SCC HDLC Mode
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
23-5
23.5
Programming the SCC HDLC Controller
HDLC mode is selected for an SCC by writing GSMR_L[MODE] = 0b0000. The HDLC controller uses
the same buffer and BD data structure as other modes and supports multi buffer operation and address
comparisons. Receive errors are reported through the RxBD; transmit errors are reported through the
TxBD.
23.6
SCC HDLC Commands
The transmit and receive commands are issued to the CPM command register (CPCR). Transmit
commands are described in
Table 23-2
.
Receive commands are described in
Table 23-3
.
Table 23-2. Transmit Commands
Command
Description
STOP
TRANSMIT
After a hardware or software reset and a channel is enabled in the GSMR, the transmitter starts
polling the first BD in the TxBD table every 64 Tx clocks, or immediately if TODR[TOD] = 1, and
begins sending data if TxBD[R] is set. If the SCC receives the
STOP
TRANSMIT
command while not
transmitting, the transmitter stops polling the BDs. If the SCC receives the command during
transmission, transmission is aborted after a maximum of 64 additional bits, the Tx FIFO is flushed,
and the current BD pointer TBPTR is not advanced (no new BD is accessed). The transmitter then
sends an abort sequence (0x7F) and stops polling the BDs.
When not transmitting, the channel sends flags or idles as programmed in the GSMR.
Note that if PSMR[MFF] = 1, multiple small frames could be flushed from the Tx FIFO; a
GRACEFUL
STOP
TRANSMIT
command prevents this.
GRACEFUL
STOP
TRANSMIT
Stops transmission smoothly. Unlike a
STOP
TRANSMIT
command, it stops transmission after the
current frame is finished or immediately if no frame is being sent. SCCE[GRA] is set when
transmission stops. HDLC Tx parameters and Tx BDs can then be updated. TBPTR points to the
next TxBD. Transmission begins once TxBD[R] of the next BD is set and a
RESTART
TRANSMIT
command is issued.
RESTART
TRANSMIT
Enables frames to be sent on the transmit channel. The HDLC controller expects this command after
a
STOP
TRANSMIT
is issued and the channel in its GSMR is disabled, after a
GRACEFUL
STOP
TRANSMIT
command, or after a transmitter error. The transmitter resumes from the current BD.
INIT
TX
PARAMETERS
Resets the Tx parameters in the parameter RAM. Issue only when the transmitter is disabled.
INIT
TX
AND
RX
PARAMETERS
resets both Tx and Rx parameters.
Table 23-3. 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.
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