[time-nuts] Raspberry Pi 4 oscillator replacement

[Hal Murray]

phk at phk.freebsd.dk said:
> I dont know if the datasheet for the Rpi4 is available to check what the
> requirements are, but you should probably expect to need some kind of PLL
> chip to deliver a clean 54 MHz on the RPi4, locked to your external
> frequency. 

Plan B would be to avoid the 10=>54 PLL chip by reprogramming the software 
that sets up the internal PLLs to run off 10 MHz rather than 54MHz.  That may 
not be possible.  It depends on how the clocking inside the ARM chip is setup.

If you are lucky, you won't need to change the kernel.  If you are half-lucky, 
you have to make a few tweaks to the kernel software and rebuild.

---------

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.

It's a chinese menu sort of deal, only more complicated.  If you get the beef 
from column A, you can't get the chicken from column B.

The basic idea is that there are more I/O devices on the chip than there are 
pins.  After reset, all the pins are setup as GPIO input so they don't 
accidentally drive an input pin.  There is a mux in front of each input signal 
to an I/O device that picks the signal from one of several pins.  There is a 
mux on the output side of each pin that selects from an output of several I/O 
devices.

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.


-- 
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