[time-nuts] OCXO Soft-Start

[Bob kb8tq]

Hi

If your MCU has a POR that resets the internal oscillator that’s an unusual part. Most
of them I’ve seen simply reset the CPU…..Once the 8 MHz crystal oscillator is running 
at 37 to 53 MHz (or KHz) it may or may not ever return to 8 MHz on it’s own. Yes, you 
need to get into some nutty (as in volt per second) rates to see a lot of this kind of thing. 
It’s not a common thing to run into on a real design and it is even less common to see
it tested for. Unfortunately there are a few fields / companies where really slow supply ramps are
“the way it’s done”.  Needless to say, they have a lot of fun getting stuff to work right.

Bob

> On Apr 13, 2017, at 1:41 PM, Scott Stobbe <scott.j.stobbe at gmail.com> wrote:
> 
> I can't say I have run into that issue with a MCU as most 21st century ones
> have a decent POR and Brown out detect (which typically burns 10x more
> current than a 32k XO + RTC, and may get switched off in battery
> applications, and then problems can occur). What does seem to come up is
> stuff hanging off non-always on power rails (ADC,DAC,Sensors,etc), leakage
> and back-feeding onto their dedicated supply has them try to startup on
> leakage but there isn't enough leakage to actually power the device. They
> may issue the on die reset once and then the supply collapses all the flops
> lose their reset state, but the IC dosen't try to reset again.
> 
> Fortunately, you can buy a bunch of LDOs which include a small discharge
> FET to help this case as well as others.
>