signal Byteorder

Overview Motorola vs. Intel-Format in Can-Frame

Some explanations of CAN byte order and bit numbering:

openxcplatform-bitnumbering

Mathworks-Side

CAN frame data is either in big endian or little endian byte format.

The bits are numbered by the file format either using MSB first (MSB0) or LSB first (LSB0).

With "consistent" format/numbering, bit numbering follows the byte format. MSB0 for big endian, LSB0 for little endian.

ietf-endianess

Usually the start bit referenced by the file formats reference the start of the signal data within the message independent of the endianess of the signal. With some formats (DBF), the startbit may always reference the LSB of the signal data.

Common formats:

KCD, SYM (consistent bit numbering)

• little,LSB0
• big,MSB0

DBC, ARXML (OSEK bit numbering)

• little,LSB0
• big,LSB0 (inconsistent)

DBF (startBit=LSB, startByte and startBit specified separately)

• little,LSB0
• big,LSB0,startBit=LSB (inconsistent)

Legacy canmatrix (yaml, json) (Note: internally canmatrix now uses consistent bit numbering)

• little,LSB0
• big,LSB0,startBit=LSB (inconsistent)

###First Example:

Frame with two 32-bit signals in motorola one can-frame

 7 6 5 4 3 2 1 0 
0>msb-----
1  signal1
2
3       ----lsb>
4>msb-----
5  signal2
6
7       ----lsb>

DBC:

BO_ 291 newFrameMotorola: 8 Vector__XXX
 SG_ signal1 : 7|32@0- (1,0) [0|0] "" Vector__XXX  (most significant bit)
 SG_ signal2 : 39|32@0- (1,0) [0|0] "" Vector__XXX (most significant bit)

candb++ views:

signal1: 24 (least significant bit?)
signal2: 56 (least significant bit?)

Dbf:

byte 4 bit 0
byte 8 bit 0

###Second Example:

 7 6 5 4 3 2 1 0 
0        >msb---
1---------lsb>
2
3       
4
5
6
7

DBC:

startbit 3
length 11

candb++ views:

Startbit 9 (least significant bit?)

dbf:

length 11
byte 2 (counting starts with byte 1)
bit 1

##found docs about kcd "Least significant bit offset of the signal relative to the least significant bit of the messages data payload." least significant bit?

estimation about dbf

least significant bit?

new formulars from dmahurin

Note the basic operations used.

1. convert from lsb0 bit numbering to msb0 bit numbering (or msb0 to lsb0)

b = b - (b % 8) + 7 - (b % 8)

2. convert from lsbit of signal data to msbit of signal data, when bit numbering is msb0

b = b + 1 - length

3. convert from msbit of signal data to lsbit of signal data, when bit numbering is msb0

b = b + length - 1

So conversion from msbit in lsb0 bit numbering to msbit in lsb0 bit numbering is:

b = b - (b % 8) + 7 - (b % 8)

b = b + length - 1

b = b - (b % 8) + 7 - (b % 8)

byte order names

little endian == Intel == MOST-SIGNIFICANT-BYTE-LAST

big endian == Motorola == MOST-SIGNIFICANT-BYTE-FIRST