RE: [RFC PATCH] watchdog: da9062: Correct the timeout values [Klartext]

[Christoph Niedermaier <cniedermaier@dh-electronics.com>]

From: Christoph Niedermaier
Sent: Monday, December 13, 2021 5:16 PM
>>>>> Thanks anyway, so now I know it must be
>>>>> problem with my DA9061 chip.
>>>>>
>>>>> @Adam
>>>>> Where can it come from?
>>>>> Can you give we a hint what to check?
>>>>
>>>> I've spoken internally and have been informed that this is down to the fact that
>>>> DA9061 runs only from an internal oscillator which may be slower. The indication
>>>> is that the values for TWDSCALE describe the window where if a kick/ping occurs
>>>> within that period then the watchdog is guaranteed *not* to timeout. The actual
>>>> timeout would be at some point after the selected timeout period, assuming no
>>>> ping/kick occurred.
>>>>
>>>> Table 8 in the datasheet specifies a minimum watchdog timeout of 2.5s (tWDMAX)
>>>> under specific operating conditions, so if the minimum 2s window was chosen
>>>> (TWDSCALE = 1) then earliest the watchdog would actually timeout, following a
>>>> ping, is 2.5s, assuming the conditions matched those described.
>>>>
>>>> If you have further questions it probably makes sense to contact Dialog/Renesas
>>>> support as they will be able to provide more detailed info on this.
>>>
>>> So a DA9061 runs only from an internal oscillator, whereas a DA9062
>>> can run on either an internal or an external oscillator. So this
>>> means that the DA9061 timeout values are differ from the DA9062
>>> with an external oscillator not only on my device but on all DA9061
>>> devices.
>>>
>>> This are the values (in seconds) in comparison:
>>> DA9062 (from driver): 0  2  4   8  16  32  65 131
>>> DA9061 (measured):    0  3  6  12  25  51 102 204
>>> =================================================
>>> Difference:           0 +1 +2  +4  +9 +19 +37 +73
>>>
>>> In my opinion, the differences in the higher values are very huge.
>>> If I expect that the watchdog triggers and I have to wait more than
>>> a minute for that to happen I ask myself is there something wrong.
>>>
>>> @Andrej
>>> I guess, you are using an external oscillator, aren't you?
>>>
>>> @Adam
>>> Is there a way to check in the driver which oscillator is in use?
>>>
>>> @Maintainers
>>> Is in the driver a need to distinguish between an external and an
>>> internal oscillator to get the timeout values more accurate?
>>>
>>
>> It would be very desirable to get timeout values more accurate.
>> I would not want to dictate how to implement it, though.
>> It could be automatically detected if that is possible, there
>> could be a devicetree clock property providing the clock
>> frequency, or maybe there is some other solution.
>>
>> Guenter
> 
> I am open for a good solution.
> Meanwhile I measured the timeout values of my 8 available DA9061
> watchdogs. I derived the following formula from the given formula
> at the data sheet and the clock divider of 2^16:
> 
> f = 2^(15+TWDSCALE) / t
> 
> Formula check with the external oscillator (32kHz) TWDSCALE=7 @ 131s:
> f = 2^(15+7) / 131 = 32017Hz (=> should be OK)
> 
> The timeouts of my 8 watchdogs (9061-AA) with TWDSCALE=7:
> t7 = 211s => 19878Hz
> t7 = 197s => 21291Hz
> t7 = 203s => 20662Hz
> t7 = 204s => 20560Hz
> t7 = 206s => 20361Hz
> t7 = 198s => 21662Hz
> t7 = 200s => 20972Hz
> 
> According to the data sheet the internal oscillator should run at 25kHz.
> The average frequency of my 8 devices is 20.6kHz. Maybe the data sheet
> Clock value is a max value. The timeout difference on my 8 devices are
> 14s. So the values vary from device to device, and maybe there is also a
> temperature component.
> 
> @Adam
> Is there a way to check which oscillator is in use?
> Is there a way to find the current oscillator frequency?
> Are there any other ideas/solutions to get the timeout values more accurate?

As I sent my E-Mail I didn't see your answer, so here it is:

> A quick scan of the DA9062 datasheet shows that reg/field EN_32K/CRYSTAL will
> indicate the presence of a 32KHz crystal oscillator. Obviously on DA9061 that
> option isn't available.
>
> I think the problem seems to lie around determining the internal oscillator's
> frequency. Datasheet references 25Mhz in Table 9 (Watchdog Electrical
> Characteristics), but that doesn't seem to tally with your timings and I don't
> see an obvious way in the regmap to calculate this at run time. *If* the
> oscillator frequency varies from part to part, or under different environmental
> conditions, then it's going to be tough to tie this down, and you wouldn't want
> to state a timeout value that's longer than reality.

@Adam
What do you think if I add also the timeout values for the internal oscillator?
Than the values would be:

t = 2^(15+TWDSCALE) / f

Internal 25kHz oscillator: 0  2  5  10  20  41  83 167

In comparison to the
external 32kHz oscillator: 0  2  4   8  16  32  65 131

So the difference to my measured values will decrease and it will improve the
current driver towards more accurate values.

Btw if I calculate the values with 32768Hz (the typical 32kHz oscillator), I get the
following values:
32768Hz oscillator:        0  2  4   8  16  32  64 128

Regards
Christoph