Communication Processor Module
MOTOROLA
MPC823e REFERENCE MANUAL
16-175
SCCs
COMMUNICATION
16
PROCESSOR MODULE
When using local loopback mode, the clock source for the transmitter and receiver must be
the same. Thus, the same baud rate generator or external CLKx pin can be used for both
the transmitter and receiver. Separate CLKx pins can be used with the transmitter and
receiver as long as the CLKx pins are connected to the same external clock signal source.
In automatic echo mode, the channel automatically retransmits the received data on a
bit-by-bit basis using whatever receive clock is provided. The receiver operates normally
and receives data if CDx
is asserted. The transmitter simply transmits the received data.
In this mode, the CTSx
signal is ignored. The echo function can be implemented in the
software by receiving buffers from a serial communication controller, linking them to transmit
buffer descriptors, and then transmitting them back out of that serial communication
controller.
In loopback/echo mode, loopback and echo operation occur simultaneously. The CDx
and
CTSx
pins are ignored. Refer to the loopback bit description for clocking requirements.
00 = Normal operation. The CTSx
and CDx signals are under automatic control.
01 = Local loopback mode.
10 = Automatic echo mode.
11 = Loopback and echo mode.
ENR—Enable Receive
This bit enables the receiver hardware state machine for a serial communication controller.
When ENR is cleared, the receiver is disabled and any data in the receive FIFO is lost. If
ENR is cleared during reception, the receiver aborts the current character. ENR may be set
or cleared, regardless of whether the serial clocks are present. Refer to
Section 16.9.14 Disabling the SCCs On-the-Fly
for a description of how to disable and
reenable a serial communication controller.
Note:
If you prefer external loopback, the DIAG field must be selected for normal
operation and an external connection must be made between the TXDx and
RXDx pins. Clocks can be generated internally using a baud rate generator or
they can be generated externally. You can physically connect the appropriate
control signals or the port C register can be used to cause the CDx and CTSx
pins to be permanently asserted to a particular serial communication controller.
Note:
The serial communication controller that controls reception provides other tools
besides the ENR bit. Some of these tools include the
ENTER HUNT MODE
command,
CLOSE RX BD
command, and the E bit of the receive buffer
descriptor.
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