XR17D158_05 Datasheet, PDF (37/73 Pages) Exar Corporation – UNIVERSAL (3.3V AND 5V) PCI BUS OCTAL UART

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REV. 1.2.2
XR17D158
UNIVERSAL (3.3V AND 5V) PCI BUS OCTAL UART
5.6 Internal Loopback
Each UART channel provides an internal loopback capability for system diagnostic. The internal loopback
mode is enabled by setting MCR register bit-4 to logic 1. All regular UART functions operate normally.
Figure 18 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, RTS# and DTR# pins are held HIGH (idle or de-asserted state), and the CTS#, DSR#
CD# and RI# inputs are ignored.
FIGURE 18. INTERNAL LOOP BACK
VCC
Transmit Shift
Register
MCR bit-4=1
Receive Shift
Register
VCC
RTS#
CTS#
DTR#
VCC
DSR#
RI#
CD#
OP1#
OP2#
TX [7:0]
RX [7:0]
RTS# [7:0]
CTS# [7:0]
DTR# [7:0]
DSR# [7:0]
RI# [7:0]
CD# [7:0]
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