MikroElektronika PIC Microcontrollers PIC18 Manual do Utilizador descarregar pdf (Página 22)

the masks and filters to determine if the message should be accepted. If a mask

bit is set to 0, that bit in the identifier is automatically accepted regardless of the

filter bit.

9.9.9 Calculating the Timing Parameters

Setting the nodes’ timing parameters is essential for the bus to operate reliably. Given

the microcontroller clock frequency and the required CAN bus bit rate, we can calculate

the values of the following timing parameters:



Baud rate prescaler value



Prop_Seg value



Phase_Seg1 value



Phase_Seg2 value



SJW value

Correct timing requires that



Prop_Seg þ Phase_Seg1  Phase_Seg2



Phase_Seg2  SJW

The following example illustrates the calculation of these timing parameters.

Example 9.2

Assuming the microcontroller oscillator clock rate is 20MHz and the required CAN bit

rate is 125KHz, calculate the timing parameters.

Solution 9.2

With a 20MHz clock rate, the clock period is 50ns. Choosing a baud rate prescaler

value of 4, from Equation (9.4), T

¼ 2

(BRP þ 1)

OSC

, gives a time

quantum of T

¼ 500ns. To obtain a nominal bit rate of 125KHz, the nominal bit

time must be:

BIT

¼ 1=0:125MHz ¼ 8ms; or 16T

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