10 looping modes, 1 automatic echo mode, 2 local loopback mode – Freescale Semiconductor MCF5480 User Manual

Page 808: Looping modes -46, Automatic echo mode -46, Local loopback mode -46

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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

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