[U-Boot-Users] i.MX31: question about CFG_HZ and CKIL

[Guennadi Liakhovetski]

On Wed, 7 May 2008, Jens Gehrlein wrote:

> in the i.MX31 processor reference manual they mention a 32 kHz clock as 
> input for several timers and controllers. Sometimes it's called 
> ipg_clk_32k. But the documentation does not clearly state, that this 
> clock actually is the CKIL.
> 
> As far as I could derive from the ADS schematics the CKIL is feeded with 
> 32768 Hz instead of 32000 Hz. On our board we use 32768 Hz either.

MCIMX31RM Rev 2.3, section 3.4.4.1.1:

<quote>
The i.MX31 and i.MX31L processors can use either a 32 kHz, 32.768 kHz or a 
38.4 kHz crystal as the external low frequency source. Throughout this 
chapter, the low frequency crystal is referred to as the 32 kHz crystal, 
even though this can refer to the 32.768 or 38.4 kHz values.
</quote>

So, so far it is allowed to use a 32768 crystal.

> If CKIL is the above mentioned 32 kHz clock, the resulting deviation in 
> the timers is "only" about 2.4% as long as CFG_HZ in mx31ads.h is 32000 
> Hz. Although it works with the current definition, I'd like to know your 
> opinions, whether it is necessary to adjust this value.

Further, it depends on the CLKSS signal, which on i.MX31ADS is connected 
to JP22, whether CKIL is used as the reference clock. Unless you override 
this choise later in software.

In any case, I think, you are right. I'll submit a patch for mx31ads 
shortly, then you'll see what you could change for your platform too.

Thanks
Guennadi
---
Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer