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dsPIC30F3014/4013

DS70138G-page 116

2010 Microchip Technology Inc.

17.5.6

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. The TXnIF flags are

read to determine which transmit buffer is

available and caused the interrupt.

Transmit Error Interrupts:

A transmission error interrupt is 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 register, CiINTF. 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 (CiINTF<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 (CiINTF<13>) indicates that the

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

exceeded 255 and the module has gone to the

bus off state.

17.6

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 Phase Segment 2

Sample Point

Propagation Segment bits

17.6.1

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 17-2.

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

sec corresponding

to a maximum bit rate of 1 MHz.

FIGURE 17-2:

CAN BIT TIMING

Input Signal

Sync

Prop

Segment

Phase

Segment 1

Phase

Segment 2

Sync

Sample Point

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