Messages in this thread | | | Date | Mon, 07 Feb 2011 13:00:57 +0100 | From | Wolfgang Grandegger <> | Subject | Re: About bittiming calculation result |
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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.
Wolfgang.
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