Can Message Identifiers - Freescale Semiconductor DSP56800E User Manual

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For more information about the bit timing requirements, see the CAN 2AB standard document or

Freescale Application Note number AN1798 (order number AN1798/D).

CAN Message Identifiers

CAN Standard 2.0 A/B defines two kinds of CAN message identifiers. Part A of the standard

defines "standard" 11 bit message identifier and part B defines "extended" 29 bit identifier. The

CAN message header contains control bit named "ID Extended" (IDE) which, when set, identifies

message with 29 bit extended identifier. In Part A-message format the IDE bit is reserved for

future use ("r1") and has always a value of zero.

The Remote Transmit Request (RTR) bit, which follows the 11 bit identifier in the standard ID

messages is replaced with "dummy" Substitute Remote Request (SRR) bit in extended 29 bit ID

messages. The SRR is always transmitted as one. For extended ID messages, 18 ID bits added and

a new RTR bit are appended after IDE bit.

There are several MSCAN registers where the message ID is to be specified. The examples are

the Receive Message Buffer ID Registers, Transmit Buffer ID Registers or ID Mask and

Acceptance Registers. Various registers containing the message ID have always the same format,

which reflects the physical bit sequence as captured-from or generated-onto the CAN bus. A care

must be taken when a numerical ID is to be supplied to registers or if a register-contained value is

to be understood as a numerical CAN message identifier. The MSCAN low-level driver as well as

this document use the term "ID-raw" (IDR) whenever the register-contained value is referenced.

The ID-raw is 32-bit value and consists of four bytes which must be treated differently when its

IDE bit (bit 19) is 0 (standard ID) or 1 (extended ID).

FREESCALE SEMICONDUCTOR

Targeting 56F8xxx Platform

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