linux-kernel - Re: About bittiming calculation result

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Message-ID: <4D4FDEF9.2030305@grandegger.com>
Date:	Mon, 07 Feb 2011 13:00:57 +0100
From:	Wolfgang Grandegger <wg@...ndegger.com>
To:	Tomoya MORINAGA <tomoya-linux@....okisemi.com>
CC:	socketcan-core@...ts.berlios.de, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: About bittiming calculation result

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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