Freescale Semiconductor PowerQUICC MPC870, Справочное руководство

Страница: 702 / 1426

SCC HDLC Mode
MPC885 PowerQUICC Family Reference Manual, Rev. 2
23-6 Freescale Semiconductor
23.7 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 23-4 .
Reception errors are described in Table 23-5 .
Table 23-4. 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
transmit and receive 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
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 20 bytes for
SCC2–SCC4. The channel also increments the retransmission counter RETRC in the parameter
RAM.
Table 23-5. 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 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.
Abort Sequence Occurs when seven or more consecutive ones are received. When this occurs while receiving a
frame, the channel closes the buffer, sets RxBD[AB] and generates a maskable RXF interrupt. The
channel also increments the abort sequence counter ABTSC. The CRC and nonoctet error status
conditions are not checked on aborted frames. The receiver then enters hunt mode.