Civil Engineering Reference
In-Depth Information
Fig. 1.19
CAN FD data frame format with 11-bit identifier and with 29-bit identifier
1.4.3
CAN FD Frame Format
The
Control Field
in standard CAN frames contains reserved bits which are speci-
fied to be transmitted
dominant
. In a CAN FD frame, the reserved bit after the
IDE
bit (11-bit Identifier) or after the
RTR
bit (29-bit Identifier) is redefined as
Extended
Data Length
(
EDL
) bit and is transmitted
recessive
. This sets the receiving
BSP
and
BTL
FSMs - Finite State Machines - into a CAN FD decoding mode.
The following bits are new in CAN FD:
•
EDL Extended Data Length
•
r1, r0
reserved, transmitted
dominant
•
BRS Bit Rate Switch
•
ESI Error State Indicator
The
DLC
values from 0000b to 1000b still code a
Data Field
length from 0 to
8 bytes, while the (in standard CAN redundant)
DLC
values from 1001b to 1111b
are redefined in CAN FD to code
Data Fields
with a length of up to 64 bytes:
DLC
1001
1010
1011
1100
1101
1110
1111
Byte
12
16
20
24
32
48
64
EDL distinguishes between the standard CAN frame format and the CAN FD frame
format.
The value of
BRS
decides whether the bit rate in the
Data-Phase
is the same as
in the
Arbitration-Phase
(
BRS dominant
) or whether the predefined faster bit rate is
used in the
Data-Phase
(
BRS recessive
).
In CAN FD frames,
EDL
is always
recessive
and followed by the
dominant
bit,
r0
. This provides an edge for resynchronization before an optional bit rate switch.
The edge is also used to measure the transceiver's loop delay for the optional
Trans-
ceiver Delay Compensation
.
