24-6
MPC860 PowerQUICC UserÕs Manual
MOTOROLA
Part V. The Communications Processor Module
Receive commands are described in Table 24-3.
24.7 Handling Errors in the SCC HDLC Controller
The SCC HDLC controller reports frame reception and transmission errors using BDs,
error counters, and the SCCE. Transmission errors are described in Table 24-4.
Reception errors are described in Table 24-5.
Table 24-3. Receive Commands
Command
Description
ENTER
HUNT
MODE
After a hardware or software reset, once an SCC is enabled in the GSMR, the receiver is
automatically enabled and uses the Þrst BD in the RxBD table. While the SCC is looking for the
beginning of a frame, that SCC is in hunt mode. The
ENTER
HUNT
MODE
command is used to force the
HDLC receiver to stop receiving the current frame and enter hunt mode, in which the HDLC
continually scans the input data stream for a ßag sequence. After receiving the command, the buffer is
closed and the CRC is reset. Further frame reception uses the next BD.
CLOSE
RXBD
Should not be used in the HDLC protocol.
INIT
RX
PARAMETERS
Resets the Rx parameters in the parameter RAM.; issue only when the receiver is disabled. Note that
INIT
TX
AND
RX
PARAMETERS
resets both Tx and Rx parameters.
Table 24-4. Transmit Errors
Error
Description
Transmitter
Underrun
The channel stops transmitting, closes the buffer, sets TxBD[UN], and generates a TXE interrupt if not
masked. Transmission resumes when a
RESTART
TRANSMIT
command is issued. The SCC1 send and
receive FIFOs are 32 bytes each; SCC2ÐSCC4 FIFOs are 16 bytes each.
CTS Lost
during Frame
Transmission
The channel stops transmitting, closes the buffer, sets TxBD[CT], and generates the TXE interrupt if
not masked. Transmission resumes after a
RESTART
TRANSMIT
command. If this error occurs on the Þrst
or second buffer of the frame and PSMR[RTE] = 1, the channel resends the frame when CTS is
reasserted and no error is reported. If collisions are possible, to ensure proper retransmission of
multi-buffer frames, the Þrst two buffers of each frame should in total contain more than 36 bytes for
SCC1 or 20 bytes for SCC2ÐSCC4. The channel also increments the retransmission counter RETRC
in the parameter RAM.
Table 24-5. Receive Errors
Error
Description
Overrun
Each SCC maintains an internal FIFO for receiving data. The CPM begins programming the SDMA
channel (if the buffer is in external memory) and updating the CRC when a full or partial FIFOÕs worth
of data (according to GSMR_H[RFW]) is received in the Rx FIFO. When an Rx FIFO overrun occurs,
the previous byte is overwritten by the next byte. The previous data byte and the frame status are lost.
The channel closes the buffer with RxBD[OV] set and generates an RXF interrupt if not masked. The
receiver then enters hunt mode. Even if an overrun occurs during a frame whose address is not
recognized, an RxBD with data length two is opened to report the overrun and the interrupt is
generated.
CD Lost
during Frame
Reception
Highest priority error. The channel stops frame reception, closes the buffer, sets RxBD[CD], and
generates the RXF interrupt if not masked. The rest of the frame is lost and other errors are not
checked in that frame. At this point, the receiver enters hunt mode.
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