Re: [PATCH] ARM: OMAP2+: timer: remove CONFIG_OMAP_32K_TIMER

[Jon Hunter <jon-hunter@ti.com>]

On 11/08/2012 12:06 PM, Hiremath, Vaibhav wrote:
> On Thu, Nov 08, 2012 at 23:28:53, Hunter, Jon wrote:
>>
>> On 11/08/2012 11:47 AM, Hiremath, Vaibhav wrote:
>>> On Thu, Nov 08, 2012 at 23:09:57, Hunter, Jon wrote:
>>
>> [snip]
>>
>>>> I think you are missing the point here. For OMAP devices we have two
>>>> main external clock sources which are the 32kHz clock and the sys_clk
>>>> (can be a range of frequencies from 12-38.4MHz for OMAP4). The point is
>>>> for OMAP these clock sources are always present and AFAIK there is no
>>>> h/w configuration that allows you not to have the 32kHz clock source.
>>>> PRCM needs it and I think for OMAP1 the reset logic needs it (if memory
>>>> serves).
>>>>
>>>> Igor was mentioning a h/w scenario where the 32kHz source is not
>>>> present. However, I am not sure which devices support this and is
>>>> applicable too.
>>>>
>>>> So we are not discussing the 32k-sync-timer here. We are discussing
>>>> whether there are any device configurations where a 32k clock source
>>>> would not be available for the gptimer.
>>>>
>>>
>>> Exactly that is the point I am trying to make here,
>>>
>>> In case of AM33xx, it is certainly possible to build the system without 
>>> this 32Khz clock. 
>>>
>>> Interesting point here is, this 32KHz clock is external clock coming from 
>>> crystal connected to in-built RTC module.
>>
>> Thanks. Looking at the AM3358 data manual it states ...
>>
>> "The OSC1 oscillator provides a 32.768-kHz reference clock to the
>> real-time clock (RTC) and is connected to the RTC_XTALIN and RTC_XTALOUT
>> terminals. OSC1 is disabled by default after power is applied. This
>> clock input is optional and may not be required if the RTC is configured
>> to receive a clock from the internal 32k RC oscillator (CLK_RC32K) or
>> peripheral PLL (CLK_32KHZ) which receives a reference clock from the
>> OSC0 input."
>>
>> So what is clear to me that an external 32k clock source is optional.
>> However, it still appears that you will always have an internal 32k
>> source and so regardless of the h/w configuration, a gptimer will always
>> have an 32k clock available on the AM335x devices. Is that correct?
>>
> 
> Internal RC oscillator is not accurate at all, so not guaranteed to give 
> accurate 32.768Hz clock. The oscillation band is from 16Khz to 64Khz.
> 
> So it may not be useful as a system clocks, right?

By the way, according to the AM3358 data manual (paragraph above), even
if there is no external 32k clock source and if you don't use the
internal 32k oscillator, you can still generate a 32k clock from the PER
PLL (CLK_32KHZ). So therefore, there should always be a 32k clock source
available for the gptimer. Is that correct?

At the end of the day, I am just trying to understand if any OMAP/AM
device supports a h/w configuration where there is no 32k clock source
available for the gptimer.

Jon