MC912DG128A Motorola, MC912DG128A Datasheet - Page 272

• Manufacturer Part Number: MC912DG128A
• Description: Microcontrollers
• Manufacturer: Motorola

Receive Structures
MSCAN Controller
MC68HC912DT128A Rev 2.0
272
This loading process lasts a definite amount of time and has to be
completed within the inter-frame sequence (IFS) in order to be able to
send an uninterrupted stream of messages. Even if this is feasible for
limited CAN bus speeds it requires that the CPU reacts with short
latencies to the transmit interrupt.
A double buffer scheme would de-couple the re-loading of the transmit
buffers from the actual message sending and as such reduces the
reactiveness requirements on the CPU. Problems may arise if the
sending of a message would be finished just while the CPU re-loads the
second buffer, no buffer would then be ready for transmission and the
bus would be released.
At least three transmit buffers are required to meet the first of above
requirements under all circumstances. The msCAN12 has three transmit
buffers.
The second requirement calls for some sort of internal prioritizing which
the msCAN12 implements with the local priority concept described
below.
The received messages are stored in a two stage input FIFO. The two
message buffers are mapped using a ping-pong arrangement into a
single memory area (see
buffer (RxBG) is exclusively associated to the msCAN12, the foreground
receive buffer (RxFG) is addressed by the CPU12. This scheme
simplifies the handler software as only one address area is applicable for
the receive process.
Both buffers have a size of 13 bytes to store the CAN control bits, the
identifier (standard or extended) and the data contents (for details see
Programmer’s Model of Message
The receiver full flag (RXF) in the msCAN12 receiver flag register
(CRFLG) (see
status of the foreground receive buffer. When the buffer contains a
correctly received message with matching identifier this flag is set.
After the msCAN12 successfully received a message into the
background buffer and if the message passes the filter, it copies the
msCAN12 Receiver Flag Register
MSCAN Controller
Figure
42). While the background receive
Storage).
(CRFLG)) signals the
MOTOROLA
4-mscan12