NI CAN arbitration id break-down

[YogeeshBK]

What is the break-down of arbitration ID for extended message?

Is it:

| x | x | 1 | 11-bits standard | 18-bits extended |

             ^                ^ ^                ^

             msb            lsb msb            lsb

or

| x | x | 1 | 18-bits extended | 11-bits standard |

             ^                ^ ^                ^

             msb            lsb msb            lsb

I couldn't find this information. I need to map CAN message structure in Freescale implementation which is as follows (32-bit):

| 11-bits standard | SRR | IDE | 18-bits extended | RTR |

 ^                ^             ^                ^

 msb            lsb             msb            lsb

where SRR => Substitute Remote Request bit (remote request for standard ID)

      IDE => Extended ID bit. 1 => extended id, 0 => standard id

      RTR => Remote Request for extended ID

Please advice.

[YogeeshBK]

The question itself was incorrect. I figured it out. There is nothing like:

| x | x | 1 | 11-bits standard | 18-bits extended |

             ^                ^ ^                ^

             msb            lsb msb            lsb

or

| x | x | 1 | 18-bits extended | 11-bits standard |

             ^                ^ ^                ^

             msb            lsb msb            lsb

it is just as follows:

for extended:

| x | x | 1 | 29-bits extended |

             ^                ^ 

             msb            lsb

for standard:

| x | x | 0 | <18-bits padded with zeroes> | 11-bits standard |

                                            ^                ^ 

                                            msb            lsb

The confusion came out from tx/rx buffer description in freescale MSCAN.

[GPIB_Guru · Accepted solution]

The CAN bus doesn't send 18 0-byte padded bytes for a standard ID.

Most hardware has a 32-bit register for the ID.

For a 29-bit ID, you fill up 28:0. For an 11-bit ID, you fill up 28:18 with the ID.

One of the upper bits 31, 30, or 29 indicates whether the ID is standard or extended for the sake of the transmission.

[YogeeshBK]

Thanks a lot. In our project, we use 29-bits (extended ID).

So, I guessed, the upper 18 bits would be padded to keep bit numbering to start from bit 0 for both standard and extended.