Re: TouCAN too slow by 2X?

Hi,

I was wondering if you had managed to obtain a similar bitrate using the
MPC565 at 56 Mhz ? I tried your settings and they work perfectly at 40 Mhz
but I am trying to find a way of running at full CPU speed.

Thanks,
Mark


Corey, Richard wrote:
> 
> 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
> 
> 
> 
> 
> 
> 
> -----------------------------------------------------------
> To learn more about Freescale Microcontrollers, please visit
> http://www.freescale.com/mcu
> 
> 
>  
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> 
> 
> 
> 
> 

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