RM0319 datasheet(201/368 Pages) STMICROELECTRONICS

201 / 368 page

RM0319

Controller area network ports (CAN)

Doc ID 022640 Rev 3

201/368

Figure 72.

Data frame format

The configuration of the message objects in the message RAM (with the exception of the

bits MsgVal, NewDat, IntPnd and TxRqst) will not be affected by resetting the chip. All the

message objects must be initialized by the CPU or they must be “not valid” (MsgVal =’0’),

and the bit timing must be configured before the CPU clears the Init bit in the CAN Control

The configuration of a message object is done by programming the mask, arbitration,

control and data fields of one of the two interface register sets to the desired values. By

writing to the corresponding IFx Command Request register, the IFx Message Buffer

registers are loaded into the addressed message object in the message RAM.

When the Init bit in the CAN Control register is cleared, the CAN Protocol Controller state

machine of the CAN Core and the message handler state machine control the CAN internal

data flow. Received messages that pass the acceptance filtering are stored into the

Message RAM, messages with pending transmission request are loaded into the CAN core

Shift register and are transmitted via the CAN bus.

The CPU reads received messages and updates messages to be transmitted via the IFx

Interface registers. Depending on the configuration, the CPU is interrupted on certain CAN

message and CAN error events.

15.4.9

Configuring a transmit object

Table 59 shows how a Transmit Object should be initialized.

The Arbitration registers (ID28-0 and Xtd bit) are given by the application. They define the

identifier and type of the outgoing message. If an 11-bit Identifier (“Standard Frame”) is

used, it is programmed to ID28-ID18. ID17-ID0 can then be disregarded.

If the TxIE bit is set, the IntPnd bit will be set after a successful transmission of the message

object.

Bus

idle

Message

identifier

Start of frame

Arbitration field

Control

field

CRC

seq.

Data field

EOF

Bus

idle

Remote

transmission

request

Delimiter bits

Intermission field

CRC sequence

111

0…..64

CRC field

Bit stuffing

End of

frame field

Acknowledgement

field

CAN data frame

Table 59.

Initialization of a transmit object

MsgVal

Arb

Data

Mask

EoB

Dir

NewDat

MsgLst

RxIE

TxIE

IntPnd

RmtEn

TxRqst

appl.

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