Re: [PATCH] i2c: mxs: fix broken timing calculation

[Marek Vasut <marex-ynQEQJNshbs@public.gmane.org>]

Dear Lothar Waßmann,

> Hi,
> 
> Marek Vasut writes:
> > Hi Lothar,
> > 
> > > The timing calculation is rather bogus and gives extremely wrong
> > > results for higher frequencies (on an i.MX28). E.g. instead of 400 kHz
> > > I measured 770 kHz.
> > > 
> > > Implement a calculation that adheres to the I2C spec and gives exact
> > > results for I2C frequencies from 12.56 kHz to 960 kHz.
> > > 
> > > Also the bus_free and leadin parameters are programmed according to
> > > the I2C spec for standard and fast mode.
> > 
> > I suspect the resulting speed is heavily dependent on hardware properties
> > of the bus. Did you have a chance to check it with a scope? I will try
> > to recheck on other boards next week.
> 
> Of course I did! I found the DS1339 RTC not working on our hardware
> with the I2C clock frequency set to 400kHz and then checked the bus
> timing. I found the SCL frequency to be 770kHz instead of 400kHz and
> 113kHz instead of 100kHz.
> On what hardware did you do your measurements?

MX28EVK and M28EVK.

> The fancy constants -2 and -7 in the calculation were derived from
> measuring the clock low and high time with low_count and high_count
> set to 1 and measuring the actual timing of the signal.
> The clock frequeny in this setup is 2.18 MHz corresponding to 11 clock
> cycles of the 24MHz clock. The LOW time was about 140ns and the HIGH
> time 318ns corresponding to 3 and 8 (instead of 1) clock cycles.
> 
> These constants could be different for different SoCs (i.MX23).
> But I don't have any hardware to verify that.

I can check it for you on MX23 next week, I have two boards with that chip with 
well accessible I2C. I am not in the office now, so this will have to wait until 
next week.

btw offtopic, I will at least try to fix the PIO in the meantime.

> Maybe some guru from Freescale can comment on this and perhaps
> document the relationship between the register contents and the actual
> timing.

Ok, you're taking a third stab at getting FSL to explain how to configure 
arbitrary clock speeds on the MXS I2C. Good luck ;-D

Best regards,
Marek Vasut