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Communication Processor Module
MOTOROLA
MPC823 USER’S MANUAL
16-233
SCC2
COMMUNICATION
16
PROCESSOR
MODULE
16.9.16.6 SCC2 HDLC COMMANDS.You can program the CPM command register
(CPCR) with the following commands to transmit data.
STOP TRANSMIT—After the hardware or software is reset and the channel is enabled
in the PSMR–SCC2 HDLC register, the channel is in transmit enable mode and starts
polling the first buffer descriptor in the table every 64 transmit clocks or immediately if
the TOD bit is set in the transmit-on-demand register (TODR). The channel STOP
TRANSMIT command disables the transmission of frames on the transmit channel. If
the SCC2 HDLC controller receives this command while a frame is transmitting,
transmission is aborted after a maximum of 64 additional bits and the transmit FIFO is
flushed. The TBPTR is not advanced, no new buffer descriptor is accessed, and no new
frames are transmitted for this channel. The transmitter transmits an abort sequence
consisting of 01111111 (if the command was given during frame transmission) and
begins transmitting flags or idles, as indicated by the PSMR–SCC2 HDLC register.
GRACEFUL STOP TRANSMIT—This command is used to stop transmission
smoothly, instead of abruptly, like the regular STOP TRANSMIT command. It stops
transmission after the current frame is finished or immediately if there is no frame being
transmitted. The GRA bit in the SCCE–HDLC is set once transmission has stopped.
Then the HDLC transmit parameters and their buffer descriptors can be modified. The
TBPTR points to the next TX buffer descriptor in the table. Transmission begins once
the R bit of the next buffer descriptor is set and the RESTART TRANSMIT command
is issued.
RESTART TRANSMIT—This command enables characters to be transmitted on the
transmit channel. The SCC2 HDLC controller expects this command after a STOP
TRANSMIT command is issued or after a GRACEFUL STOP TRANSMIT command is
issued or a transmitter error occurs. The SCC2 HDLC controller resumes transmission
from the current TBPTR in the channel TX buffer descriptor table.
INIT TX PARAMETERS—This command initializes all transmit parameters in this serial
channel parameter RAM to their reset state and should only be issued when the
transmitter is disabled. The INIT TX AND RX PARAMS command can be used to reset
the transmit and receive parameters.
Note: If the MFF bit in the PSMR–SCC2 UART is set, one or more small frames can
be flushed from the transmit FIFO. Issue the GRACEFUL STOP TRANSMIT
command to stop this from occurring.