Re: [RFC 2/2] clk: vc5: Add support for optional load capacitance

[Adam Ford <aford173@gmail.com>]

On Tue, Jan 12, 2021 at 10:45 AM Luca Ceresoli <luca@lucaceresoli.net> wrote:
>
> Hi Adam,
>
> On 11/01/21 17:40, Adam Ford wrote:
> > On Sat, Jan 9, 2021 at 12:02 PM Luca Ceresoli <luca@lucaceresoli.net> wrote:
> >>
> >> Hi Adam,
> >>
> >> On 09/01/21 04:00, Adam Ford wrote:
> >>> On Fri, Jan 8, 2021 at 4:49 PM Luca Ceresoli <luca@lucaceresoli.net> wrote:
> >>>>
> >>>> Hi Adam,
> >>>>
> >>>> On 06/01/21 18:39, Adam Ford wrote:
> >>>>> There are two registers which can set the load capacitance for
> >>>>> XTAL1 and XTAL2. These are optional registers when using an
> >>>>> external crystal.  Parse the device tree and set the
> >>>>> corresponding registers accordingly.
> >>>>
> >>>> No need to repeat the first 2 sentences, they are already in patch 1.
> >>>
> >>> The reason I did that was because if someone does a git log on the
> >>> individual file, they'd see the comment.  While it's redundant not, it
> >>> might not be as obvious in the future when looking back.   Not
> >>> everyone reviews the history of the binding, but the source files' git
> >>> logs usually have some value.   However, if you want me to drop it or
> >>> rephrase it, I can do that.
> >>
> >> Makes sense, I had never considered that before.
> >>
> >>>>> +static int vc5_map_cap_value(u32 femtofarads)
> >>>>> +{
> >>>>> +     int mapped_value;
> >>>>> +
> >>>>> +     /* The datasheet explicitly states 9000 - 25000 */
> >>>>> +     if ((femtofarads < 9000) || (femtofarads > 25000))
> >>>>> +             return -EINVAL;
> >>>>> +
> >>>>> +     /* The lowest target we can hit is 9430, so exit if it's less */
> >>>>> +     if (femtofarads < 9430)
> >>>>> +             return 0;
> >>>>> +
> >>>>> +     /*
> >>>>> +      * According to VersaClock 6E Programming Guide, there are 6
> >>>>> +      * bits which translate to 64 entries in XTAL registers 12 and
> >>>>> +      * 13. Because bits 0 and 1 increase the capacitance the
> >>>>> +      * same, some of the values can be repeated.  Plugging this
> >>>>> +      * into a spreadsheet and generating a trendline, the output
> >>>>> +      * equation becomes x = (y-9098.29) / 216.44, where 'y' is
> >>>>> +      * the desired capacitance in femtofarads, and x is the value
> >>>>> +      * of XTAL[5:0].
> >>>>> +      * To help with rounding, do fixed point math
> >>>>> +      */
> >>>>> +     femtofarads *= 100;
> >>>>> +     mapped_value = (femtofarads - 909829) / 21644;
> >>>>
> >>>> Thanks for the extensive comment, but I am confused. Not by your code
> >>>> which is very clean and readable, but by the chip documentation
> >>>> (disclaimer: I haven't read it in full depth).
> >>>
> >>> I was confused too since the datasheet and programmers manual differ a bit.
> >>>>
> >>>> The 5P49V6965 datasheet at page 17 clearly states capacitance can be
> >>>> increased in 0.5 pF steps. The "VersaClock 6E Family Register
> >>>> Descriptions and Programming Guide" at page 18 shows a table that allows
> >>>> 0.43 pF. Can you clarify how the thing works?
> >>>
> >>> I used the Versaclock 6E doc which is based on the following:
> >>>
> >>> BIT 5 - Add 6.92pF
> >>> BIT 4 - Add 3.46pF
> >>> BIT 3 - Add 1.73pF
> >>> BIT 2 - Add 0.86pF
> >>> Bit 1 - Add 0.43pF
> >>> Bit 0 - Add 0.43pF
> >>>
> >>> Because the Datasheet starts at 9pF, the math I used, assumes these
> >>> numbers are added to 9pF.
> >>> Because the datasheet shows the increments are in .5pF increments, the
> >>> 430nF seems close.  The datasheet shows 9pF - 25pF and based on the
> >>> programmer table, we could get close to 25pF by enabling all bits and
> >>> adding 9pF, however the math doesn't quite hit 25pF.
> >>>
> >>> For what it's worth I needed around 11.5pF, and with this patch, the
> >>> hardware engineer said our ppm went from around 70 ppm to around 4ppm.
> >>
> >> Did he measure what happens if you set the register according to the 0.5
> >> pF interpretation? Does it improve? I understand the difference is
> >> probably olwer than the noise, but who knows.
> >>
> >>>>> +
> >>>>> +     /*
> >>>>> +      * The datasheet states, the maximum capacitance is 25000,
> >>>>> +      * but the programmer guide shows a max value is 22832,
> >>>>> +      * so values higher values could overflow, so cap it.
> >>>>> +      */
> >>>>
> >>>> The 22832 limit is if you assume 0.43 pF steps. Assuming 0.5 pF steps
> >>>> leads to 25000. Now I am more confused than before.
> >>>
> >>> I agree.  It would be nice to get some clarification from Renesas.
> >>
> >> Definitely. Do you have access to some support from them?
> >
> > Luca,
> >
> > We reached out to  Renesas with the following questions:
> >
> > 1)
> > I'm seeing a discrepancy between the datasheet and programming guide
> > we have for the Versaclock 6e in regard to the crystal load
> > programming registers.  The datasheet for the 5P49V6965A000NLGI
> > indicates a 9pF minimum with 0.5pF steps, while the programming guide
> > says that the lower two register bits both add 0.43pF, which would
> > make the equation:
> >
> > Ci = 9pF + 0.43pF * XTAL[5:1] instead of
> > Ci = 9pF + 0.5pF * XTAL[5:0] as is published in the datasheet.
> >
> > 2)  The programming guide shows that the default setting is 01b, but
> > the note says it's reserved, use D1 D0 = 00.  Can you confirm that we
> > should set switch mode to 00 instead of the default 01?
> >
> > And we got the following answers:
> >
> > 1)
> >      The first one with 0.43pF steps is the correct one. Ci = 9pF +
> > 0.43pF * XTAL[5:1]
> >      0.5pF steps was the design target.  When measuring actual
> > silicon, we found 0.43pF steps.
> >
> >      There are 6 bits reserved for the CL setting but bits 0 and 1
> > have the same 0.43pF step.  So it is actually 5 bits with an extra LSB
> > of 0.43pF.
> >
> > 2)
> >       Please use D1 D0 = 01.   The “00” is a typo…..
>
> Great thing you got all those info from Renesas!
>
> >
> > Based on the above response I think we should always assume XTAL bit 0
> > is 0, and only use XTAL[5:1] which should make the math go easier,
> > because the desired value in femtofarads would just be offset by 9000
> > and divided by 430 and that value would be shifted 3 places instead fo
> > two, and the  fixed-point math calculation can go away.
> >
> > In addition to that, I would also need to make sure that D0 is set to
> > 1, so instead of just writing the shifted XTAL value, I would also
> > have to do a logic OR with 1 to set the low bit.
> >
> > I talked with the hardware guys from work who also suggested that we
> > always write the same value to X1 and X2, so I can remove the X1 and
> > X2 references from the bindings.
> >
> > Does that work for you?
>
> Yes.
>
> We are only losing the ability to set 9 + (0.43 * 32) pF using all bits.

We'd be doing this with XTAL[5:1] which mathematically makes more sense.

> I'm OK with that. Should it be needed in the future we can just add it
> as a special case, maybe just add a comment saying that, like "XTAL[5:0]
> = b111111 not supported".

XTAL[0] won't be supported at all by my updated algorithm, not just b111111

adam
>
> --
> Luca