Datasheet

Copyright © 2013 ARM Ltd. All rights reserved
CAN: Controller Area Network Lab using ST STM32 Cortex-M processors. www.keil.com
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CAN FD: CAN with Flexible Data Rate:
CAN FD is a new extension to the standard CAN 2.0 protocol. For more information search the internet for the files
can_fd_spec.pdf and can_fd.pdf . CAN FD was created by Robert Bosch GmbH.
The CAN frequency and bit overhead added to information carrying bits (ID + data bytes) (also called payload) are
limitations to the effective maximum data rate transmission. CAN FD is one solution to this.
In a CAN system, once the frequency is chosen and implemented, it is difficult to change. Changes are easier if you know
exactly what nodes are in a system such as in a passenger vehicle. In systems where the nodes can be supplied from different
manufacturers depending on customer options or added by after-market users, the change problem is usually a problem.
This is certainly true for SAE J1939. J1939 is used in heavy duty trucks, buses, marine and in construction and farm
equipment. It uses CAN at 250 Kbps. This speed is not fast enough for larger, more bus intensive J1939 systems. CAN FD
might be a good solution as systems can slowly migrate from CAN to CAN FD.
CAN FD provides:
1. Up to 64 bytes of data bytes. Remember regular CAN has from 0 to 8 data bytes.
2. It is possible to increase the bus speed during the transmission of the DLC, data and CRC fields and before ACK bit.
This time period occurs just after arbitration is complete.
Features of CAN FD:
1. A CAN FD controller will be different than regular CAN. It can also transmit and receive regular CAN frames.
2. Uses the same physical layer as regular CAN.
3. Can use the same transceiver although specialized ones might be made available.
4. CAN FD can be used for specific applications such as programming, large data transfers or general use.
5. When CAN FD is transmitting, regular CAN must be in standby. CAN and FD collisions will result in Bus errors.
6. Regular CAN uses bits 0000 through 1000 of the 4 bits of DLC (Data Length Code). CAN FD adds 1001 through
1111 to extend the data field.
7. Reserve bit R0 (in 11 bit) or R1 (in 29 bit) are used to signify the frame is CAN FD. This is the EDL bit.
8. Three new bits are added: They are added just before the DLC field.
a. EDL: (Extended Data Length): specifies a CAN or CAN FD frame (EDL was called R0 or R1).
b. BRS: (Bit Rate Switch): switches the bit rate after arbitration and before ACK bit.
c. ESI: (Error State Indicator): denotes if the node is in error-active or error-passive mode.
Selection of Data Bytes:
The DLC has 4 bits: b0000 through b1000 are used by regular CAN and CAN FD to signify from 0 to 8 data bytes.
CAN FD uses b1001 through b1111. Each of these bits represents not one byte as in regular CAN, but map into a table. This
makes a total of up 64 data bytes. See this table to the right: With this scheme it is not possible to select some byte numbers.
The rest of the story- The Details:
There are many details covered in the Bosch and other documents. When this was
written, no CAN FD controllers are yet available but NXP might be first. For details visit
http://can-newsletter.org/ and search for FD TechDay.
Other useful network protocols:
Here are some other protocols that might prove useful to complement CAN:
FlexRay: High speed dual channel Time Triggered network. You can use one or two
channels. www.flexray.com It is used for redundancy in safety critical applications.
LIN: Single wire network using a common UART. Nearly any controller can be used to
implement LIN. LIN is a very low cost network. It is often used as a sub-network to CAN. See www.lin-subbus.org
Safe-by-Wire: A very reliable network used to activate vehicle airbags. Search the web for safebywire.pdf.
Ethernet: It is very common to use CAN for a small, local network (such as on one machine) and then use a gateway to
ethernet for the long distances or to connect to the other machines. It can handle the traffic load of multiple CAN networks.
Wireless: WiFi, ZigBee, Bluetooth and NFC (Near Field Communication) are all useful in small networks attached to CAN.
DLC Number of Data Bytes
1001 12
1010 16
1011 20
1100 24
1101 32
1110 48
1111 64