XR88C681_06 Datasheet, PDF (75/101 Pages) Exar Corporation – CMOS Dual Channel UART (DUART)

XR88C681
RXDn
Receive Shift Register
Receive Holding
Register
TXCn
Transmit Shift Register
V_CC
TXDn
Transmit Holding
Register
8
8
To Data Bus
(To be read by the CPU)
From Data Bus.
(Parallel Data from CPU)
Figure 40. A Block Diagram depicting Local Loopback Mode Operation
In this mode:
1. The transmitter output is internally connected to the
receiver input.
2. The transmit clock is used for the receiver.
3. The channel’s TXDn output is held marking (high).
4. The channel’s RXDn input is ignored.
5. The transmitter is enabled, but the received need not
be enabled.
6. CPU to transmitter and receiver communications
continue normally.
Remote Loopback Mode.
This mode is selected by setting MR2n[7:6] = 11.
Figure 41 presents a diagram depicting Remote
Loopback Mode operation.
Rev. 2.11
75