[time-nuts] Thought experiment on a low cost timing board

John Pettitt jpp at cloudview.com
Sat Feb 25 18:50:56 EST 2006

Tim Shoppa wrote:
> Hal Murray <hmurray at suespammers.org> wrote:
>>> PCI 3.3v board with:
>>>     10Mhz OCXO (provision for external clock source?)
>>>     Uart (serial is an endangered on many PC's)
>>>     Free running counter driven from the OCXO and readable by PC
>>>     inputs to latch the counter (how many?) with the latched result
>>> also readable (for PPS) 
>> I've been daydreaming along the same line but haven't hit a combination of 
>> ideas that feels good enough to actually do anything.
>> My list has a FPGA in the middle.  The problem with that is that newer FPGAs 
>> don't work on 5V PCI which is what all of my PCs have.  The only 3V slots 
>> I've seen are in high end (noisy, expensive) servers.  The PCI slots on 
>> Soekris boards are 3V.  That adds another option to think about.
>> There are several possibilities for dealing with 5V PCI.
>> [ BGA's, Gold Fingers, etc.]
> My gut feeling: back up a little bit. Figure out how to do what you
> want without a PCI bus, without gold fingers, without BGA's, etc.
I had several goals in mind when I asked the initial question:

1) a low cost high stability ntp stratum 1 clock board - something that 
when added to a sub $100 gps would yield a really stable time source for 
ntp.  To do this it really needs to let the main cpu read the counters 
in a predictable, consistent time which is why I wanted PCI (3v because 
I have soekris boxes).   

2) it would also be nic to be able to add a low cost frequency synth to 
the design.

What I've seen so far there are designs out there based on various cpu 
cores and or f/c pga chips that are close to what I'm thinking.  The 
replies from the list have been really helpful - I'm going to do some 
more research and look closely at the existing designs and see what I 
can adapt.

What prompted this in the first place was the horrible temperature 
sensitivity my soekris boxes exhibit - I can only keep them to within 
10us of the gps 95% of the time and the occasional 100us excursion is 
not uncommon.   I want to be stable to the limit of the gps I'm using - 
for no other reason than the belief that I can do it for way less than 
commercial devices using open source software and low cost hardware.  In 
other works I'm hacking for the sake of it :-)


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