SCC Transparent Mode
MPC885 PowerQUICC Family Reference Manual, Rev. 2
28-6
Freescale Semiconductor
28.5
SCC Transparent Parameter RAM
For transparent mode, the protocol-specific area of the SCC parameter RAM is mapped as in
Table 28-2
.
CRC_P and CRC_C overlap with the CRC parameters for the HDLC-based protocols. However, this
overlap is not detrimental since the CRC constant is used only for the receiver and the CRC preset is used
only for the transmitter, so only one entry is required for each. Thus, the user can choose an HDLC
transmitter with a transparent receiver or a transparent transmitter with an HDLC receiver.
28.6
SCC Transparent Commands
The following transmit and receive commands are issued to the CP command register.
Table 28-3
describes transmit commands.
Table 28-2. SCC Transparent Parameter RAM Memory Map
Offset
1
1
From SCC base address. SCC base = IMMR + 0x3D00 (SCC2) or 0x3E00 (SCC3) or 0x3F00 (SCC4)
Name
Width
Description
0x 30
CRC_P
Long
CRC preset for totally transparent. For the 16-bit CRC-CCITT, initialize with 0x0000_FFFF.
For the 32-bit CRC-CCITT, initialize with 0xFFFF_FFFF and for the CRC-16, initialize with
ones (0x0000_FFFF) or zeros (0x0000_0000).
0x 34
CRC_C Long
CRC constant for totally transparent receiver. For the 16-bit CRC-CCITT, initialize with
0x0000_F0B8. For the 32-bit CRC-CCITT, CRC_C initialize with 0xDEBB_20E3 and for
the CRC-16, which is normally used with BISYNC, initialize with 0x0000_0000.
Table 28-3. Transmit Commands
Command
Description
STOP
TRANSMIT
After hardware or software is reset and the channel is enabled in the GSMR, the channel is in
transmit enable mode and starts polling the first BD every 64 clocks (or immediately if TODR[TOD]
= 1).
STOP
TRANSMIT
disables frame transmission on the transmit channel. If the transparent
controller receives the command during frame transmission, transmission is aborted after a
maximum of 64 additional bits and the transmit FIFO is flushed. The current TxBD pointer (TBPTR)
is not advanced, no new BD is accessed and no new buffers are sent for this channel. The
transmitter will send idles.
GRACEFUL
STOP
TRANSMIT
Stops transmission smoothly, rather than abruptly, in much the same way that the regular
STOP
TRANSMIT
command stops. It stops transmission after the current frame finishes or immediately if no
frame is being sent. A transparent frame is not complete until a BD with TxBD[L] set has its buffer
completely sent. SCCE[GRA] is set once transmission stops; transmit parameters and their BDs can
then be modified. The current TxBD pointer (TBPTR) advances to the next TxBD in the table.
Transmission resumes once TxBD[R] is set and a
RESTART
TRANSMIT
command is issued.
RESTART
TRANSMIT
Reenables transmission of characters on the transmit channel. The transparent controller expects
it after a
STOP
TRANSMIT
command is issued (at which point the channel is disabled in SCCM), after
a
GRACEFUL
STOP
TRANSMIT
command is issued, or after a transmitter error. The transparent
controller resumes transmission from the current TBPTR in the channel TxBD table.
INIT
TX
PARAMETERS
Initializes all transmit parameters in the serial channel parameter RAM to reset state. Issue only
when the transmitter is disabled.
INIT
TX
AND
RX
PARAMETERS
resets receive and transmit
parameters.
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