CTS#
DTR#
DSR#
RI#
CD#
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