CP3SP33SMSSLASHNOPB datasheet(174/407 Pages) TI1

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ERROR FRAME

ERROR

FLAG

< 6

ECHO

ERROR FLAG

ERROR

DELIMITER

DATA FRAME OR

REMOVE FRAME

An error frame can start anywhere within a frame

INTER-FRAME OR

OVERLOAD FRAME

Note:

d = dominant

r = recessive

DS024

CP3SP33

SNOSCW5 – MAY 2013

www.ti.com

22.2.3.3 Error Frame

As shown in Figure 22-7, the Error Frame consists of the error flag and the error delimiter bit fields. The

error flag field is built up from the various error flags of the different nodes. Therefore, its length may vary

from a minimum of six bits up to a maximum of twelve bits depending on when a module has detected the

error. Whenever a bit error, stuff error, form error, or acknowledgment error is detected by a node, the

node starts transmission of an error flag at the next bit. If a CRC error is detected, transmission of the

error flag starts at the bit following the acknowledge delimiter, unless an error flag for a previous error

condition has already been started.

If a device is in the error active state, it can send a “dominant” error flag, while a error passive device is

only allowed to transmit “recessive” error flags. This is done to prevent the CAN bus from getting stuck

due to a local defect. For the various CAN device states, refer to Section 22.2.4.

Figure 22-7. Error Frame

22.2.3.4 Overload Frame

As shown in Figure 22-8, an overload frame consists of the overload flag and the overload delimiter bit

fields. The bit fields have the same length as the error frame field: six bits for the overload flag and eight

bits for the delimiter. The overload frame can only be sent after the end of frame (EOF) field and in this

way destroys the fixed form of the intermission field. As a result, all other nodes also detect an overload

condition and start the transmission of an overload flag. After an overload flag has been transmitted, the

overload frame is closed by the overload delimiter.

NOTE

A CAN module never initiates an overload frame due to its inability to process an incoming

message. However, it is able to recognize and respond to overload frames initiated by other

devices.

174

Dual CAN Interfaces

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