70353C datasheet(34/76 Pages) MICROCHIP

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dsPIC33E/PIC24E Family Reference Manual

DS70353C-page 21-34

21.6

TRANSMITTING ECAN MESSAGES

A node originating a message is a transmitter of that message. The node remains a transmitter

until the bus becomes Idle or the unit loses arbitration. Figure 21-8 illustrates a typical ECAN

transmission process.

Message Buffers 0-7 (located in device RAM) are configured to transmit or receive CAN

messages using the TX/RX Buffer Selection bit (TXENn) in the corresponding ECAN TX/RX

Buffer m Control register (CiTRmnCON<7>). If the TXENn bit is set, the message buffer is

configured for transmission. For the layout of standard and extended frames in the message

buffer, and the states of IDE, SRR, RTR, RB0 and RB1 bits for Standard Data, Extended Data,

Standard Remote, or Extended Remote frames as per the CAN Specification, refer to 21.2 “CAN

Message Formats”.

21.6.1

Message Transmission Flow

To transmit a message over the CAN bus, the user application must perform these tasks:

Configure a message buffer for transmission and assign a priority to the buffer.

Write the CAN message in the message buffer located in device RAM.

Set the transmit request bit for the buffer to initiate message transmission.

The Message transmission is initiated by setting the Message Send Request bit (TXREQm) in

the ECAN Transmit/Receive Control register (CiTRmnCON<3>). The TXREQm bit is cleared

automatically after the message is transmitted. Before the SOF is sent, all the buffers ready for

transmission are examined to determine which buffer has the highest priority. The transmit buffer

with the highest priority is sent first.

Each of the transmit message buffers can be assigned to any of the four user application-defined

priority levels using the TXnPRI<1:0> bits (CiTRmnCON<1:0>).

TXnPRI<1:0> Message Transmission Priority selections are:

• 11 = The transmit message has the highest priority

• 10 = The transmit message has intermediate high priority

• 01 = The transmit message has intermediate low priority

• 00 = The transmit message has the lowest priority

There is a natural order of priority for message buffers that are assigned to the same user

application-defined priority level. The message buffer seven has the highest natural order of

priority. The user application-defined priority levels override the natural order of priority.

Figure 21-8:

ECAN™ Transmission

A code example to transmit a standard frame using message buffer 0 is shown in Example 21-1.

Note:

Setting the TXREQm bit when TXENn bit is ‘0’ will result in unpredictable module

behavior.

Word 0

SRR IDE

Word 1

Word 2

EID<5:0>

DLC<3:0>

RTR RB1

SID<10:0>

EID<17:6>

FILHIT<4:0>

Data Byte 1

Data Byte 3

Data Byte 5

Data Byte 7

Data Byte 0

Data Byte 2

Data Byte 4

Data Byte 6

Word 7

SID SRR EID RTR

SID RTR DLC

DLC

Message Buffer 0

Message Buffer 1

Message Buffer 2

Message Buffer 3

(TX)

Message Buffer 4

Message Buffer 5

Message Buffer 6

Message Buffer 7

Extended

Standard

ECAN™ Data Frames

Word 3

Word 4

Word 5

Word 6

RB0

Device RAM

Transmit Message

Identifier

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