[time-nuts] Raspberry Pi 4 oscillator replacement

[Bob kb8tq]

Hi

A lot depends on the intended application of the chip. For
a general purpose MCU that could go into a wide range 
of things and that has very few peripherals,  1 to 40 MHz
might be just fine.  There are chips out there with much
wider clock input ranges.

Toss in USB and that locks in a clock tree output. Pile on
various Video formats and they need this or that specific
output frequency. Various speeds of Ethernet need this or
that. Fancy audio may adds in its needs. These are outputs 
from the clock system rather than inputs, but they do impact 
what inputs will work.

A general purpose device may still allow a range of inputs
and then play games to get the needed outputs. An on chip
VCO and a variety of dividers feeding a PLL are one common
setup. There may be multiple sets of this and that onboard. 

The issue becomes that only very specific individual frequencies 
in the specified range will work. Many of those specific frequencies 
may be quite strange looking …. 

On a purpose built device ( = one intended use) it is not 
uncommon to see them saving die space on the clock tree. 
You have one target input that it will accept. If you are lucky, 
it might accept 2X  and 4X that frequency. I suspect that this 
approach also makes the part a bit easier to test / validate. 

None of this is to say that the RPi will not work at (say)
50 MHz in. It is very likely that it will.  It is a pretty good bet 
that a whole bunch of peripherals will have issues if you do. 

Bob

> On Feb 5, 2021, at 2:44 AM, Poul-Henning Kamp <phk at phk.freebsd.dk> wrote:
> 
> --------
> Trent Piepho writes:
>> On Thu, Feb 4, 2021 at 4:09 PM Hal Murray <hmurray at megapathdsl.net> wrote:
> 
>> 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.
> 
> Usually if you read very closely, you will find a wide range is OK
> but a footnote to the effect that they only warrant USB will work at
> the following specific frequencies.
> 
> -- 
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG         | TCP/IP since RFC 956
> FreeBSD committer       | BSD since 4.3-tahoe    
> Never attribute to malice what can adequately be explained by incompetence.
> 
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