Communication Processor Module
MOTOROLA
MPC823 USER’S MANUAL
16-169
SCC2
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 CD is asserted. The transmitter simply transmits the received data.
In this mode, the CTS signal is ignored. The echo function can be implemented in the
software by receiving buffers from a serial communication controller, linking them to TX
buffer descriptors, and then transmitting them back out of that serial communication
controller.
In loopback/echo mode, loopback and echo operation occur simultaneously. The CD and
CTS pins are ignored. Refer to the loopback bit description for clocking requirements.
00 = Normal operation. The CTS and CD 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 the 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
reenable the serial communication controller.
Note: If you prefer external loopback, the DIAG field should be selected for normal
operation and an external connection should be made between the TXD2 and
RXD2 pins. Clocks can be generated internally using a baud rate generator or
generated externally. You can physically connect the appropriate control signals
or the port C register can be used to cause the CD and CTS pins to be
permanently asserted to the 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 RX buffer descriptor.