CP3BT13 datasheet(86/232 Pages) NSC

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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 39, 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 39.

Message Storage with BUFFLOCK Enabled

16.5

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 40.

Note: The hidden receive buffer must not be accessed by

the CPU.

Figure 40.

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

11111111

00000

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

XXXXXXXX

01010

10101010

XXXXXXXX

01010

BUFFER1_ID

BUFFER13_ID

10101010

XXXXXXXX

01010

10101010

BUFFER0_ID

XXXXXXXX

01010

10101010

DS035

Hidden

Receive

Buffer

CR16CAN

Buffer 0

BUFFER_ID

Buffer 13

BUFFER_ID

Buffer 14

BUFFER_ID

DS036

CP3BT13 Datasheet(PDF) 86 Page - National Semiconductor (TI)

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