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dsPIC30F4011/4012

DS70135G-page 134

2010 Microchip Technology Inc.

19.5.6

TRANSMIT INTERRUPTS

Transmit interrupts can be divided into 2 major groups,

each including various conditions that generate

interrupts:

Transmit Interrupt

At least one of the three transmit buffers is empty (not

scheduled) and can be loaded to schedule a message

for transmission. Reading the TXxIF flags will indicate

which transmit buffer is available and caused the

interrupt.

Transmit Error Interrupts

A transmission error interrupt will be indicated by the

ERRIF flag. This flag shows that an error condition

occurred. The source of the error can be determined by

checking the error flags in the CAN Interrupt Status reg-

ister, C1INTF. The flags in this register are related to

receive and transmit errors.

Transmitter warning interrupt.

The TXWAR bit indicates that the Transmit Error

Counter has reached the CPU warning limit of 96.

Transmitter error passive.

The TXEP bit (C1INTF<12>) indicates that the

Transmit Error Counter has exceeded the error

passive limit of 127 and the module has gone to

error passive state.

Bus off.

The TXBO bit (C1INTF<13>) indicates that the

Transmit Error Counter (TERRCNT<7:0>) has

exceeded 255 and the module has gone to bus off

state.

19.6

Baud Rate Setting

All nodes on any particular CAN bus must have the

same nominal bit rate. In order to set the baud rate, the

following parameters have to be initialized:

Synchronization Jump Width

Baud Rate Prescaler

Phase Segments

Length Determination of Phase2 Seg

Sample Point

Propagation Segment Bits

19.6.1

BIT TIMING

All controllers on the CAN bus must have the same

baud rate and bit length. However, different controllers

are not required to have the same master oscillator

clock. At different clock frequencies of the individual

controllers, the baud rate has to be adjusted by

adjusting the number of time quanta in each segment.

The nominal bit time can be thought of as being divided

into separate non-overlapping time segments. These

segments are shown in Figure 19-2.

Synchronization segment (Sync Seg)

Propagation time segment (Prop Seg)

Phase segment 1 (Phase1 Seg)

Phase segment 2 (Phase2 Seg)

The time segments and also the nominal bit time are

made up of integer units of time called time quanta or

TQ. By definition, the nominal bit time has a minimum

of 8 TQ and a maximum of 25 TQ. Also, by definition,

the minimum nominal bit time is 1

μsec, corresponding

to a maximum bit rate of 1 MHz.

FIGURE 19-2:

CAN BIT TIMING

Input Signal

Sync

Prop

Segment

Phase

Segment 1

Phase

Segment 2

Sync

Sample Point

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