[time-nuts] A look inside the DS3231

Pete Stephenson pete at heypete.com
Sun Jul 30 06:23:17 EDT 2017

On Sun, Jul 30, 2017, at 11:15 AM, Attila Kinali wrote:
> On Sat, 29 Jul 2017 20:32:30 +0200
> Pete Stephenson <pete at heypete.com> wrote:
> > - There's several square grids of circles-in-squares circuit elements. I
> > have no idea what these are.
> If you look closely, these are actually suqares-in-squares.
> I am not sure, but my guess would be that these are the
> capacitor banks for the correction of the oscillator frequency.

True, the larger ones are squares-in-squares, but the smaller ones to
the left look like circles-in-octagons, but I find it hard to see the
details of the smaller features.

Either way, I should probably stare less through microscope eyepieces.
It seems to stress the eyes a bit.

> > - 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.

> > 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

As you say, minimizing part count keeps the price down and makes the
design simpler.


Pete Stephenson

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