CAN Protocol | Zero to Hero Complete Course | Chapter 2 | CAN Frames Standard vs Extended & versions

Summary

The instructor introduces the main Classical CAN frame types, explains the structure and purpose of their fields, compares standard and extended identifiers, and discusses error handling, overload frames, and CAN 2.0 versions.

CAN Frame Types

Four frame types are covered:

• Data frame: Carries payload data from one node to other nodes.
• Remote frame: Requests data from a node; it contains no payload.
• Error frame: Signals a detected communication error and can interrupt the current transmission.
• Overload frame: Delays the next data or remote frame when a node needs extra time to process messages.

Data Frame Structure

A CAN data frame consists of the following fields:

• SOF (Start of Frame): One dominant bit (0) that marks the start of a frame and helps nodes synchronize.
• Arbitration field: Contains the message identifier and the RTR bit. The identifier is 11 bits in a standard frame and 29 bits in an extended frame.
• Control field: Includes the identifier-extension indicator, reserved bit(s), and DLC.
• Data field: Carries the payload. Classical CAN supports up to 8 bytes (64 bits); the DLC indicates how many bytes are used.
• CRC field: Contains a 15-bit CRC sequence and a one-bit delimiter. The receiver calculates and checks the CRC to detect corruption. The delimiter is recessive (1).
• Acknowledgement field: Contains a one-bit ACK slot followed by a one-bit delimiter. The transmitter sends the ACK slot recessive; a receiver that has accepted the frame can overwrite it with a dominant bit.
• EOF (End of Frame): Seven recessive bits.
• Interframe space: Three recessive bits that separate consecutive frames.

The instructor suggests describing a frame in an interview by identifying each field’s name, size, and value or function.

Standard and Extended Frames

Feature Standard format Extended format Identifier length 11 bits 29 bits: 11-bit base ID plus 18-bit extension Identifier-extension bit (IDE) Dominant (0) Recessive (1) Additional fields — SRR bit and an additional reserved bit RTR position After the 11-bit identifier After the full 29-bit identifier

The instructor gives the identifier capacities as 2,048 standard IDs and more than 536 million extended IDs. Extended format is useful when a network needs a larger identifier space; standard format is sufficient when fewer identifiers are needed.

Arbitration Between Formats

When a standard and extended frame share the same 11-bit base identifier and are transmitted together, the standard frame wins arbitration. The formats differ at the arbitration bits following the base identifier, allowing the bus to resolve which frame continues.

Purpose of Key Fields

• RTR (Remote Transmission Request): Distinguishes a data frame from a remote frame. It is dominant (0) in a data frame and recessive (1) in a remote frame.
• IDE: Identifies whether the frame uses the standard or extended identifier format.
• Reserved bits: Set aside for possible future use. The instructor describes them as transmitted dominant in the format covered.
• DLC (Data Length Code): Indicates the number of payload bytes, from 0 to 8 in Classical CAN.
• Data field: Holds the actual information, such as temperature, vehicle speed, or fuel level.
• CRC: Checks whether the received frame has been corrupted.
• ACK: Lets the transmitter know that at least one receiver accepted the frame.

Remote Frames

A remote frame requests data from another node:

1. The requesting node sends a remote frame with the identifier associated with the desired data.
2. It sets RTR recessive (1) and includes no data field.
3. The DLC indicates the requested data length.
4. The data source responds with a data frame using the matching identifier.

If a data frame and a remote frame with the same identifier arbitrate against each other, the data frame wins because its RTR bit is dominant.

Error Frames and Superposition Bits

A node that detects a bus error sends an error frame. The instructor distinguishes between:

• Active error frame: An active node sends six dominant bits as its error flag, followed by an eight-bit recessive delimiter.
• Passive error frame: A passive node sends a six-bit recessive error flag, followed by a recessive delimiter.

Superposition Concept

Superposition bits arise when an active node receives an error flag and also detects an error—for example, when six consecutive dominant bits violate the stuffing rule.

• The receiving active node may begin sending its own active error flag.
• If its dominant bits conflict with the first node’s recessive delimiter, the first node waits while the second node sends its flag.
• These waiting bits are called superposition bits.
• Once the flags are complete, the nodes can send the recessive delimiters.

The video walks through two cases:

1. Active error frame received by an active node: Both nodes send active error flags. One may wait for the other’s flag to finish before sending the delimiters.
2. Passive error frame received by an active node: The passive node sends recessive bits while the active node may respond with dominant bits. The passive node waits for the active flag to finish.

The instructor emphasizes that superposition depends on an active node receiving the error frame; it does not occur in the same way if all relevant nodes are passive.

Overload Frames

An overload frame gives a node extra time when it cannot process incoming messages quickly enough.

• Its format is described as similar to an active error frame: a six-dominant-bit overload flag followed by an eight-recessive-bit delimiter.
• It can delay the next data or remote frame and temporarily stop normal traffic.
• A node may send up to two consecutive overload frames.
• It may begin during the first or second bit of the preceding frame’s EOF.
• Unlike an active error frame, an overload frame does not affect the error counters.
• The video notes that overload frames are uncommon with modern CAN controllers.

CAN 2.0 Versions

The video describes three versions according to their handling of standard and extended identifiers:

• CAN 2.0A: Supports 11-bit standard frames. An extended frame is treated as a bus error.
• CAN 2.0B passive: Supports standard frames; an extended frame is tolerated or ignored rather than causing an error.
• CAN 2.0B active: Supports both standard 11-bit and extended 29-bit frames.

Speaker and Source

One instructor/presenter from the Professor Of Automotive channel explains the material using CAN bus diagrams and examples.