Need a clarification regarding I2C bus frequency in FreeBSD

[Daniel Braniss]

> On 20 Aug 2018, at 19:13, Gary Jennejohn <gljennjohn at gmail.com> wrote:
> 
> On Mon, 20 Aug 2018 07:16:15 -0700
> Mark Millard via freebsd-hackers <freebsd-hackers at freebsd.org <mailto: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.
> 
yup, I connected the logic analyzer to check the frequency, which
was not changing, later I confirmed by looking at the source that it’s set
at a constant 100KHz.

> -- 
> Gary Jennejohn