[time-nuts] How to properly characterize 32kHz oscillators manually and with a microcontroller?

[Nick Sayer]

> 
> For more precise stuff timing, I have a Thunderbolt and other goodies.
> (Speaking of which, I really need to figure out how to use the
> Thunderbolt as an external clock source for my Arduinos.)
> 

I don’t think you can do it for an Arduino without hardware changes. The CLKI pin is shared with XTAL1. But you can clock almost any AVR from an external clock source as long as the Vcc vs frequency “safe operating area” is respected (see the datasheet for your particular device). Note that if you fuse the device for an external clock source of any kind, that source must be present during (non-HV) programming as well as operation.

I use a DC blocking cap and self-biased inverter nowadays as an input conditioner for external clock inputs. It allows me to accept sine as well as square inputs if necessary. My principle use for this is my Crazy Clock calibrator, which I clock from one of my GPSDOs. It’s not 100% reliable, and I’m not sure why. Every once in a while, the AVR will lock up, even with the watchdog turned on. The square wave on CLKI looks *perfect*. I can only guess that there’s a glitch every once in a while that screws something up, but I haven’t figured it out yet.