Need a clarification regarding I2C bus frequency in FreeBSD

[Gary Jennejohn]

On Mon, 20 Aug 2018 07:16:15 -0700
Mark Millard via freebsd-hackers <freebsd-hackers at freebsd.org> wrote:

> On 2018-Aug-20, at 6:18 AM, Ian Lepore <ian at freebsd.org> wrote:
> 
> > On Mon, 2018-08-20 at 11:13 +0300, Daniel Braniss wrote:  
> >>   
> >>> 
> >>> On 20 Aug 2018, at 09:49, Daniel Braniss <danny at cs.huji.ac.il> wrote:
> >>>   
> >>>> . . .  
> >>> 
> >>> hi,
> >>> I have similar issues with the allwinner/twsi but I do have a Saleae Logic and here is a nice picture:  
> >> ah, maybe this is better:
> >> 	https://cs.huji.ac.il/~danny/Screen%20Shot%202018-08-20%20at%2011.06.43.png  
> > . . .
> > This has nothing to do with the twsi driver, this is about the ig4
> > driver (found in sys/dev/ichiic).
> > 
> > That screenshot seems to show a bus running at 100KHz like it should
> > (although the 62:38 duty cycle is a bit suspicious).  
> 
> Being a logic analyzer display, it my just be that the threshold
> was off from the optimal value. The waveform shape is not really
> visible.
> 
> The logic analyzer output also shows a thick "rising" edge without the
> uparrow symbol. My guess would be that is a rising/falling/rising
> sequence that on the scale in use does not show space between edges. In
> other words: a glitch on the leading edge side of the intended pulse.
> This too might be tied to the threshold used vs . the actual signal
> properties: no way to tell from what is shown.
> 

I have two of these logic analyzers and they definitely do a
major clean up of the signals displayed.

Things like overshoot and ringing, which can be seen on an
oscilloscope, do not appear on what the logic analyzer displays.

I suspect the purpose of the trace was simply to show the 100KHz
SCL.

-- 
Gary Jennejohn