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From: Marc Kleine-Budde <mkl@pengutronix.de>
To: Vincent MAILHOL <mailhol.vincent@wanadoo.fr>
Cc: linux-can <linux-can@vger.kernel.org>,
	"Stefan Mätje" <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
Date: Mon, 16 Aug 2021 16:30:52 +0200	[thread overview]
Message-ID: <20210816143052.3brm6ny26jy3nbkq@pengutronix.de> (raw)
In-Reply-To: <CAMZ6RqK0vTtCkSM7Lim2TQCZyYTYvKYsFVwWDnyNaFghwqToXg@mail.gmail.com>

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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:
> 
> | SSP = TDCV + TDCO
> 
> 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...)
> 
> 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.
> 
> Finally, I did a bit of research and found that:
> http://ww1.microchip.com/downloads/en/DeviceDoc/Section_56_Controller_Area_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.
> 
> It reads:
> | TDCMOD<1:0>: Transmitter Delay Compensation Mode bits
> | Secondary Sample Point (SSP).
> | 10 = Auto; measure delay and add CFDxDBTCFG.TSEG1; add TDCO
> | 11 = Auto; measure delay and add CFDxDBTCFG.TSEG1; add TDCO
> | 01 = Manual; Do not measure, use TDCV plus TDCO from the register
> | 00 = Disable
> 
> | TDCO<6:0>: Transmitter Delay Compensation Offset bits
> | Secondary Sample Point (SSP). Two's complement; offset can be
> positive, zero, or negative.
> | 1111111 = -64 x SYSCLK
> | .
> | .
> | .
> | 0111111 = 63 x SYSCLK
> | .
> | .
> | .
> | 0000000 = 0 x SYSCLK
> 
> 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:
> 
> | SSP = TDCV (measured delay) + CFDxDBTCFG.TSEG1 (sample point) + TDCO
> 
> Which means this is a relative TDCO.
> 
> 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

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
Embedded Linux                   | https://www.pengutronix.de  |
Vertretung West/Dortmund         | Phone: +49-231-2826-924     |
Amtsgericht Hildesheim, HRA 2686 | Fax:   +49-5121-206917-5555 |

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  reply	other threads:[~2021-08-16 14:30 UTC|newest]

Thread overview: 36+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-15  3:32 [PATCH v5 0/7] add the netlink interface for CAN-FD Transmitter Delay Compensation (TDC) Vincent Mailhol
2021-08-15  3:32 ` [PATCH v5 1/7] can: netlink: allow user to turn off unsupported features Vincent Mailhol
2021-08-19  7:45   ` Marc Kleine-Budde
2021-08-19  9:24     ` Vincent MAILHOL
2021-08-19 10:07       ` Marc Kleine-Budde
2021-08-15  3:32 ` [PATCH v5 2/7] can: bittiming: allow TDC{V,O} to be zero and add can_tdc_const::tdc{v,o,f}_min Vincent Mailhol
2021-08-16  8:42   ` Marc Kleine-Budde
2021-08-16 10:24     ` Vincent MAILHOL
2021-08-16 12:25       ` Marc Kleine-Budde
2021-08-16 12:33         ` Marc Kleine-Budde
2021-08-16 13:43           ` Marc Kleine-Budde
2021-08-16 15:49             ` Vincent MAILHOL
2021-08-16 16:04               ` Vincent MAILHOL
2021-08-17  1:12                 ` Vincent MAILHOL
2021-08-17 20:01               ` Marc Kleine-Budde
2021-08-18  9:22                 ` Vincent MAILHOL
2021-08-18 12:29                   ` Marc Kleine-Budde
2021-08-18 14:17                     ` Vincent MAILHOL
2021-08-20  6:12                       ` Vincent MAILHOL
2021-08-16 14:10           ` Vincent MAILHOL
2021-08-16 14:30             ` Marc Kleine-Budde [this message]
2021-08-17  9:43               ` Marc Kleine-Budde
2021-08-16 13:49         ` Stefan Mätje
2021-08-16 15:56           ` Vincent MAILHOL
2021-08-15  3:32 ` [PATCH v5 3/7] can: bittiming: change unit of TDC parameters to clock periods Vincent Mailhol
2021-08-15  3:32 ` [PATCH v5 4/7] can: dev: add can_tdc_get_relative_tdco() helper function Vincent Mailhol
2021-08-16 11:27   ` Marc Kleine-Budde
2021-08-15  3:32 ` [PATCH v5 5/7] can: netlink: add interface for CAN-FD Transmitter Delay Compensation (TDC) Vincent Mailhol
2021-08-17 19:55   ` Marc Kleine-Budde
2021-08-18  8:08     ` Vincent MAILHOL
2021-08-18  8:19       ` Marc Kleine-Budde
2021-08-18  8:37         ` Vincent MAILHOL
2021-08-18  8:43           ` Marc Kleine-Budde
2021-08-15  3:32 ` [PATCH v5 6/7] can: netlink: add can_priv::do_get_auto_tdcv() to retrieve tdcv from device Vincent Mailhol
2021-08-15  3:32 ` [PATCH v5 7/7] can: etas_es58x: clean-up documentation of struct es58x_fd_tx_conf_msg Vincent Mailhol
2021-08-19 10:08   ` Marc Kleine-Budde

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