Subject: Re: [Patch v2 1/2] 5200/mpc: improve i2c bus error recovery

From: "Albrecht Dreß" <[email-masked]>
Date: Thu, 18 Feb 2010 16:04:16 +0100 (CET)

Hi Joakim:

[snip]
> >   static void mpc_i2c_fixup(struct mpc_i2c *i2c)
> >   {
> > -   writeccr(i2c, 0);
> > -   udelay(30);
> > -   writeccr(i2c, CCR_MEN);
> > -   udelay(30);
> > -   writeccr(i2c, CCR_MSTA | CCR_MTX);
> > -   udelay(30);
> > -   writeccr(i2c, CCR_MSTA | CCR_MTX | CCR_MEN);
> > -   udelay(30);
> > -   writeccr(i2c, CCR_MEN);
> > -   udelay(30);
> > +   int k;
> > +   u32 delay_val = 1000000 / i2c->real_clk + 1;
> > +
> > +   if (delay_val < 2)
> > +      delay_val = 2;
> > +
> > +   for (k = 9; k; k--) {
> > +      writeccr(i2c, 0);
> > +      writeccr(i2c, CCR_MSTA | CCR_MTX | CCR_MEN);
> > +      udelay(delay_val);
> > +      writeccr(i2c, CCR_MEN);
> > +      udelay(delay_val << 1);
> > +   }
> >   }
> 
> I am curious, didn't old method work with by just wrapping
> a for(k=9; k; k--) around it? How did the wave form look?

Sure does that work!  The waveform was somewhat "streched", mainly due to the delays between some of the writeccr() calls which don't change the sda/scl lines.  Unfortunately I didn't take shots from the scope.

However, for *one* cycle, the old code needed (only counting the udelay's) 150 us.  For 9 cycles, it's 1.35 ms, which isn't really nice ;-).  At 375 kHz real clock rate, delay_val is 3, i.e. each cycle consumes 9 us, or 81 us for the whole fixup procedure.  If the clock is slower, the gain is of course a lot smaller, and at 20.5 kHz each cycle again needs 150 us...

My feeling is that the delays used in the old code are just "some" values which work for sure, to if you like, my change is basically optimisation...

BTW, related to your earlier question, I checked the timings recorded with the scope at 100 and at 20 kHz against the nxp's "I2C bus specification and user manual", rev. 03 - everything seems to be fine.

Thanks, Albrecht.

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