DSPIC30F datasheet(111/220 Pages) MICROCHIP

111 / 220 page

 2004 Microchip Technology Inc.

Preliminary

DS70116C-page 109

dsPIC30F5011/5013

• 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 deter-

mined by checking the bits in the CAN Interrupt

Status register, CiINTF.

- 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 RXnOVR 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.

17.5

Message Transmission

17.5.1

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.

17.5.2

TRANSMIT MESSAGE PRIORITY

Transmit priority is a prioritization within each node of

the pending transmittable messages. There are

4 levels

transmit

priority.

TXPRI<1:0>

(CiTXnCON<1:0>, where n = 0, 1 or 2 represents a par-

ticular 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.

17.5.3

TRANSMISSION SEQUENCE

To initiate transmission of the message, the TXREQ bit

(CiTXnCON<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

(CiTXnCON<6>), TXLARB (CiTXnCON<5>) and TXERR

(CiTXnCON<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 TXIE was set.

If the message transmission fails, one of the error con-

dition flags will be set, and the TXREQ bit will remain

set indicating that the message is still pending for trans-

mission. 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.

17.5.4

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 (CiCTRL<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 TXnIF flag is not

automatically set.

17.5.5

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 (CiINTF<5>) and the TXWAR bit

(CiINTF<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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