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. > _______________________________________________ > time-nuts mailing list -- firstname.lastname@example.org > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- email@example.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.