TouCAN too slow by 2X?

Hi

I thought I had set up TouCAN on an MPC565 for 500 KBPS (2 us per 
bit), but I see 4 us per bit with a scope.  

Did anyone else find a factor-of-two bitrate surprise going from the 
MPC565 User Manual to reality?

Or would some kind person point out where I'm confused?

SYSTEM CLOCK = f_CLK = 40 MHz
PRESDIV=3, so divide by 4.
Therefore, the TouCAN Sclock = 10 MHz = 1/10th us per TimeQuantum

20 Time Quanta per bit x 1/10 us/Tq  should =  2 us / bit = 500 KBPS
However, a scope tells me each bit is 4 us (250 KBPS, too slow).

We checked SYSTEM_CLOCK several ways, including what we get from the 
decrementer and what we see on the external bus (we divided by 2 for 
a 20 MHz external bus).

I've looked at CANCTRL1 & CANCTRL2 with a BDM probe, and PRESDIV=3.
If I poke PRESDIV to 1, my CAN bit rate jumps to what I thought it 
should have been when PRESDIV was 3.

Here is my count-on-fingers logic for 20 Tq per bit:

20 time quanta per bit =
SynchSeg + (PROPSEG+1) + (PSEG1+1) + (PSEG2+1) = 
    1    +      8      +      7    +     4      (actual Tq)
    -           7             6          3   (bitfield values)

PRESDIV = 3
PROPSEG = 7
PSEG1   = 6 
PSEG2   = 3 


Any suggestions would be very welcome.  This seems too simple to get 
wrong, but I did!

Rick Corey






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