[U-Boot] [PATCH 3/3] fsl_i2c: Impl. AN2919, rev 5 to calculate FDR/DFSR

[Joakim Tjernlund]

Wolfgang Grandegger <wg at denx.de> wrote on 21/09/2009 13:59:04:
>
> Joakim Tjernlund wrote:
> > Wolfgang Grandegger <wg at denx.de> wrote on 21/09/2009 12:53:36:
> >> Hi Joakim,
> >>
> >> Joakim Tjernlund wrote:
> >>> The latest AN2919 has changed the way FDR/DFSR should be calculated.
> >>> Update the driver according to spec. However, Condition 2
> >>> is not accounted for as it is not clear how to do so.
> >> I compared rev. 5 of AN2919 with rev. 3 and, as you pointed out, it puts
> >> additional constraints on how to select dfsr and fdr. Especially dfsr
> >> should not exceed a certain, frequency dependent value: dfsr <= 50 /
> >> period-in-ns. Therefore, I expected problems with divider values from
> >> the table which high dfsr values. I did your "=> date;date;date;date"
> >> test on a MPC8548 board using dfsr=43 and fdr=7 but it did not fail.
> >> According to the rev. 5, dfsr is not allowed to be greater than 8.
> >> Your patch works fine on this board as well. I have no time for a more
> >> thorough testing with different CPUs and frequencies. Anyhow...
> >
> > Yes, I too notice that higher dfsr values than the spec says works, in
> > fact my board needs at least dfsr 8 to be 100% stable but the spec says
> > no more that 6 for my board. I suspect that we should enforce a minimum
> > value of 8 to be on the safe side, especially as the HW default is 0x10.
>
> You say that the new constraints introduced by rev. 5 are not even
> correct!? Well, these modifications are obscure anyhow.

My board has a higher rise time than the I2C spec(1 us) so it may
be the reason for me. It *looks* like a higher dfsr makes the controller
tolerate more "noise". Where does the dfsr=1 come from in the first place?
It seems random and is very far from HW default.