7/14

OVERVIEW OF ENHANCED CAN CONTROLLERS FOR ST7 AND ST9 MCUS

Figure 1. Figure 1: bxCAN Block Diagram

3.3 TRANSMISSION HANDLING

Three transmit mailboxes are provided to the application to set-up messages for transmission.

Three mailboxes reduce software queuing and allow deterministic transmission. The Trans-

mission Scheduler decides which mailbox has to be transmitted first according to the priority

rules. Transmit Priority Rules BxCAN provides two modes for the transmit priority:

s

In identifier mode when more than one transmit mailbox are pending, the identifier of the

message stored in the mailbox gives the transmission order. The message with the lowest

identifier value has the highest priority according to the arbitration of the CAN protocol.

s

In FIFO mode the priority order is given by the transmit request order. This mode is well

suited for segmented transmission.

Mailbox 2

Mailbox 1

7

6

5

CAN 2.0B Active Core

Mailbox 0

Transmission

Acceptance Filters

Tx Mailboxes

Master Control

Scheduler

Master Status

Transmit Control

Transmit Status

Transmit Priority

Receive FIFO

Error Status

Error Int. Enable

Tx Error Counter

Rx Error Counter

Diagnostic

Bit Timing

Filter Mode

Filter Config.

Page Select

Interrupt Enable

Mailbox 0

1

2

Receive FIFO 1

4

3

2

1

Filter

0

Mailbox 0

1

2

Receive FIFO 0