CP3SP33SMSSLASHNOPB datasheet(182/407 Pages) TI1

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Buffer 0

BUFFER_ID

Buffer 13

BUFFER_ID

GMASK1

GMASK2

Buffer 14

BUFFER_ID

BMASK1

BMASK2

DS032

CP3SP33

SNOSCW5 – MAY 2013

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22.3 Message Transfer

Each CAN module has access to 15 independent message buffers, which are memory mapped in RAM.

Each message buffer consists of 8 different 16-bit RAM locations and can be individually configured as a

receive message buffer or as a transmit message buffer.

A dedicated acceptance filtering procedure enables software to configure each buffer to receive only a

single message ID or a group of messages. One buffer uses an independent filtering procedure, which

provides the possibility to establish a BASIC-CAN path.

For reception of data frame or remote frames, a 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 alternate way to schedule a transmission is the automatic answer to remote frames. In the latter case,

the CAN module will schedule every buffer for transmission to respond to remote 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.

22.4 Acceptance Filtering

Two 32-bit masks are used to filter unwanted messages from the CAN bus: GMASK and BMASK.

Figure 22-15 shows the mask and the buffers controlled by the masks.

Figure 22-15. Acceptance Filtering

Acceptance filtering of the incoming messages for the buffers 0...13 is performed by means of a global

filtering mask (GMASK) and by the buffer ID of each buffer. Acceptance filtering 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 configured 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.

182

Dual CAN Interfaces

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