Freescale Semiconductor, Inc.
7.3.3 Looping Modes
Each serial module channel can be configured to operate in various looping modes as
shown in Figure 7-7. These modes are useful for local and remote system diagnostic
functions. The modes are described in the following paragraphs with further information
available in 7.4 Register Description and Programming.
The channel'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.
7.3.3.1 AUTOMATIC ECHO MODE. In this mode, the channel automatically retransmits
the received data on a bit-by-bit basis. The local CPU32-to-receiver communication
continues normally, but the CPU32-to-transmitter link is disabled. While in this mode,
received data is clocked on the receiver clock and retransmitted on TxDx. The receiver
must be enabled, but the transmitter need not be enabled.
Since the transmitter is not active, the SR TxEMP and TxRDY bits 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.
7.3.3.2 LOCAL LOOPBACK MODE. In this mode, TxDx is internally connected to RxDx.
This mode is useful for testing the operation of a local serial 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 CPU32-to-receiver
communications continue normally in this mode. While in this mode, the RxDx input data
is ignored, the TxDx is held marking, and the receiver is clocked by the transmitter clock.
The transmitter must be enabled, but the receiver need not be enabled.
7.3.3.3 REMOTE LOOPBACK MODE. In this mode, the channel automatically transmits
received data on the TxDx output on a bit-by-bit basis. The local CPU32-to-transmitter link
is disabled. This mode is useful in testing receiver and transmitter operation of a remote
channel. While in this mode, the receiver clock is used for the transmitter.
Since the receiver is not active, received data cannot be read by the CPU32, and the error
status conditions are inactive. Received parity is not checked and is not recalculated for
transmission. Stop bits are transmitted as received. A received break is echoed as
received until the next valid start bit is detected.
7- 14
MC68340 USER’S MANUAL
MOTOROLA
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