[time-nuts] Raspberry Pi 4 oscillator replacement

[Trent Piepho]

On Thu, Feb 4, 2021 at 4:09 PM Hal Murray <hmurray at megapathdsl.net> wrote:
> In case anybody isn't familiar with ARM SOC chips, they typically have a layer
> of muxes between the external pins and the internal I/O devices.  I don't know
> if the chip used in the Pi-4 works this way.  Quite likely.
>
>
> The system designer has to find a combination of mux settings that works for
> the application.  Leftover pins can be used as GPIO.  And the clocking is
> tangled up in there.

While true for many pins, usually quite a few are not muxed.  Those
deemed necessary for all designs or with specialized drivers.  So
typically there will be no mux for power pins, dram interface, xtal
inputs, and high speed differential data, like USB3, PCIe, SGMII, MIPI
CSI or DSI.  Or perhaps muxing between different high speed serial
interfaces.

It seems to be very common for ARM SoC chips to be designed to be
clocked by either an XTAL and a built in oscillator, or via an
external clock generator connected to the same pin.  But, I've never
seen a SoC where this pin could be muxed.

I've not seen one where the input clock frequency had much of a range.
It might be 24-26 MHz, but never 10 - 52 MHz.  At least documented.
But the clock is invariably an input to a programmable PLL or PLLs,
and those PLLs are likely programmable with a rather wider range of
multipliers and dividers than required for the limited documented
input frequency range.

One might have better luck with a chip from Ti or NXP, since, unlike
Broadcom, they have documentation on how their clocks and PLLs work.