On 18.06.2021 20:17:51, Vincent MAILHOL wrote:

quoted

quoted

I just noticed in the mcp2518fd data sheet:

| bit 14-8 TDCO[6:0]: Transmitter Delay Compensation Offset bits;
| Secondary Sample Point (SSP) Two’s complement; offset can be positive,
| zero, or negative.
|
| 011 1111 = 63 x TSYSCLK
| ...
| 000 0000 = 0 x TSYSCLK
| ...
| 111 1111 = –64 x TSYSCLK

Have you takes this into account?

I have not. And I fail to understand what would be the physical
meaning if TDCO is zero or negative.

The mcp25xxfd family data sheet says:

| SSP = TDCV + TDCO

quoted

TDCV indicates the position of the bit start on the RX pin.

If I understand correctly in automatic mode TDCV is measured by the CAN
controller and reflects the transceiver delay. I don't know why you want
to subtract a time from that....

The rest of the relevant registers:

| TDCMOD[1:0]: Transmitter Delay Compensation Mode bits; Secondary Sample Point (SSP)
| 10-11 = Auto; measure delay and add TDCO.
| 01 = Manual; Do not measure, use TDCV + TDCO from register
| 00 = TDC Disabled
| 
| TDCO[6:0]: Transmitter Delay Compensation Offset bits; Secondary Sample Point (SSP)
| Two’s complement; offset can be positive, zero, or negative.
| 011 1111 = 63 x TSYSCLK
| ...
| 000 0000 = 0 x TSYSCLK
| ...
| 111 1111 = –64 x TSYSCLK
| 
| TDCV[5:0]: Transmitter Delay Compensation Value bits; Secondary Sample Point (SSP)
| 11 1111 = 63 x TSYSCLK
| ...
| 00 0000 = 0 x TSYSCLK

quoted

If TDCO is zero, the measurement occurs on the bit start when all
the ringing occurs. That is a really bad choice to do the
measurement. If it is negative, it means that you are measuring the
previous bit o_O !?

I don't know...

quoted

Maybe I am missing something but I just do not get it.

I believe you started to implement the mcp2518fd.

No I've just looked into the register description.

quoted

Can you force a
zero and a negative value and tell me if the bus is stable?

Actually, ISO 11898-1 specifies that the "SSP position should be
at least 0 to 63 minimum time quanta". This means that we can
have SSP = TDCV + TDCO = 0. In my implementation, I used 0 as a
reserved value for TDCV and TDCO. To comply with the standard, I
now need to allow both TDCV and TDCO to be zero and add a new
field in struct tdc to manage the automatic/manual options.

That said, these zero values still make no sense to me. Why would
someone do the measurement on the bit edge?

Concerning the negative values, the ISO standard says nothing
about it.  If you are using the automatic measurement, a negative
TDCO is impossible to use. TDCV is measured on every bit. When
the measurement is done, it is too late to subtract from it (or
maybe the mcp2518fd has a time machine built in?).

:)

If you are
using the manual mode for TDCV, just choose two positive values
so that TDCV + TDCO = SSF.

Marc

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