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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