MikroElektronika PIC Microcontrollers PIC18 Manual de usuario descargar pdf (Pagina 23)

The Sync_Segment is 1T

. Choosing 2T

for the Prop_Seg, and 7T

for Phase_Seg1

leaves 6T

for Phase_Seg2 and places the sampling point at 10T

at the end of

Phase_Seg1.

By the rules described earlier, the SJW can be the maximum allowed (i.e., 4). However,

a large SJW is only necessary when the clock generation of different nodes is not

stable or accurate (e.g., if ceramic resonators are used). Typically, a SJW of 1 is

enough. In summary, the required timing parameters are:

Baud rate prescaler (BRP) ¼ 4

Sync_Seg ¼ 1

Prop_Seg ¼ 2

Phase_Seg1 ¼ 7

Phase_Seg2 ¼ 6

SJW ¼ 1

The sampling point is at 10T

which corresponds to 62.5% of the total bit time.

There are several tools available for free on the Internet for calculating CAN bus timing

parameters. One such tool is the CAN Baud Rate Calculator, developed by Artic

Consultants Ltd (http://www.articconsultants.co.uk). An example using this tool

follows.

Example 9.3

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

bit rate is 125KHz, calculate the timing parameters using the CAN Baud Rate

Calculator.

Solution 9.3

Figure 9.14 shows the output of the CAN Baud Rate Calculator program. The device

type is selected as PIC18Fxxx8, the oscillator frequency is entered as 20MHz, and the

CAN bus baud rate is entered as 125KHz.

Clicking the Calculate Settings button calculates and displays the recommended timing

parameters. In general, there is more than one solution, and different solutions are given

in the Calculated Solutions field’s drop-down menu.

In choosing Solution 2 from the drop-down menu, the following timing parameters are

recommended by the program:

www.newnespress.com

497Advanced PIC18 Projects—CAN Bus Projects

1 2 ... 18 19 20 21 22 23 24 25 26 27 28 ... 39 40

Sin comentarios

MikroElektronika PIC Microcontrollers PIC18 Manual de usuario Pagina 23