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Communications Processor (CP)
4.5.6.2 Maximum Receive Buffer Length Register (MRBLR)
Each SCC has one MRBLR that is used to define the receive buffer length for that SCC. The
MRBLR defines the maximum number of bytes that the IMP will write to a receive buffer on
that SCC before moving to the next buffer. The IMP may write fewer bytes to the buffer than
MRBLR if a condition such as an error or end of frame occurs, but it will never write more
bytes than the MRBLR value. Thus, buffers supplied by the user for use by the IMP should
always be of size MRBLR (or greater) in length.
The transmit buffers for an SCC are not affected in any way by the value programmed into
MRBLR. Transmit buffers may be individually chosen to have varying lengths, as needed.
The number of bytes to be transmitted is chosen by programming the data length field in the
Tx BD.
MRBLR was not intended to be changed dynamically while an
SCC is operating. However, if it is modified in a single bus cycle
with one 16-bit move (NOT two 8-bit back-to-back bus cycles),
then a dynamic change in receive buffer length can be success-
fully achieved, which occurs when the CP moves control to the
next Rx BD in the table. Thus, a change to MRBLR will not have
an immediate effect. To guarantee the exact Rx BD on which the
change will occur, the user should change MRBLR only while
the SCC receiver is disabled (see 4.5.6 SCC Parameter RAM
Memory Map).
The MRBLR value should be greater than zero in all modes. In
the HDLC and transparent modes, the MRBLR should have an
even value.
4.5.6.3 Receiver Buffer Descriptor Number (RBD#)
The RBD# for each SCC channel defines the next BD to which the receiver will move data
when it is in the IDLE state or defines the current BD during frame processing. The RBD# is
the BD offset from the SCC base in the Rx BD table. For Rx BD 0, RBD# = $00; for Rx BD
1, RBD# = $08, etc. Upon reset, the CP main controller sets this register to zero. The user
can change this register only after the ENR bit is clear and after the ENTER HUNT MODE
command has been issued. In most applications, this parameter will never need to be mod-
ified by the user.
4.5.6.4 Transmit Buffer Descriptor Number (TBD#)
The TBD# for each SCC channel defines the next BD from which the transmitter will move
data when it is in the IDLE state or defines the current BD during frame transmission. The
TBD# is the BD offset from the SCC base in the Tx BD table. For Tx BD 0, TBD# = $40; for
Tx BD 1, TBD# = $48, etc. Upon reset, the CP main controller sets this register to $40. The
user can change this register only after the STOP TRANSMIT command has been issued.
In most applications, this parameter will never need to be modified by the user.
4-36
NOTE
NOTE
MC68302 USER'S MANUAL
MOTOROLA
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