COP888EB datasheet(28/75 Pages) NSC

28 / 75 page

Functional Block Description of

the CAN Interface (Continued)

Bus synchronization of the device is this case is done in the

following way:

If the output was disabled (TxEN1, TxEN0 = “0”) and either

TxEN1 or TxEN0, or both are set to 1, the device will not start

transmission or reception of a frame until eleven consecutive

“recessive” bits have been received.

A “bus off” condition will also cause the output drivers Tx1

and Tx0 to be at TRI-STATE (independent of the status of

TxEN1 and TxEN0). The device will switch from “bus off” to

“error active” mode as described under the FMOD-bit de-

scription (see Can Bus Control register). This will ensure that

the device is synchronized to the bus, before starting to

transmit or receive.

For information on bus synchronization and status of the

CAN related registers after external reset refer to the RESET

section.

ON-CHIP VOLTAGE REFERENCE

The on-chip voltage reference is a ratiometric reference. For

electrical characteristics of the voltage reference refer to the

electrical specifications section.

ANALOG SWITCHES

Analog switches are used for selecting between Rx0 and

REF and between Rx1 and VREF.

Basic CAN Concepts

The following paragraphs provide a generic overview of the

basic concepts of the Controller Area Network (CAN) as de-

scribed in

Chapter 4 of ISO/DIS11519-1. Implementation re-

lated issues of the National Semiconductor device will be

discussed as well.

This device will process standard frame format only. Ex-

tended frame formats will be acknowledged, however the

data will be discarded. For this reason the description of

frame formats in the following section will cover only the

standard frame format.

The following section provides some more detail on how the

device will handle received extended frames:

If the device’s remote identifier acceptance filter bit (RIAF) is

set to “1”, extended frame messages will be acknowledged.

However, the data will be discarded and the device will not

reply to a remote transmission request received in extended

frame format. If the device’s RIAF bit is set to “0”, the upper

7 received ID bits of an extended frame that match the de-

vice’s receive identifier (RID) acceptance filtler bits, are

stroed in the device’s RID register. However, the device does

not reply to an RTR and any data is discarded. The device

will only acknowledge the message.

MULTI-MASTER PRIORITY BASED BUS ACCESS

The CAN protocol is message based protocol that allows a

total of 2032 (= 2

11 −16) different messages in the standard

format and 512 million (= 2

29 −16) different messages in the

extended frame format.

MULTICAST FRAME TRANSFER BY

ACCEPTANCE FILTERING

Every CAN Frame is put on the common bus. Each module

receives every frame and filters out the frames which are not

required for the module’s task.

REMOTE DATA REQUEST

A CAN master module has the ability to set a specific bit

called the “remote transmission request bit” (RTR) in a

frame. This causes another module, either another master or

a slave, to transmit a data frame after the current frame has

been completed.

SYSTEM FLEXIBILITY

Additional modules can be added to an existing network

without a configuration change. These modules can either

perform completely new functions requiring new data or pro-

cess existing data to perform a new function.

SYSTEM WIDE DATA CONSISTENCY

As the CAN network is message oriented, a message can be

used like a variable which is automatically updated by the

controlling processor. If any module cannot process informa-

tion it can send an overload frame. The device is incapable

of initiating an overload frame, but will join a overload frame

initiated by another device as required by CAN specifica-

tions.

NON-DESTRUCTIVE CONTENTION-BASED

ARBITRATION

The CAN protocol allows several transmitting modules to

start a transmission at the same time as soon as they moni-

tor the bus to be idle. During the start of transmission every

node monitors the bus line to detect whether its message is

overwritten by a message with a higher priority. As soon as a

transmitting module detects another module with a higher

priority accessing the bus, it stops transmitting its own frame

and switches to receive mode. For illustration see

Figure 18.

AUTOMATIC RETRANSMISSION OF FRAMES

If a data or remote frame is overwritten by either a higher-

prioritized data frame, remote frame or an error frame, the

transmitting module will automatically retransmit it. This de-

vice will handle the automatic retransmission of up to two

data bytes automatically. Messages with more than 2 data

bytes require the user’s software to update the transmit reg-

isters.

ERROR DETECTION AND ERROR SIGNALING

All messages on the bus are checked by each CAN node

and acknowledge if they are correct. If any node detects an

error it starts the transmission of an error frame.

Switching Off Defective Nodes

There are two error counters, one for transmitted data and

one for received data, which are incremented, depending on

the error type, as soon as an error occurs. If either counter

goes beyond a specific value the node goes to an error state.

A valid frame causes the error counters to decrease.

The device can be in one of three states with respect to error

handling:

•

Error active

An error active unit can participate in bus communication

and sends an active (“dominant”) error flag.

•

Error passive

An error passive unit can participate in bus communica-

tion. However, if the unit detects an error it is not allowed

to send an active error flag. The unit sends only a passive

(“recessive”) error flag.

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