Modes of Operation
MSC8144E Reference Manual, Rev. 3
Freescale Semiconductor
20-23
20.4.3 Loop Operation
To help isolate system problems, the Loop mode is sometimes used to check software without
changing the physical connections in the external system. In Loop mode, the transmitter output is
connected to the receiver input internally. The
URXD
signal is disconnected from the external
connection which then becomes available for use as a GPIO signal. Clearing the data direction bit
of
UTXD
disconnects the transmitter output from the external connection.
To enable loop operation, set the SCICR[LOOPS] bit and clear SCICR[RSRC]. Setting the
SCICR[LOOPS] bit disables the path from
URXD
to the external output connection. Clearing the
SCICR[RSRC] bit connects the transmitter output to the receiver input. Both the transmitter and
receiver must be enabled (SCICR[TE] = 1 and SCICR[RE] = 1) for loop operation.
20.4.4 Stop Mode
The UART stops its clock to provide reduced power consumption when the GCR1[UART_STC]
bit is set (see Section 8.2.1, General Configuration Register 1 (GCR1), on page 8-2). When the
UART enters Stop mode, the states of the UART registers are unaffected. The UART registers
cannot be accessed during Stop mode. When the UART_STC bit is cleared, UART operation
resumes. Entering Stop mode during a transmission or reception results in invalid data.
Therefore, disable the receiver and transmitter (SCICR[TE] = 0, SCICR[RE] = 0) before entering
Stop mode.
20.4.5 Receiver Standby Mode
Refer to Section 21.2.7, Receiver Wake-Up.
Figure 20-20. Loop Operation (LOOPS = 1, RSRC = 0)
UTXD
URXD
General-Purpose I/O
Transmitter
Receiver
SCIDDR[DDRTX]
GPIO
UART
Содержание MSC8144E
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