Frame Layout - Little or Big Endian

A CAN frame consists of three sections:

• Identifier
• Data
• Data length code

An identifier can be of standard or extended type or can be a remote transmission request.

To differentiate the types the following addition to the identifier is implemented:

•  bit31: reserved (always 0)
•  bit30: marks a remote transmission request (1=rtr, 0=data frame)
•  bit29: marks an extended identifier (1=extended, 0=standard)
•  bit28-0: the identifier (standard or extended)

Note: The layers dont use of bit 29 and bit 30, but this bits can be useful in callback- or hook-functions.

The functions CanFrmGet/Set support little or big endianess for bit- and byte-granularity. How bytes are ordered in both formats is well defined, therefore this chapter outlines only the formats for BIT-granularity of signals.

As an example for little endian format we have a 32 bit signal which starts at bit 29 in a 8 byte CAN msg:If we set the signal to 0x12345678 an send the CAN message with all other bits set to 0, we will receive a CAN message with 8 data bytes: 00 00 00 00 CF 8A 46 02.

If we look a the last 5 bytes in bit-format:

The red-colored bits are not part of the signal and can be dismissed.As an example for big endian format we have a 32 bit signal which also starts at bit 29 in a 8 byte CAN msg:If we set the signal to 0x12345678 an send the CAN message with all other bits set to 0, we will receive a CAN message with 8 data bytes: 00 00 00 00 48 D1 59 E0.

If we look a the last 5 byte in bit-format:

Shift the bits 2 times to the right and you will get the signal value back: