That device also has analog circuitry for the oscillator itself and the temperature sensor and the temperature compensation. I believe I have read an app note some time ago, it may have been from Maxim describing a kind of ring oscillator being used as a temperature sensor which drew much less power than a bandgap or a PN junction and directly produced a digital output. The DAC itself, or whatever circuit they use for temp compensation also has analog components and must use pico power. Quite amazing.
On Jul 30, 2017 7:13 AM, "Attila Kinali" <att...@kinali.ch> wrote: > On Sun, 30 Jul 2017 12:23:17 +0200 > Pete Stephenson <p...@heypete.com> wrote: > > > > > - I find it remarkable that this circuit can operate on less than a > > > > microamp during normal usage, including temperature conversion. > > > > > > That's not so remarkable. If you make the transistors long, then > > > you get very low leakage. Couple that with small clock frequency > > > and you use very little current. Modern ICs only use so much current > > > because they have so many transistors, which are also optimized > > > for being fast, rather then low leakage. > > > > Good point! I admit the details of optimizing transistors for different > > purposes is beyond my ken, and I appreciate the insight. > > > There are multiple optimization points. One is to select a prodcution > process that is optimized for low leakage. I.e. thick gate oxide > and high threshold voltage. Both of these parameters imply higher > suplly voltage. > Then, in the design, you make your transistors long and large. > > The problem here is, that power consumption scales proportional > to the square of supply voltage, the gate capacitance and the > switching frequency. This means, if you choose a low leakage > process, and thus high supply voltage, your power consumtion > will go up. The same goes for choosing large transistors. > Hence it becomes a trade-off between static (leakage) and > dynamic (through gate capacitance) power consumption. > > > > > > The DS3231 has on-board temperature monitoring to correct the crystal > > > > frequency: is this something where they would have bothered putting a > > > > separate sensor next to the crystal itself, or are the die and the > > > > crystal are close enough and in the same package that they could use > an > > > > on-die sensor like a diode and call that "good enough"? > > > > > > My guess would be that it's a PN-junction or a bandgap temperature > > > sensor somewhere on the chip. Adding another part increases the cost > > > of production quite considerably. > > > > Indeed. At first glance, I was surprised not to see tiny discrete > > capacitors within the chip package itself, as I assumed (incorrectly) > > that getting sufficient capacitance to steer a crystal a little would > > require larger capacitors than could be easily put on a die, but then I > > remembered that each LSB in the aging register only changes the > > frequency by 0.1ppm at 25C, so that wouldn't need a large amount of > > capacitance. > > As a rule of thumb, you can assume that in an "old" (aka large node size) > process the gate capacitance is approximately 1nF per mm^2. So, you can > build quite easily 10-100pF of capacitors on-chip. > > > Attila Kinali > -- > You know, the very powerful and the very stupid have one thing in common. > They don't alters their views to fit the facts, they alter the facts to > fit the views, which can be uncomfortable if you happen to be one of the > facts that needs altering. -- The Doctor > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/ > mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.