The main limitation of the traditional CAN network is the restricted bandwidth, which is the maximum of 1 Mbit/s on a 40-meter CAN-bus. The CAN-Flexible Data Rate nothing but the CAN-FD protocol is a new standard developed by Bosch in the year of 2011 aiming to increase the bandwidth of the CAN bus while retaining the core characteristics of the traditional CAN bus. Then after one year of lab tests, they have released it in the year of 2012. The bandwidth restriction in CAN arises due to the arbitration feature
On a CAN bus, multiple nodes are allowed to transmit at the same time before the higher priority message wins the arbitration. Suppose the sender A does at ID 7 In order for the arbitration scheme to work for all communication on the bus. Then the signals from a node must be able to propagate through the entire length of the CAN bus and back again. In order to ensure corresponding bits are compared in the arbitration scheme, even for the nodes, which are furthest away from each other.
What Is The Need Of CAN-FD Protocol?
The automotive technologies are changing in every year as the customer expectation is increasing. If you will look onto the vehicle there are so many features are adding in automotive ECU. To prevent this they want a protocol that can send a bigger data packet in a single frame and also the data rate is 5-10 Mbps. In the current times, only the CAN protocol mostly used for ECU communication. It is nothing like there is no other protocol like FlexRay, MOST, etc. available but the CAN is better than them if you will compare them with cost and safety. Even if you also can get a better data rate but it will be high cost. There is the main reason also that there are most of the vehicles are now having CAN protocol. The OEM does not want to change it which will cost them both from customer and development. Even if the OEM will change that since there is no way of option, then what Robot BOSCH will do if everyone will remove the CAN? How they will earn the money?
Then the Robot BOSCH started to research and development of advanced CAN protocol called CAN-FD protocol. The CAN-FD protocol has a lot of advanced features over the standard classical CAN protocol. Let us discuss below the difference between the CAN protocol and CAN-FD protocol.
CAN-FD Physical Layer Design
The CAN-FD protocol physical layer design is different from standard classical CAN protocol. The CAN-FD controller is not that much change but in transceiver design, it is totally different to make the compatibility of new high bandwidth and payload support.
Difference Between The CAN and CAN-FD Protocol
| SL NO | CAN Protocol | CAN-FD Protocol |
|---|---|---|
| 1 | It has maximum of 8- bytes of data can be send in a single frame | It has maximum of 64- bytes of data can be send in a single frame |
| 2 | It has a maximum of 1Mbps data rate at 40 meters of the bus cable | It has 2-8 Mbps data rate at 40 meters of the bus cable |
| 3 | CAN supports constant bit rate. | CAN-FD has supported a dual bit rate. It has some setup to set the low data rate before the data field and high datarate when it will send the data field. |
| 4 | CAN protocol can not be used in CAN-FD network. | CAN-FD protocol can be used in CAN network with some changes. |
| 5 | CAN physical layer is low cost | CAN-FD physical layer is high cost |
| CAN supports supports lower order CRC polynomials with 15-bit only. | CAN-FD supports higher-order CRC polynomials with 15, 17, or 21-bit | |
| CAN is not having the dynamic stuff bits in CRC field | CAN-FD has extra inclusion of dynamic stuff bits in the CRC field called stuff bit counter. | |
| CAN has remote frame | CAN-FD is not having any error frame. It uses the standard classical CAN error frame whenever |
CAN-FD Message Frame Format
The CAN-FD protocol is having its own frame format. Though there is not a lot of changes, some changes are there to support the new features as well as how it can also work with standard classical CAN network. Except for the Data frame, other frames are the same so let us discuss the CAN-FD data frame. If you will look at the below figure there are some fields have different. I would like to request you that if you have any doubts about the data frame please go to my CAN protocol tutorial. Here we will only discuss the advanced fields added in CAN-FD protocol.
Extended Data Length (EDL):
