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Date:   Mon, 16 Aug 2021 16:30:52 +0200
From:   Marc Kleine-Budde <mkl@pengutronix.de>
To:     Vincent MAILHOL <mailhol.vincent@wanadoo.fr>
Cc:     linux-can <linux-can@vger.kernel.org>,
        Stefan =?utf-8?B?TcOkdGpl?= <Stefan.Maetje@esd.eu>,
        netdev <netdev@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v5 2/7] can: bittiming: allow TDC{V,O} to be zero and add
 can_tdc_const::tdc{v,o,f}_min
Message-ID: <20210816143052.3brm6ny26jy3nbkq@pengutronix.de>
References: <20210815033248.98111-1-mailhol.vincent@wanadoo.fr>
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 <CAMZ6RqK5t62UppiMe9k5jG8EYvnSbFW3doydhCvp72W_X2rXAw@mail.gmail.com>
 <20210816122519.mme272z6tqrkyc6x@pengutronix.de>
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On 16.08.2021 23:10:29, Vincent MAILHOL wrote:
[...]
> After the discussion I had with Stefan, I assumed mcp251xxfd also
> used relative TDCO.  However, in the mcp15xxfd family manual,
> Equation 3-10: "Secondary Sample Point" on page 18 states that:
>=20
> | SSP =3D TDCV + TDCO
>=20
> As I commented above, this is the formula of the absolute
> TDCO. Furthermore, in the example you shared, TDCO is
> 16 (absolute), not 0 (relative).

ACK

> *BUT*, if this is the absolute TDCO, I just do not get how it can
> be negative (I already elaborated on this in the past: if you
> subtract from TDCV, you are measuring the previous bit...)
>=20
> Another thing which is misleading to me is that the mcp15xxfd
> family manual lists the min and max values for most of the
> bittiming parameters but not for TDCO.
>=20
> Finally, I did a bit of research and found that:
> http://ww1.microchip.com/downloads/en/DeviceDoc/Section_56_Controller_Are=
a_Network_with_Flexible_Data_rate_DS60001549A.pdf

Interesting. This data sheet is older than the one of the mcp2518fd.

> This is *not* the mcp25xxfd datasheet but it is still from
> Microship and as you will see, it is mostly similar to the
> mcp25xxfd except for, you guessed it, the TDCO.
>=20
> It reads:
> | TDCMOD<1:0>: Transmitter Delay Compensation Mode bits
> | Secondary Sample Point (SSP).
> | 10 =3D Auto; measure delay and add CFDxDBTCFG.TSEG1; add TDCO
> | 11 =3D Auto; measure delay and add CFDxDBTCFG.TSEG1; add TDCO
> | 01 =3D Manual; Do not measure, use TDCV plus TDCO from the register
> | 00 =3D Disable
>=20
> | TDCO<6:0>: Transmitter Delay Compensation Offset bits
> | Secondary Sample Point (SSP). Two's complement; offset can be
> positive, zero, or negative.
> | 1111111 =3D -64 x SYSCLK
> | .
> | .
> | .
> | 0111111 =3D 63 x SYSCLK
> | .
> | .
> | .
> | 0000000 =3D 0 x SYSCLK
>=20
> Here, you can clearly see that the TDCO has the exact same range
> as the one of the mcp25xxfd but the description of TDCMOD
> changes, telling us that:
>=20
> | SSP =3D TDCV (measured delay) + CFDxDBTCFG.TSEG1 (sample point) + TDCO
>=20
> Which means this is a relative TDCO.
>=20
> I just do not get how two documents from Microchip can have the
> TDCO relative range of -64..63 but use a different formula. I am
> sorry but at that point, I just do not understand what is going
> on with your controller...

Me neither. I'll ask my microchip contact.

regards,
Marc

--=20
Pengutronix e.K.                 | Marc Kleine-Budde           |
Embedded Linux                   | https://www.pengutronix.de  |
Vertretung West/Dortmund         | Phone: +49-231-2826-924     |
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