AN1077 データシートの表示（PDF） - STMicroelectronics

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AN1077

OVERVIEW OF ENHANCED CAN CONTROLLERS FOR ST7 AND ST9 MCUS

STMicroelectronics 

OVERVIEW OF ENHANCED CAN CONTROLLERS FOR ST7 AND ST9 MCUS

3.5 RECEPTION HANDLING

Figure 3. Message Filtering and Storage

Identifier

Message Received

Ctrl

Data

Identifier

Identifier

Identifier

0

1

2 Identifier #2 Match

Receive FIFO

Message

Stored

Identifier

n

Identifier n+1

Mask

Identifier n+m

Mask

No Match

Found

Message Discarded

n: number of single identifiers to receive

m: number of identifier groups to receive

n and m values depend on the configuration of the filters

For the reception of CAN messages bxCAN provides two FIFOs. Each FIFO can store 3 com-

plete CAN messages without CPU intervention. Each filter can be independently associated

with FIFO 0 or FIFO 1.

This structure reduces the real-time constraint on message reception on the application side.

In order to reduce CPU load, simplify the software and guarantee data consistency, the FIFO

is managed completely by hardware. The application accesses the messages stored in the

FIFO through the FIFO output mailbox.

Message Prioritization

A drawback of the FIFO architecture is that at reception time the application does not know the

level of priority of these messages. Thus all messages have to be handled with the same “ur-

gency”. To address this issue bxCAN provides two independent FIFOs. This allows differenti-

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