Hi

Assuming you are going to run it off a battery. What’s the self discharge
rate on a reasonable battery? 

Bob

> On Mar 6, 2018, at 8:34 PM, Attila Kinali <att...@kinali.ch> wrote:
> 
> On Tue, 6 Mar 2018 22:59:34 +0000
> Mark Sims <hol...@hotmail.com> wrote:
> 
>> Sparkfun is selling an interesting RTC clock chip board. 
>> It draws 22 nA.  It has a rather novel clock generator...
>> a tuning fork crystal disciplines an RC oscillator every few minutes.
>> They claim 3 minutes per year drift.
> 
> *Sigh* There are a couple of things wrong with that description.
> 
> 1) The part is from Microcrystal, so you can believe the specs
> they publish with high confidence. This also means this is probably
> the exact same chip you will find in Swiss quartz watches.
> (Microcrystal is part of the Swatch group and the main supplier
> of 32kHz oscillators and electronics for the Swiss watch industry)
> 
> 2) The 22nA is the _typical_ consumption in auto-cal mode.
> Max (the number you should design with) is 32nA. Keep in
> mind this number is for the chip alone, no external connection.
> It also includes the implicit condition that all input pins are
> at valid voltage levels. If a pull up/down resistor is too weak
> (because you tried to safe a few more nA) it will result in
> pins being driven by leakage currents, possibly reaching invalid
> voltage levels, which in turn will cause shot-through currents
> through the input stages, increasing the power consumption 10 fold
> at least, 1000 fold easily.
> 
> 3) The current ratings are at 25°C. Going higher means also
> an increase in power consumption. How much, is not specified.
> 
> 4) The 3minutes per year number comes from the +/-3ppm first year
> aging. This is, as usual, at 25°C and is on top of the typ. +/2ppm
> time accuracy. So, the real accuracy is more like 5min after the first
> year... when running from Xtal all the time!
> Which has a typ. 60nA/max 80nA current spec!
> Taking temperature into consideration, assuming something in the order
> of 10 minutes per year is probably more realistic... if you stay
> close to 25°C. The quadratic nature of the temperature dependence
> for tuning fork X-cut crystals causes high deviations pretty quickly
> (In one of the devices I designed, the 10ppm spec of the crystal
> suddenly became 300ppm when taking the whole temperature range at
> which it had to operate into consideration)
> 
> 5) The precision of the auto-cal mode is not specified. It can be
> litterally anything. Especially considering that the datasheet
> talks about using it "several hours at Backup Supply Voltage," 
> i.e. as an emergency measure when the normal power supply is lost.
> 
>                       Attila Kinali
> 
> -- 
> <JaberWorky>  The bad part of Zurich is where the degenerates
>                throw DARK chocolate at you.
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