Anelok: oscillator choices

Felix sucotronic at gmail.com
Mon Jun 2 02:27:11 EDT 2014


Wow, it's the first time I hear of FLL generators, but the idea seems good
enough to use a low power crystal for main clk source :D
Btw, the jitter is too much for certification, but, how much is the
tolerance of usb controllers chips? Will they be able to handle it?


On Mon, Jun 2, 2014 at 4:30 AM, Werner Almesberger <werner at almesberger.net>
wrote:

> The original design had only one crystal, on the RF chip. The RF chip
> would then generate a clock signal for the SoC. The Soc needs a
> precise clock for two purposes:
>
> - for USB, and
> - for timekeeping.
>
> Since it would be wasteful to run the RF chip all the time just to
> provide the clock (*), the idea was to calibrate the low-power RC
> oscillator in the KL26 with it, and then keep time based on that,
> with occasional wakeup and recalibration.
>
> (*) The AT86RF232 draws about 330 uA when idle. The CC2543 draws
>     about 3.1 mA in the lowest power mode in which the 32 MHz crystal
>     oscillator runs.
>
>
> If we want to have a "hard" rfkill switch that shorts the RF SoC's
> power supply to ground, the KL26 also has to be able to run without
> the RF SoC. It therefore needs a local crystal.
>
> The KL26 offers two choices: a high-frequency crystal that draws
> between 200 uA and 4 mA just for the oscillator, depending on speed
> and configuration, or a low-frequency crystal (32.768 kHz) that can
> work with as little as 0.5 uA (25 uA in high gain mode).
>
> 0.5 uA is interesting. This is low enough that the crystal oscillator
> could run continuously, providing very accurate timekeeping.
>
> Since I have a few small 32.768 kHz crystals lying around, I tought
> I'd give it a try in the next board version.
>
>
> Alas, one can't have more than on such clock source. So if we use a
> 32.768 kHz crystal, there can be no external clock input.
>
> This means that have to generate also the USB clock from 32.768 kHz.
> Unfortunately, the PLL can't do that. But there is a
> "Frequency-Locked Loop" (FLL) that can - with the one gotcha that
> Freescale state that
>
> "The MCGFLLCLK does not meet the USB jitter specifications for
>  certification."
>
> But it's clearly designed to do just that, and people seem to be
> using it in this way.
>
> So let's see how it behaves.
>
> - Werner
>
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-- 
Felix
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