RE: About bittiming calculation result

[Tomoya MORINAGA]

On Tuesday, February 08, 2011 12:53 AM,  Wolfgang Grandegger wrote:

> BTW, it's always possible to specify optimized bit-timing 
> parameters directly, e.g. the following seem better:
> 
>    800000     60  12    4    4   4   3  793650  0.8% 80.0% 81.0%  1.2%
> 
> You could set these with:
> 
>   $ ip link set can0 type can \
>     tq 60 prop-seg 12 phase-seg1 4 phase-seg2 4 sjw 4

I can confirm 800K comms works well using the above.

I wish Can-core could calculate like above.

>> seg1/seg2/sjw/prop_seg must be more than 1 ?
BTW, according to EG20T PCH data sheet, 
CAN bit-timing parameters(BRP, Prop_Seg, Phase_Seg1, Phase_Seg2, SJW) must not be set 0.

Thanks,
-----------------------------------------
Tomoya MORINAGA
OKI SEMICONDUCTOR CO., LTD.


> -----Original Message-----
> From: Wolfgang Grandegger [mailto:wg@xxxxxxxxxxxxxx] 
> Sent: Tuesday, February 08, 2011 12:53 AM
> To: Tomoya MORINAGA
> Cc: socketcan-core@xxxxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; 
> linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: About bittiming calculation result
> 
> On 02/07/2011 01:00 PM, Wolfgang Grandegger wrote:
> > Hi Tomoya,
> > 
> > On 02/07/2011 12:38 PM, Tomoya MORINAGA wrote:
> >> Hi,
> >>
> >> I have a question for bittiming-value calculated by Can-core.
> >>
> >> In case setting like below,
> >>  - ip link set can0 type can bitrate 800000
> >>  - clock=50MHz
> >>  - Use pch_can
> >>
> >> Can-core calculates like below
> >> brp=21
> >> seg1=1
> >> seg2=1
> >> sjw=1
> >> prop_seg=0
> >>
> >> Is "prop_seg=0" true ?
> > 
> > Well, only prop_seg+phase_seg=tseg1 is relevant and the 
> pch_can driver 
> > sets the allowed minimum "tseg1_min1" currently to 1:
> > 
> > static struct can_bittiming_const pch_can_bittiming_const = {
> >         .name = KBUILD_MODNAME,
> >         .tseg1_min = 1,
> >         .tseg1_max = 16,
> >         .tseg2_min = 1,
> >         .tseg2_max = 8,
> >         .sjw_max = 4,
> >         .brp_min = 1,
> >         .brp_max = 1024, /* 6bit + extended 4bit */
> >         .brp_inc = 1,
> > };
> > 
> >> seg1/seg2/sjw/prop_seg must be more than 1 ?
> > 
> > Then "tseg1_min" should be set to *2*.
> > 
> >> Also I can see the following kernel error log.
> >> bitrate error 0.7%
> > 
> > A clock frequency of 50 MHz is sub-optimal for CAN and some 
> bit-rates 
> > cannot be reproduced properly. Here is the output of the can-utils 
> > program "can-calc-bit-timing" (with an entry for the pch-can added):
> > 
> > $ ./can-calc-bit-timing pch-can
> > Bit timing parameters for pch-can with 50.000000 MHz ref clock
> > nominal                                 real Bitrt   nom  real SampP
> > Bitrate TQ[ns] PrS PhS1 PhS2 SJW BRP Bitrate Error SampP 
> SampP Error CNF1 CNF2 CNF3
> > 1000000    100   3    3    3   1   5 1000000  0.0% 75.0% 
> 70.0%  6.7% 0x05 0x92 0x02
> >  800000    420   0    1    1   1  21  793650  0.8% 80.0% 
> 66.6% 16.8% 0x15 0xff 0x00
> >  500000    100   8    8    3   1   5  500000  0.0% 87.5% 
> 85.0%  2.9% 0x05 0xbf 0x02
> >  250000    500   3    3    1   1  25  250000  0.0% 87.5% 
> 87.5%  0.0% 0x19 0x92 0x00
> >  125000    500   6    7    2   1  25  125000  0.0% 87.5% 
> 87.5%  0.0% 0x19 0xb5 0x01
> >  100000    500   8    8    3   1  25  100000  0.0% 87.5% 
> 85.0%  2.9% 0x19 0xbf 0x02
> >   50000   2500   3    3    1   1 125   50000  0.0% 87.5% 
> 87.5%  0.0% 0x7d 0x92 0x00
> >   20000   2500   8    8    3   1 125   20000  0.0% 87.5% 
> 85.0%  2.9% 0x7d 0xbf 0x02
> >   10000  12500   3    3    1   1 625   10000  0.0% 87.5% 
> 87.5%  0.0% 0x71 0x92 0x00
> > 
> > As you can see, especially 800000 gives rather bad results.
> 
> BTW, it's always possible to specify optimized bit-timing 
> parameters directly, e.g. the following seem better:
> 
>    800000     60  12    4    4   4   3  793650  0.8% 80.0% 81.0%  1.2%
> 
> You could set these with:
> 
>   $ ip link set can0 type can \
>     tq 60 prop-seg 12 phase-seg1 4 phase-seg2 4 sjw 4
> 
> Wolfgang.
> 

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