MikroElektronika PIC Microcontrollers PIC18 Manuel d'utilisateur télécharger pdf (Page 15)

In order to compensate for phase shifts between the oscillator frequencies of nodes

on a bus, each CAN controller must synchronize to the relevant signal edge of the

received signal. Two types of synchronization are defined: hard synchronization and

resynchronization. Hard synchronization is used only at the beginning of a message

frame, when each CAN node aligns the Sync_Seg of its current bit time to the

recessive or dominant edge of the transmitted start of frame. According to the

rules of synchronization, if a hard synchronization occurs, there will not be a

resynchronization within that bit time.

With resynchronization, Phase_Seg1 may be lengthened or Phase_Seg2 may be

shortened. The amount of change in the phase buffer segments has an upper bound

given by the synchronization jump width (SJW). The SJW is programmable between

1 and 4, and its value is added to Phase_Seg1 or subtracted from Phase_Seg2.

9.8 PIC Microcontroller CAN Interface

In general, any type of PIC microcontroller can be used in CAN bus–based projects, but

some PIC microcontrollers (e.g., PIC18F258) have built-in CAN modules, which can

simplify the design of CAN bus–based systems. Microcontrollers with no built-in CAN

modules can also be used in CAN bus applications, but additional hardware and

software are required, making the design costly and also more complex.

Figure 9.11 shows the block diagram of a PIC microcontroller–based CAN bus

application, using a PIC16 or PIC12-type microcontroller (e.g., PIC16F84) with no

CAN Node

CAN Bus

CAN

Transceiver

MCP2551

PIC12/16

Series 8-bit

microcontroller

CAN

Controller

MCP2515

SPI

Figure 9.11: CAN node with any PIC microcontroller

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489Advanced PIC18 Projects—CAN Bus Projects

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MikroElektronika PIC Microcontrollers PIC18 Manuel d'utilisateur Page 15