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

[Wolfgang Grandegger]

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.

> Anyhow it is better now than before.

At least for your board. Let's keep an eye on people reporting I2C
problems for these processors on the U-Boot and linuxppc-dev ML.

Wolfgang.