TMS320F28379D: TMS28379D Can bit Timing

electrocc

Hi,

I want to use the CAN module with a bit rate = 125kbit/s ; if I use the following function, it works :

CANBitRateSet(CANB_BASE, 200000000, 125000);

But I don't like to use function, I prefer programming directly registers, it's more flexible I think.
This is the configuration :

EALLOW;
// Configure the eCAN timing
// CANB Bit-Clock Source Select Bit:
ClkCfgRegs.CLKSRCCTL2.bit.CANBBCLKSEL = 0;      // 00 = PERx.SYSCLK (default on reset)
EDIS;

CanbRegs.CAN_BTR.bit.BRPE = 4; // Baud Rate Prescaler Extension - Valid programmed values are 0 to 15.
CanbRegs.CAN_BTR.bit.BRP = 19; // Baud Rate Prescaler - Value by which the CAN_CLK frequency is divided for generating the bit time quanta. The bit time is built up from a multiple of this quanta.

CanbRegs.CAN_BTR.bit.TSEG1 = 12; // Or 11 because The actual TSeg1 value interpreted for the Bit Timing will be the programmed TSeg1 value + 1.
CanbRegs.CAN_BTR.bit.TSEG2 = 1;   // Or 0 because The actual TSeg2 value interpreted for the Bit Timing will be the programmed TSeg2 value + 1.

With this settings, I have a bit time (on the oscilloscope) of about 23µs (about 45kHz) instead of bit time of 8µs (125kHz).

Can you explain where am I wrong?

time quantum = SYSCLK / ((BRP + 1)*(BRPE + 1)) = 200MHz / (20 * 5) = 2MHz = 0,5µs

bit time = [TSEG1 + TSEG2 + 3] * t_q = 16 * t_q = 8µs ?!?

Thanks for your help

Regards

over 5 years ago

0 Vince over 5 years ago

TI__Mastermind 19795 points

Hello,

Thanks for your question! You are correct in how this is meant to be viewed (that the time quanta per bit is different than the bit timing itself). I definitely understand the desire to use the registers directly to modify these values for better flexibility. Because of this, rather than using just the registers from the code snippet above, I would suggest using the following resource to dig into the timing control of the device:

"Calculator for CAN Bit Timing Parameters": http://www.ti.com/lit/an/sprac35/sprac35.pdf

Direct link to the calculator (also linked in the document): http://www.ti.com/lit/zip/sprac35

This calculator shows the suggested parameters based on a given bit rate/time quanta/etc, with many details on what to set in the device based on that.

Let me know if this helps with your development process!

Regards,

Vince

0 electrocc over 5 years ago in reply to Vince

Expert 2450 points

Hi,

Thanks for this very helpful document. Now I have the good period on the oscilloscope.

But I just don't understand how I can find the good result with calculations..

Regards

C2000™︎ microcontrollers

C2000 microcontrollers forum

TMS320F28379D: TMS28379D Can bit Timing