Motorola MPC8260 User Manual

21-6

MOTOROLA

Receive commands are described in Table 21-3.

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. 

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.

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.

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.