UART ModulesMCF5253 Reference Manual, Rev. 1Freescale Semiconductor 15-1115.3.3.2 Local Loopback ModeIn this mode, TxD is internally connected to RxD. This mode is useful for testing the operation of a localUART module channel by sending data to the transmitter and checking data assembled by the receiver. Inthis manner, correct channel operations can be assured. Both transmitter and CPU-to-receivercommunications continue normally in this mode. While in this mode, the RxD input data is ignored, theTxD is held marking, and the receiver is clocked by the transmitter clock. The transmitter must be enabledbut not the receiver.15.3.3.3 Remote Loopback ModeIn this mode, the channel automatically transmits received data on the TxD output on a bit-by-bit basis.The local CPU-to-transmitter link is disabled. This mode is useful for testing remote channel receiver andtransmitter operation. While in this mode, the receiver clocks the transmitter.NOTEBecause the receiver is not active, the CPU cannot read received data. Allstatus conditions are inactive. Received parity is not checked and is notrecalculated for transmission. Stop bits are transmitted as received. Areceived break is echoed as received until the next valid start bit is detected.Figure 15-7. Looping Modes Functional DiagramCPURxTxCPUCPURxRxTxTx(a) Automatic Echo(b) Local Loopback(c) Remote LoopbackRxD InputRxD InputRxD InputTxDOutputTxDOutputTxDOutputDisabledDisabledDisabledDisabledDisabled
