SCC Asynchronous HDLC Mode and IrDA
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
25-7
Transmit commands are described in
Table 25-3
. 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 8 transmit clocks,
or immediately if TODR[TOD] = 1, and begins sending data if TxBD[R] is set.
Table 25-4
describes receive commands. After hardware or software is reset and a channel is enabled in
the GSMR, reception begins with the first BD in the RxBD table.
25.12 Handling Errors in the Asynchronous HDLC Controller
The asynchronous HDLC controller reports frame reception and transmission error conditions using the
channel BDs and the asynchronous HDLC event register (SCCE).
Table 25-5
describes transmit errors.
Table 25-3. Transmit Commands
Command
Description
STOP
TRANSMIT
Sends the asynchronous HDLC abort sequence (0x7D;0x7E for PPP, 0x7D; 0xC1 for IrLAP) and
disables data transmission. If the asynchronous HDLC controller receives this command during
frame transmission, the abort sequence is put in the FIFO and the transmitter does not try to
send more data from the current BD or advance to the next TxBD. The BD to be terminated is
indicated by the TBPTR entry in the parameter RAM table.
Note that unlike with other SCC protocols, the
STOP
TRANSMIT
command does not flush the FIFO.
Up to 16 characters can be sent ahead of the abort sequence unless GSMR_H[TFL] = 1.
GRACEFUL
STOP
TRANSMIT
Not supported by the asynchronous HDLC controller.
RESTART
TRANSMIT
Reenables transmission of characters; the asynchronous HDLC controller expects it after a
STOP
TRANSMIT
command or transmitter error. The controller continues sending from the first character
in the buffer using the current TxBD (pointed to by TBPTR).
INIT
TX
PARAMETERS
Initializes all Tx parameters in this channel’s parameter RAM to reset state. It must be issued
only when the transmitter is disabled. The
INIT
TX
AND
RX
PARAMETERS
command resets both Tx
and Rx parameters.
Table 25-4. Receive Commands
Command
Description
ENTER
HUNT
MODE
Forces the asynchronous HDLC controller to close the current RxBD, if it is in use, and enter
hunt mode. Reception resumes after the controller finds a frame preceded by one or more
opening flags.
CLOSE
RXBD
Not supported by the asynchronous HDLC controller.
INIT
RX
PARAMETERS
Initializes all Rx parameters in the channel’s parameter RAM to reset state. Issue only when the
receiver is disabled. The
INIT
TX
AND
RX
PARAMETERS
command resets both Tx and Rx
parameters.
Table 25-5. Transmit Errors
Error Description
CTS Lost during
Frame Transmission
The channel stops sending the buffer, closes it, sets SCCE[TXE] and TxBD[CT]. The channel
resumes sending from the next TxBD after a
RESTART
TRANSMIT
command is issued.
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