MC68000 MOTOROLA [Motorola, Inc], MC68000 Datasheet - Page 359

• Manufacturer Part Number: MC68000
• Manufacturer: MOTOROLA [Motorola, Inc]

in the buffer or when certain events, such as an error or an end-of-frame, are detected.
Whatever the reason, the buffer is then said to be closed, and additional data will be stored
using the next BD. Whenever the CP needs to begin using a BD because new data is arriv-
ing, it will check the empty bit of that BD. If the current BD is not empty, it will report a busy
error. However, the CP will not move from the current BD until it becomes empty. When the
CP sees the “wrap” bit set in a BD, the CP goes back to the beginning of the BD table after
use of this BD is complete. After using a BD, the CP sets its empty bit to not-empty; thus,
the CP will never use a BD twice. The BD must be processed by the M68000 core before
being used again.
D.3.3 New Pointers
To control the buffering of the SCCs, the three BD pointers to be used by software are de-
fined. Two pointers are used for the transmit BDs, and one pointer is used for the receive
BDs (see Figure D-5).
New transmit data (NTD) shows the next transmit BD that will be receiving data. This is the
first pointer to move in the transmit process.
Confirm transmit data (CTD) shows the next transmit BD that will be confirmed. To confirm
a buffer, check for errors after transmission and then mark the BD as available for USQ. This
is the last pointer to move in the transmit process.
MOTOROLA
TBD#
NTD
CTD
Figure D-5. Pointer during Execution
TRANSMIT BD TABLE
R = 0
R = 0
R = 1
R = 1
R = 0
R = 0
R = 0
R = 0
MC68302 USER’S MANUAL
5
6
7
3
4
0
1
2
IN USE BY SCC
RBD#
PRD
RECEIVE BD TABLE
E = 1
E = 1
E = 0
E = 0
E = 1
E = 1
E = 1
E = 1
5
6
7
0
2
3
4
1
MC68302 Applications
D-9