MC68302 Applications
D-8
MC68302 USER’S MANUAL
MOTOROLA
Figure D-4. Transmit and Receive BD Tables
The TBD# and RBD# are pointers used by the SCC to show which BD the SCC is currently
using (or which one it will be using next). TBD# and RBD# always sequence around the
queue of BDs in a circular fashion and are initialized to the first BD as shown.
D.3.2 MC68302 Buffer Processing
The communications processor (CP) processes the transmit BDs in a straightforward fash-
ion. Once the transmit side of an SCC is enabled, the CP starts with the first BD in that SCC's
transmit BD table, periodically checking the ready bit to see if that BD is ready. Once that
BD is ready, the CP will process that BD, reading a word at a time from its associated buffer,
doing certain required protocol processing on the data, and moving resultant data to the
SCC transmit FIFO. When the first buffer has been processed, the CP moves on to the next
BD, again waiting for that BD's ready bit to be set. Thus, the CP does no look-ahead BD
processing, nor does it skip over BDs that are not ready. When the CP sees the wrap bit set
in a BD, it goes back to the beginning of the BD table after processing of this BD is complete.
After using a BD, the CP sets its ready bit to not-ready; thus, the CP will not use a BD twice.
The BD must be confirmed by the M68000 core before being used again.
The CP uses the receive BDs in a similar fashion. Once the receive side of an SCC is en-
abled, the CP starts with the first BD in that SCC's receive BD table. Once data arrives from
the serial line into the SCC, the CP performs certain required protocol processing on the
data and moves the resultant data (either by bytes or by words, depending on the protocol)
to the buffer pointed to by the first BD. Use of a BD is complete when there is no more room
R = 0
TRANSMIT BD TABLE
TBD#
E = 1
RECEIVE BD TABLE
RBD#
R = 0
R = 0
R = 0
R = 0
R = 0
R = 0
R = 0
0
1
2
3
4
5
6
7
E = 1
E = 1
E = 1
E = 1
E = 1
E = 1
E = 1
0
1
2
3
4
5
6
7
