National CP3BT26 manual Acceptance Filtering, Example 1 Acceptance of a Single Identifier

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.

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.

GMASK1		GMASK2
00000000	00000000	00000000	00000
BUFFER_ID1		BUFFER_ID2
10101010	10101010	10101010	10101

CP3BT26

19.4	ACCEPTANCE FILTERING		Accepted ID
Two 32-bit masks are used to filter unwanted messages		10101010	10101010	10101010	10101
from the CAN bus: GMASK and BMASK. Figure 48 shows					DS033
the mask and the buffers controlled by the masks.

Buffer 0

BUFFER_ID

GMASK1

GMASK2

Buffer 13

BUFFER_ID

Buffer 14

BMASK1

BUFFER_ID

BMASK2

DS032

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.

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.

GMASK1		GMASK2
00000000	11111111	00000000	00000
BUFFER_ID1		BUFFER_ID2
10101010	10101010	10101010	10101
	Accepted ID Group
10101010	XXXXXXXX	10101010	10101

DS034

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

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