Message Buffers

25.3.1.2 Fields for Extended Format Frames

Table 25-5describes the message buffer fields used only for extended identifier format frames.

 

 

Table 25-5. Extended Format Frames

 

 

 

Field

 

Description

 

 

ID[28:18]/[17:15]

Contains the 14 most significant bits of the extended identifier, located in the ID HIGH word of the

 

message buffer.

 

 

Substitute

Contains a fixed recessive bit, used only in extended format. Should be set to one by the user for

Remote Request

Tx buffers. It will be stored as received on the CAN bus for Rx buffers. This is a bit in the ID HIGH

(SRR)

word of the message buffer.

 

 

ID Extended (IDE)

If extended format frame is used, this field should be set to one. If zero, standard format frame

 

should be used. This is a bit in the ID HIGH word of the message buffer.

 

 

ID[14:0]

Bits [14:0] of the extended identifier, located in the ID LOW word of the message buffer.

 

 

Remote

This bit is located in the least significant bit of the ID LOW word of the message buffer;

Transmission

0

Data Frame

Request (RTR)

1

Remote Frame.

 

 

 

25.3.1.3 Fields for Standard Format Frames

Table 25-6describes the message buffer fields used only for standard identifier format frames.

 

 

Table 25-6. Standard Format Frames

 

 

 

Field

 

Description

 

 

ID[28:18]

Contains bits [28:18] of the identifier, located in the ID HIGH word of the message buffer. The four

 

least significant bits in this register (corresponding to the IDE bit and ID[17:15] for an extended

 

identifier message) must all be written as logic zeros to ensure proper operation of the FlexCAN.

 

 

RTR

Remote Transmission Request. This bit is located in the ID HIGH word of the message buffer.

 

0

data frame

 

1

remote frame.

 

If the FlexCAN transmits this bit as a one and receives it as a zero, an “arbitration loss” is

 

indicated. If the FlexCAN transmits this bit as a zero and receives it as a one, a bit error is

 

indicated.

 

If the FlexCAN transmits a value and receives a matching response, a successful bit transmission

 

is indicated.

 

 

16-Bit Time Stamp

The 16-bit time stamp, located in the ID LOW word of the message buffer, is not needed for

 

standard format, and is used in a standard format buffer to store the 16-bit value of the

 

free-running timer. The timer value is captured at the beginning of the identifier field of the frame

 

on the CAN bus.

 

 

 

25.3.2 Message Buffer Memory Map

The message buffer memory map starts at an offset of 0x80 from the FlexCAN’s base address (0x1C_0000). The 256-byte message buffer space is fully used by the 16 message buffer structures.

MOTOROLA

Chapter 25. FlexCAN

25-7

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Image 533
Motorola MCF5281, MCF5282 Message Buffer Memory Map, Fields for Extended Format Frames, Fields for Standard Format Frames