Custom calypso board

[Christian Vogel]

Hi all,

On Sun, Jul 15, 2012 at 11:51:22PM +0200, Harald Welte wrote:
> More interesting is the clock source, where I would like to see a
> switchable clock source between:
>  * regular inetrnal VCTCXO
>  * external clock input (U.FL)

on the compal phones, a stand-alone oscillator (with control input) is
used, so I think you'll want to copy that. There are tiny single gate
multiplexers (e.g. 74LVC1G157) where NXP specifies a propagation delay
of 13ns max (125°C temperature, 1.65V supply), so it should certainly
work at 26Mhz (RITA's input wouldn't need the full voltage swing
anyway).

> Does anyone have a good idea how to implement the actual switch?  I
> think it would be best to have it software-switchable from a GPIO...

Switching the clock will produce a glitch, but we should be able
to run the ARM core from the 32kHz clock crystal during that period,
I think (but will have to check) would we really want to toggle
between the two inputs? Do we want to autodetect a valid ext. refclock?

On Mon, Jul 16, 2012 at 10:11:50PM +0200, Mathias K. wrote:
> http://www.micrel.com/page.do?page=product-info/multiplexers.jsp

The micrel parts are really impressive, but much too fast for our
purpose (>2GHz).

	Chris