16-03-2010, 03:38 PM
[attachment=2676]
implimentation of can using vhdl
Presented by:
N.D Ramesh
Mohit Gupta
Arun Batra
Introduction to CAN
CAN is a multi-master broadcast serial bus standard for connecting electronic control units (ECUs).
Each node is able to send and receive messages, but not simultaneously.
The devices that are connected by a CAN network are typically sensors, actuators and control devices. A CAN message never reaches these devices directly, but instead a host-processor and a CAN Controller is needed between these devices and the bus.
We will talk about¦
Introduction to CAN
General Features
CAN messages
Bitwise arbitration
Error Detection and Processing
CAN synchronization
Each node requires¦
Host-processor
The host-processor decides what received messages mean, and which messages it wants to transmit itself.
CAN Controller (hardware with a synchronous clock)
Receiving: the CAN Controller stores received bits from the bus until an entire message is available, that can then be fetched by the host.
Sending: the host-processor stores its transmit-messages into a CAN Controller, which transmits the bits serially onto the bus.
Transceiver (possibly integrated into the CAN Controller)
Receiving: it adapts signal levels from the bus, to levels that the CAN Controller expects and has protective circuitry that protect the CAN Controller.
Sending: it converts the transmit-bit signal received from the CAN Controller into a signal that is sent onto the bus.
General Features
General Features
Extensive ERROR checking
Five different checks
Every connected node participates
Data consistency secured
A message is accepted by all nodes or none
Types of Frame
Data Frame
Remote Frame
Error Frame
Overload Frame
Interframe
CAN Messages
A CAN frame
Priority and/or Identification
Control and Data length field
Control Field
Main function Data Length Code DLC
DLC can have the value 0 to 8
Two bits are reserved and used to indicate Extended frames
In Standard frames the reserved bits are fix dominant bits
Data field
CRC field
Acknowledgement field
Fixed value bits
CAN Remote Frame Std
CAN Data Frame Extended
Bit-wise Arbitration
Error Detection and Processing
All microcontrollers conforming in the CAN protocol must have two separate internal counters, the Ëœtransmit error counterâ„¢ and the Ëœreceive error counterâ„¢, with the task of counting the errors occurring in transmission and reception, respectively.
If the message is correctly transmitted or received, the counter in question is decremented and if the message contains errors, the counter in question is incremented.
Error counters do not non use proportional counting methods.
Error Detection and Processing
There are two kinds of error flags:
Active error flags,
Passive error flags.
Adding up the Score¦
Types of Errors
A node can identify five types of errors for transmitted or received frames. These errors are defined as
Form error,
Stuff Error,
CRC Error,
Bit Error and
Acknowledgment Error.
A Transmitter detects Bit error and Acknowledgment Error, while a Receiving Node detects Form Error, Stuff Error and CRC Error.
Bit Error
A transmitter monitors the bus while transmitting the frame. The purpose is to stop transmitting the message if the transmitting node loses the arbitration.
If the transmitter transmits a recessive bit and receives a dominant bit or vice-versa after the RTR bit it identifies the bit error and generates an error frame immediately after the bit with the error.
Acknowledgment Error
When a transmitter sends a frame on the CAN bus all receivers who receive the frame correctly send a dominant
Acknowledgment bit after the CRC delimiter bit. If the transmitter does not see an Acknowledgment bit it sends an error frame following the Acknowledgment bit position.
Form Error
The CRC Delimiter, Ack Delimiter, End of Frame, Intermission Frame and Overload Frames have a fixed format.
When a receiver receives a bit that is not recessive it generates an error frame immediately following the bit with error.
CRC Error
Each CAN frame includes a 15 bit CRC at the end. These CRC bits are calculated for bits beginning at Start of Frame bit to last bit of the Data Frame.
If any errors are found in the calculated CRC of the received frame an Active Error or Passive Error Frame is generated immediately after the Ack Delimiter Bit.
Stuff Error
The part of the CAN frame between the Start of Frame (SOF) and the CRC delimiter is covered by bit stuffing rules. The CAN bus does not have synchronization information. The nodes synchronize themselves by changes in the bit levels. Therefore, the CAN frame follows the NRZ-5 coding rule.
If the sixth bit is of the same level as the preceding five bits, the receiving controller generates the error frame immediately after the sixth bit of the same level.
BIT Synchronization
BIT TIMING
A bit time is built up by four parts:
Synch_Seg, Prop_Seg, Phase_Seg1 and Phase_Seg2
Those parts are built up by a number of time cycles
BIT TIME
Stuffing
