National CP3BT26 - Manual (Page 120)

www.national.com 120
CP3BT26
buffer status field. With this lock function, software has the
capability to save several messages with the same identifier
or same identifier group into more than one buffer. For ex-
ample, a buffer with the second highest priority will receive
a message if the buffer with the highest priority has already
received a message and is now locked (provided that both
buffers use the same acceptance filtering mask). As shown in Figure 51, several messages with the same ID
are received while BUFFLOCK is enabled. The filtering
mask of the buffers 0, 1, 13, and 14 is set to accept this mes-
sage. The first incoming frame will be received by buffer 0.
Because buffer 0 is now locked, the next frame will be re-
ceived by buffer 1, and so on. If all matching receive buffers
are full and locked, a further incoming message will not be
received by any buffer.
Figure 51. Message Storage with BUFFLOCK Enabled
19.5 RECEIVE STRUCTURE
All received frames are initially buffered in a hidden receive
buffer until the frame is valid. (The validation point for a re-
ceived message is the next-to-last bit of the EOF.) The re-
ceived identifier is then compared to every buffer ID
together with the respective mask and the status. As soon
as the validation point is reached, the whole contents of the
hidden buffer are copied into the matching message buffer
as shown in Figure 52.
Note: The hidden receive buffer must not be accessed by
the CPU.
Figure 52. Receive Buffer
The following section gives an overview of the reception of
the different types of frames.
The received data frame is stored in the first matching re-
ceive buffer beginning with buffer 0. For example, if the mes-
sage is accepted by buffer 5, then at the time the message
will be copied, the RX request is cleared and the CAN mod-
ule will not try to match the frame to any subsequent buffer.
10101010 01010 Received ID
GMASK
10101010 10101010
11111111 00000 00000000 00000000
11111111 00000 BMASK
BUFFER14_ID Saved when buffer
is empty
Saved when buffer
is empty
Saved when buffer
is empty
Saved when buffer
is empty
00000000 00000000
XXXXXXXX 01010 10101010 10101010
XXXXXXXX 01010 BUFFER1_ID
BUFFER13_ID
10101010 10101010
XXXXXXXX 01010 10101010 10101010
BUFFER0_ID XXXXXXXX 01010 10101010 10101010
DS035
Hidden
Receive
Buffer
CR16CAN
Buffer 0
BUFFER_ID
Buffer 13
BUFFER_ID
Buffer 14
BUFFER_ID
DS036