CAN communication at 1 Mb/s

[dastek]

Hi,

I have a 9s12XEQ384 running on a 5 MHz resonator and I have CAN communication using TJA1040 and TJA1042 that works at 250 Kb/s and 500 Kb/s.
I now need to make it work at 1 Mb/s.

I can see that I need to change my 5 MHz resonator to a 8 MHz resonator.

Now the settings for CAN_BTR0 and CAN_BTR1 is where I am confused.

For 250 Kb/s I have

#define    BTR0_250        0x41            // 5 MHz reference
#define    BTR1_250        0x34            // Sample at 4 / 5 = 80%

for 500 Kb/s I have

#define    BTR0_500        0xC0            // Suggested by Radek - Works
#define    BTR1_500        0x34            // Suggested by Radek - Works

When I use your CAN calculator (ver 7) I get

#define    BTR0_1000       0x40            // 8 MHz reference
#define    BTR1_1000       0x94            // 3 samples / bit

But when I go back to using a 5 MHz resonator on the calculator it says

For 250 Kb/s

#define    BTR0_250        0x41
#define    BTR1_250        0x16    or     0x96

for 500 Kb/s

#define    BTR0_500        0x40
#define    BTR1_500        0x16    or     0x96

Now I have confirmed the 250 and 500 KB/s as working using CAN_BTR1 = 0x34

So which is correct?  And should I use the above for 1 Mb/s ?

Regards,
Wade Whitehorn

[lama]

Hi,

Radek used older version of the calculator where I programmed the sample point as close as possible  to 50%. Moreover, I am not sure whether he used the right version. There was some error I can not remember.

Here is a calculation he probably used (after my call with him):

If you want to follow changes you can change Propagation delay of the interface, bus length, sample point,...the changes are clearly visible in the right part of the table -> length of segment changes.

If I use TJA1040 propagation delay 255ns, different bus length and sample point 80% then relations between values change.

So that's the reason.

Note:  You can see better oscillator must be used in the second case.

Best regards,

Ladislav