On 27 September 2016 at 16:49, Greg Ungerer <g...@uclinux.org> 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
