About CAN's least Baudrate !

Started by Wang dawei December 20, 2002
Hi everybody;

I am using the CAN bus. To be compatible with old communication products.
I need the to run the CAN at 5-100k baudrate. But with the IPBUS clock, the
least baudrate is 40M/28*64>20k. Any comments appreciated!

I am using 56F807

Many thanks;

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You can use the PLL Clock Out Divide bits (PLLCOD[1:0] - Bits 11-10) in the PLL
Divide-By Register (PLLDB), to divide the PLL output by 1, 2, 4, or 8, so you
can get IPBus clock frequencies of 40, 20, 10, or 5 MHz. Please see Section
15.6.2 "PLL Divide-By Register (PLLDB)", and Section 15.6.2.3 "PLL Clock Out
Divide (PLLCOD[1:0] - Bits 11-10)", in the DSP56F80x User's Manual.

The 5 MHz IPBus clock would give you a lowest CANbus frequency of:
( 5MHz / ( 24 * 64 ) ) = 3255.2 Hz (approximately)

The only drawback to the above, is the reduced operating frequency of the whole
chip. Using the EXTAL clock (which is another possibility for the CANbus clock
source) is NOT recommended, as there have been several reports of CANbus system
failures when using EXTAL as the CANbus clock source.

Please also see the following message I posted to this discussion group:
October 1, 2002, at about 8:01am:
"RE: [motoroladsp] MSCAN problem"

The following Application Notes and FAQs have some highly useful information for
anyone writing code for the MSCAN (Motorola Scalable Controller Area Network)
module. Although they were written for other devices, most of this information
does apply to the DSP56F80x MSCAN module, especially regarding how the
interrupts should be handled. I found that the DSP56F80x User's Manual didn't
have enough details on how the error interrupts should be handled.

AN2283/D Motorola Scalable Controller Area Network (MSCAN) Interrupts
AN1798/D CAN Bit Timing Requirements
AN2255/D MSCAN Low-Power Applications
EB181/D FAQs - M68HC05 Family MCAN Module

You can get the above documents by following these links:
http://e-www.motorola.com/brdata/PDFDB/docs/AN2283.pdf
http://e-www.motorola.com/brdata/PDFDB/docs/AN1798.pdf
http://e-www.motorola.com/brdata/PDFDB/docs/AN2255.pdf
http://e-www.motorola.com/brdata/PDFDB/docs/EB181.pdf

Motorola has many links to Application Notes for the DSP56F80x DSP chips, just
go to their website at:
http://e-www.motorola.com/webapp/sps/site/homepage.jsp?nodeIdM0ylgz6wY

Then select the chip you are using (for example, DSP56F807):
- In the Search area (top right) enter DSP56F807 and press Go
- Click on the link to "DSP56F807 Product Summary Page"
- On this page scroll down to "DSP56F807 Documentation"
- You will see a section titled "Application Note", this has links to all the
Application Notes that apply to the DSP56F807 chip I hope this information is helpful to you. Regards,

Art Johnson
Senior Systems Analyst
PMC Prime Mover Controls Inc.
3600 Gilmore Way
Burnaby, B.C., Canada
V5G 4R8
Phone: 604 433-4644
FAX: 604 433-5570
Email:
http://www.pmc-controls.com
-----Original Message-----
From: Wang dawei [mailto:]
Sent: Thursday, December 19, 2002 5:42 PM
To:
Subject: [motoroladsp] About CAN's least Baudrate ! Hi everybody;

I am using the CAN bus. To be compatible with old communication products.
I need the to run the CAN at 5-100k baudrate. But with the IPBUS clock, the
least baudrate is 40M/28*64>20k. Any comments appreciated!

I am using 56F807

Many thanks;

_________________________________________________________________
нʹ MSN Messenger: http://messenger.msn.com/cn _____________________________________
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