National CP3BT26 manual Receive Structure, 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.

Received ID	01010	10101010	10101010	10101010
GMASK
	00000	11111111	00000000	00000000
BUFFER0_ID
	01010	XXXXXXXX	10101010	10101010	Saved when buffer
					is empty
BUFFER1_ID	01010	XXXXXXXX	10101010	10101010	Saved when buffer
BUFFER13_ID					is empty
	01010	XXXXXXXX	10101010	10101010	Saved when buffer
					is empty
BMASK	00000	11111111	00000000	00000000
BUFFER14_ID
	01010	XXXXXXXX	10101010	10101010	Saved when buffer
					is empty
					DS035

Figure 51. Message Storage with BUFFLOCK Enabled

19.5RECEIVE STRUCTURE

All received frames are initially buffered in a hidden receive buffer until the frame is valid. (The validation point for a re-

Note: The hidden receive buffer must not be accessed by the CPU.

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.

CR16CAN

Hidden

Receive

Buffer

Buffer 0

BUFFER_ID

Buffer 13

BUFFER_ID

Buffer 14

BUFFER_ID

DS036

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.

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