Freescale Semiconductor MCF5480 - Manual (Page 808)

MCF548x Reference Manual, Rev. 3
26-46 Freescale Semiconductor
26.4.10 Looping Modes
The UART can be configured to operate in various looping modes as shown in Figure 26-42 . These modes
are useful for local and remote system diagnostic functions, and can be used in modem mode and IrDA
mode as well as UART mode. The modes are described in the following paragraphs.
Figure 26-42. Looping Modes Functional Diagram
The UART's transmitter and receiver should both be disabled when switching between modes. The
selected mode is activated immediately upon mode selection, regardless of whether a character is being
received or transmitted.
26.4.10.1 Automatic Echo Mode
In the automatic echo mode, the UART automatically retransmits the received data on a bit-by-bit basis.
The local CPU-to-receiver communication continues normally, but the CPU-to-transmitter link is
disabled. While in this mode, received data is clocked on the receiver clock and retransmitted on TxD. The
receiver must be enabled, but the transmitter need not be enabled.
Because the transmitter is not active, the TxEMP and TxRDY bits in USR are inactive, and data is
transmitted as it is received. Received parity is checked, but not recalculated for transmission. Character
framing is also checked, but stop bits are transmitted as received. A received break is echoed as received
until the next valid start bit is detected.
26.4.10.2 Local Loopback Mode
TxD is internally connected to RxD in the local loopback mode. This is useful for testing the operation of
a local UART module channel by sending data to the transmitter and checking data assembled by the
receiver. In this manner, correct channel operations can be assured. Also, both transmitter and
CPU-to-receiver communications continue normally in this mode. While in this mode, the RxD input data
RX
TX
disabled
PSCnRXD
PSCnTXD
Automatic echo
RX
TX
disabled
PSCnRXD
PSCnTXD
Local loop back
RX
TX
disabled
PSCnRXD
PSCnTXD
Remote loop back
disabled