Communications Processor (CP)
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MC68302 USER’S MANUAL
MOTOROLA
X—External Buffer
0 = The buffer associated with this BD is in internal dual-port RAM.
1 = The buffer associated with this BD is in external memory.
W—Wrap (Final BD in Table)
0 = This is not the last BD in the Tx BD table.
1 = This is the last BD in the Tx BD table. After this buffer has been used, the CP will
transmit data from the first BD in the table.
NOTE
The user is required to set the wrap bit in one of the first eight
BDs; otherwise, errant behavior may occur.
I—Interrupt
0 = No interrupt is generated after this buffer has been serviced.
1 = The TX bit in the UART event register will be set when this buffer has been serviced
by the CP, which can cause an interrupt.
Bits 8–1—Reserved for future use.
CR—Clear-to-Send Report
This bit allows a choice of no delay between buffers transmitted in UART mode, versus a
more accurate CTS lost error reporting and two bits of idle between buffers.
0 = The buffer following this buffer will be transmitted with no delay (assuming it is
ready), but the CT bit may not be set in the correct Tx BD, or may not be set at all
in a CTS lost condition. The user is advised to monitor the CTS bit in the UART
event register for an indication of CTS lost, in addition to the CT bits in the Tx BDs.
The CTS bit will always be set properly.
1 = Normal CTS lost (CT bit) error reporting, and two bits of idle occur between back-
to-back buffers.
If the DIAG1–DIAG0 bits in the SCM are set to software operation (rather than normal op-
eration), then this bit only affects the delay between buffers, not the CTS reporting, and
would normally be set to zero.
A—Address
This bit is valid only in multidrop mode (UM0 = 1).
0 = This buffer contains data only.
1 = Set by the M68000 core, this bit indicates that this buffer contains address charac-
ter(s). All the buffer's data will be transmitted as address characters.
P—Preamble
0 = No preamble sequence is sent.
1 = The UART sends one preamble sequence (9 to 13 ones) before sending the data.
The following bits are written by the CP after it has finished transmitting the associated data
buffer.