Motorola MPC823e Reference Manual page 694

Communication Processor Module
You can program the CPM command register with the following commands to receive data.
• ENTER HUNT MODE—After the hardware or software is reset and the channel is
enabled in the PSMR–SCC HDLC register, the channel is in receive enable mode and
uses the first buffer descriptor in the table. The ENTER HUNT MODE command is
generally used to force the HDLC receiver to stop receiving the current frame and enter
hunt mode. In hunt mode, the SCCx HDLC controller continually scans the input
datastream for the flag sequence. After receiving the command, the current receive
buffer is closed and the CRC is reset. Additional frame reception uses the next buffer
descriptor.
• CLOSE RX BD—This command must not be used in the HDLC protocol.
• INIT RX PARAMETERS—This command initializes all the receive parameters in this
serial channel parameter RAM to their reset state and must only be issued when the
receiver is disabled. The INIT TX AND RX PARAMS command can be used to reset
the receive and transmit parameters.
16.9.16.7 SCCx HDLC CONTROLLER ERRORS.The SCCx HDLC controller reports
frame reception and transmission error conditions using the channel buffer descriptors, error
counters, and SCCE–HDLC register. The following transmission errors can be detected by
the SCCx HDLC controller.
• Transmitter Underrun Error — When this error occurs, the channel terminates buffer
transmission, closes the buffer, sets the UN bit in the TX buffer descriptor, and
generates the TXE interrupt if it is enabled. The channel continues transmitting after it
receives the RESTART TRANSMIT command. The transmit FIFO size is 32 bytes for
each serial communication controller.
• CTS Lost During Frame Transmission Error — When this error occurs, the channel
terminates buffer transmission, closes the buffer, sets the CT bit in the TX buffer
descriptor, and generates the TXE interrupt if it is enabled. The channel continues
transmitting after it receives the RESTART TRANSMIT command. If this error occurs
on the first or second buffer of the frame and the RTE bit in the PSMR–SCC HDLC is
set, the channel retransmits the frame when the CTSx signal becomes active again.
When you are working in SCCx HDLC mode with the possibility of a collision, in order
to ensure the retransmission method functions properly, the first and second data
buffers must contain more than 36 bytes of data and 20 bytes of data if multiple buffers
per frame are used. The channel also increments the retransmission counter. This
requirement does not apply to small frames that consist of a single buffer.
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