National CP3BT26 - Manual (Page 119)

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CP3
BT26
independent filtering procedure, which provides the possi-
bility to establish a BASIC-CAN path.
For reception of data frame or remote frames, the CAN
module follows a “receive on first match” rule which means
that a given message is only received by one buffer: the first
one which matches the received message ID.
The transmission of a frame can be initiated by software
writing to the transmit status and priority register. An alter-
nate way to schedule a transmission is the automatic an-
swer to remote frames. In the latter case, the CAN module
will schedule every buffer for transmission to respond to re-
mote frames with a given identifier if the acceptance mask
matches. This implies that a single remote frame is able to
poll multiple matching buffers configured to respond to the
triggering remote transmission request.
19.4 ACCEPTANCE FILTERING
Two 32-bit masks are used to filter unwanted messages
from the CAN bus: GMASK and BMASK. Figure 48 shows
the mask and the buffers controlled by the masks.
Figure 48. Acceptance Filtering
Acceptance filtering of the incoming messages for the buff-
ers 0...13 is performed by means of a global filtering mask
(GMASK) and by the buffer ID of each buffer. Acceptance fil-
tering of incoming messages for buffer 14 is performed by a
separate filtering mask (BMASK) and by the buffer ID of that
buffer.
Once a received object is waiting in the hidden buffer to be
copied into a buffer, the CAN module scans all buffers con-
figured as receive buffers for a matching filtering mask. The
buffers 0 to 13 are checked in ascending order beginning
with buffer 0. The contents of the hidden buffer are copied
into the first buffer with a matching filtering mask.
Bits holding a 1 in the global filtering mask (GMASK) can be
represented as a “don’t care” of the associated bit of each
buffer identifier, regardless of whether the buffer identifier bit
is 1 or 0.
This provides the capability to accept only a single ID for
each buffer or to accept a group of IDs. The following two ex-
amples illustrate the difference.
Example 1: Acceptance of a Single Identifier
If the global mask is loaded with 00h, the acceptance filter-
ing of an incoming message is only determined by the indi-
vidual buffer ID. This means that only one message ID is
accepted for each buffer.
Figure 49. Acceptance of a Single Identifier
Example 2: Reception of an Identifier Group
Set bits in the global mask register change the correspond-
ing bit status within the buffer ID to “don’t care” (X). Messag-
es which match the non-“don’t care” bits (the bits
corresponding to clear bits in the global mask register) are
accepted.
Figure 50. Acceptance of a Group of Identifiers
A separate filtering path is used for buffer 14. For this buffer,
acceptance filtering is established by the buffer ID in con-
junction with the basic filtering mask. This basic mask uses
the same method as the global mask (set bits correspond to
“don’t care” bits in the buffer ID).
Therefore, the basic mask allows a large number of infre-
quent messages to be received by this buffer.
Note: If the BMASK register is equal to the GMASK regis-
ter, the buffer 14 can be used the same way as the buffers
0 to 13.
The buffers 0 to 13 are scanned prior to buffer 14. Subse-
quently, the buffer 14 will not be checked for a matching ID
when one of the buffers 0 to 13 has already received an ob-
ject.
By setting the BUFFLOCK bit in the configuration register,
the receiving buffer is automatically locked after reception of
one valid frame. The buffer will be unlocked again after the
CPU has read the data and has written RX_READY in the
Buffer 0
BUFFER_ID
Buffer 13
BUFFER_ID
GMASK1
GMASK2
Buffer 14
BUFFER_ID
BMASK1
BMASK2
DS032
GMASK1
00000000 00000000
GMASK2
00000 00000000
BUFFER_ID1
10101010 10101010
BUFFER_ID2
10101 10101010
10101010 10101010
Accepted ID
10101 10101010
DS033
GMASK1
11111111 00000000
GMASK2
00000 00000000
BUFFER_ID1
10101010 10101010
BUFFER_ID2
10101 10101010
XXXXXXXX 10101010
Accepted ID Group
10101 10101010
DS034