Motorola MPC8260 - Manual (Page 614)

21-6 MPC8260 PowerQUICC II UserÕs Manual MOTOROLA
Part IV. Communications Processor Module
Receive commands are described in Table 21-3.
21.7 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 21-4.
Reception errors are described in Table 21-5.
Table 21-3. 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 21-4. 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 SCC send and
receive FIFOs are 32 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 SCC or
20 bytes for SCC. The channel also increments the retransmission counter RETRC in the parameter
RAM.
Table 21-5. Receive Errors
Error Description
Overrun Each SCC maintains an internal FIFO for receiving data. The CP 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.