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.
_______________________________________________
Socketcan-core mailing list
[email protected]
https://lists.berlios.de/mailman/listinfo/socketcan-core
