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 <[email protected]> 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 > > _______________________________________________ > Qi Hardware Discussion List > Mail to list (members only): [email protected] > Subscribe or Unsubscribe: > http://lists.en.qi-hardware.com/mailman/listinfo/discussion > -- Felix
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