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Sheet目录1917

35页 36页 37页 38页 [39页]40页 41页 42页 43页

2010 Microchip Technology Inc.

DS70135G-page 133

dsPIC30F4011/4012

19.4.6.3

Receive Error Interrupts

A receive error interrupt will be indicated by the ERRIF

bit. This bit shows that an error condition occurred. The

source of the error can be determined by checking the

bits in the CAN Interrupt Status register, C1INTF.

Invalid message received.

If any type of error occurred during reception of

the last message, an error will be indicated by the

IVRIF bit.

Receiver overrun.

The RXxOVR bit indicates that an overrun

condition occurred.

Receiver warning.

The RXWAR bit indicates that the Receive Error

Counter (RERRCNT<7:0>) has reached the

warning limit of 96.

Receiver error passive.

The RXEP bit indicates that the Receive Error

Counter has exceeded the error passive limit

of 127 and the module has gone into error passive

state.

19.5

Message Transmission

19.5.1

TRANSMIT BUFFERS

The CAN module has three transmit buffers. Each of

the three buffers occupies 14 bytes of data. Eight of the

bytes are the maximum 8 bytes of the transmitted mes-

sage. Five bytes hold the standard and extended

identifiers and other message arbitration information.

19.5.2

TRANSMIT MESSAGE PRIORITY

Transmit priority is a prioritization within each node of the

pending transmittable messages. There are 4 levels of

transmit priority. If TXPRI<1:0> (C1TXxCON<1:0>, where

x = 0, 1 or 2, represents a particular transmit buffer) for a

particular message buffer is set to ‘11’, that buffer has the

highest priority. If TXPRI<1:0> for a particular message

buffer is set to ‘10’ or ‘01’, that buffer has an intermediate

priority. If TXPRI<1:0> for a particular message buffer is

‘00’, that buffer has the lowest priority.

19.5.3

TRANSMISSION SEQUENCE

To initiate transmission of the message, the TXREQ bit

(C1TXxCON<3>) must be set. The CAN bus module

resolves any timing conflicts between setting of the

TXREQ bit and the Start-of-Frame (SOF), ensuring

that if the priority was changed, it is resolved correctly

before the SOF occurs. When TXREQ is set, the

TXABT (C1TXxCON<6>), TXLARB (C1TXxCON<5>)

and

TXERR

(C1TXxCON<4>)

flag

bits

are

automatically cleared.

Setting TXREQ bit simply flags a message buffer as

enqueued for transmission. When the module detects

an available bus, it begins transmitting the message

which has been determined to have the highest priority.

If the transmission completes successfully on the first

attempt, the TXREQ bit is cleared automatically and an

interrupt is generated if TXxIE was set.

If the message transmission fails, one of the error

condition flags will be set and the TXREQ bit will

remain set, indicating that the message is still pending

for transmission. If the message encountered an error

condition during the transmission attempt, the TXERR

bit will be set and the error condition may cause an

interrupt. If the message loses arbitration during the

transmission attempt, the TXLARB bit is set. No

interrupt is generated to signal the loss of arbitration.

19.5.4

ABORTING MESSAGE

TRANSMISSION

The system can also abort a message by clearing the

TXREQ bit associated with each message buffer. Set-

ting the ABAT bit (C1CTRL<12>) will request an abort

of all pending messages. If the message has not yet

started transmission, or if the message started but is

interrupted by loss of arbitration or an error, the abort

will be processed. The abort is indicated when the

module sets the TXABT bit, and the TXxIF flag is not

automatically set.

19.5.5

TRANSMISSION ERRORS

The CAN module will detect the following transmission

errors:

Acknowledge Error

Form Error

Bit Error

These transmission errors will not necessarily generate

an interrupt but are indicated by the transmission error

counter. However, each of these errors will cause the

transmission error counter to be incremented by one.

Once the value of the error counter exceeds the value

of 96, the ERRIF (C1INTF<5>) and the TXWAR bit

(C1INTF<10>) are set. Once the value of the error

counter exceeds the value of 96, an interrupt is

generated and the TXWAR bit in the error flag register

is set.

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