P2 PLL clock setup documentation question

rogloh · 2020-07-25 06:51

I'm making some PLL changes in my video driver and saw this in the SPIN2 document but wonder if it is correct.

In the third case in this table with both the PLL and the direct clock input XI (no crystal) in use, should the CC_SS bits be %01_10 or should they be %01_11 to make use of the PLL as the clock source? Other information I found is shown below, but it still isn't clear. Why would we select %10 for %SS as the clock source if the PLL is being used as the source, or does the %SS=%10 also let you enable the PLL as the clock source but just disables the internal oscillator for the crystal? Maybe this documentation is incorrect or incomplete, or I am just reading it wrong.

rogloh · 2020-07-25 07:03

Also it would be good for nested driver COGs needing access to the PLL to get access to the _xtlfreq or _xinfreq settings, otherwise they need to specify it themselves and there is a risk they will differ from the top level settings. Problems can happen if they need to be specified in multiple files when values get changed.

So what happens in SPIN2 when a nested driver COG tries to make use of these _xtlfreq or _xinfreq names when they might already not be setup by the top level file? Are they still available to compile in but are set to zero, or will they default to 20MHz, when unspecified for example? I'm trying to figure out if my code can even use them when changing the PLL.

My current code looks like this but it would be nice to tidy it up and somehow make it automatic and not need changing based on the board setup if it could be obtained from the top level SPIN file clock information.

CON
   #0, CLKSRC_XTAL, CLKSRC_XIN
    ' select one of these based on your HW
    'CLKIN_HZ =  _xtalfreq
    'CLKIN_HZ =  _xinfreq
    CLKIN_HZ = 20000000 ' 20MHz default

    CLOCKSRC = CLKSRC_XTAL ' enable for crystal
    'CLOCKSRC = CLKSRC_XIN ' enable for direct clock in (no crystal)

PUB computeClockMode(desiredHz) : mode
    '<snip>

    ' final clock mode format is this #%0000_000E_DDDD_DDMM_MMMM_MMMM_PPPP_CCSS
    if mode
        ' also set 15 or 30pF capacitor loading based on input crystal frequency
        mode |= (1<<24) ' enable PLL
        if (CLOCKSRC == CLKSRC_XTAL) ' enable oscillator and caps for crystal
            mode |= (CLKIN_HZ < 16000000) ? %1111 : %1011
        else
            mode |= %0111 ' don't enable oscillator

evanh · 2020-07-25 07:35

That's for external clock gen modules in place of the crystal. They can go straight into the XI pin, XO becomes unused. The PLL can still work from those as well - Which would then be %01_11.

EDIT: Oh, the description talks about PLL. Hmm, yeah, could do with some editing. The fifth entry is the mode I was thinking of.

EDIT2: The first entry of the table is also extraneous as it is covered by the second entry.

rogloh · 2020-07-25 07:41

Yes I am using %01_11 in my code above for direct clock + PLL which I think is correct. The %01_10 setting shown in the table seems like it wouldn't select the PLL as the clock source, even though the example mentioned it used the PLL.

P2 PLL clock setup documentation question

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