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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