
21-6
MPC8260 PowerQUICC II User’s 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 first 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 flag 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 first 
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 first 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 FIFO’s 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.