[PATCH 2/2] ARM: imx: improve precision of AV PLL to 1 Hz

[Lothar Waßmann]

Hi,

On Thu,  6 Oct 2016 13:12:11 +0200 Emil Lundmark wrote:
> The audio and video PLLs are designed to have a precision of 1 Hz if some
> conditions are met. The current implementation only allows a precision that
> depends on the rate of the parent clock. E.g., if the parent clock is 24
> MHz, the precision will be 24 Hz; or more generally the precision will be
> 
>     p / 10^6 Hz
> 
> where p is the parent clock rate. This comes down to how the register
> values for the PLL's fractional loop divider is chosen.
> 
> The clock rate calculation for the PLL is
> 
>     PLL output frequency = Fref * (DIV_SELECT + NUM / DENOM)
> 
> or with a shorter notation
> 
>     r = p * (d + a / b)
> 
> In addition to all variables being integers, we also have the following
> conditions:
> 
>     27 <= d <= 54
> 
>     0 <= a <= 2^30-1
>
Wrong. 'a' is a _signed_ number (see below)!
So this should be:
-2^29 <= a < 2^29

>     0 <  b <= 2^30-1
>     a < b
> 
> Here, d, a and b are register values for the fractional loop divider. We
> want to chose d, a and b such that f(p, r) = p, i.e. f is our round_rate
> function. Currently, d and b are chosen as
> 
>     d = r / p
>     b = 10^6
> 
> hence we get the poor precision. And a is defined in terms of r, d, p and
> b:
> 
>     a = (r - d * p) * b / p
> 
> I propose that if p <= 2^30-1 (i.e., the max value for b), we chose b as
> 
>     b = p
> 
According to the Reference Manual the "Absolute value should be less
than denominator"
|18.7.9 Numerator of Audio PLL Fractional Loop Divider Register
|           (CCM_ANALOG_PLL_AUDIO_NUM)
|This register contains the numerator (A) of Audio PLL fractional loop divider.(Signed
|number), absolute value should be less than denominator
|Absolute value should be less than denominator


Lothar Waßmann