Re: [Qemu-devel] [PATCH] m68k: change default system clock for m5208evb

[Peter Maydell]

On 27 September 2016 at 16:49, Greg Ungerer <address@hidden> wrote:
> On 27/09/16 23:27, Laurent Vivier wrote:
>> It is better because 166.67 MHZ is clearly a rounded value computed from
>> the period: 1000000000/6000000 = 166.666666666666666666666666...
>
> Perhaps it is, but again it is not documented that way.
> All the 5208 documentation talks in terms of frequency.
> Would it not be clearer to define it in the same way that
> the documentation lists?
>
> Prime example from the M5208 Reference Manual regarding
> the PLL settings:
>
> . Voltage controlled oscillator range from 350 MHz to 540 MHz, resulting in a 
> core frequency
>   (fvco ÷ 3 (or fvco ÷ 4)) of 87.5 MHz to 166.67 MHz (maximum rated for 
> device)

Documentation quite often describes things in ways which
aren't what the underlying hardware actually does -- there's
an art to reading it and figuring out what's really going
on under the hood :-)

In the text you list here it says specifically that the
87.5 and 166.67 MHz frequencies are the results of
dividing the fvco clock by 3 or 4, which is obviously
 350 / 4 == 87.5 (for the low end)
 500 / 3 == 166.666666... (for the high end)

(If you care you can probably work through what the PLL
registers are set to that generates the 500MHz from
the crystal frequency.)

thanks
-- PMM