6 ms·
> So what else could it be? Well, at the heart of every electronic device is a clock. Traditionally, these are quartz oscillators, crystals that vibrate at a sp
by blattimwind 8y ago
> So what else could it be? Well, at the heart of every electronic device is a clock. Traditionally, these are quartz oscillators, crystals that vibrate at a specific predictable frequency—generally 32 kHz.
Watch crystals run at 32[.768] kHz because you can divide with a binary counter by 2^16 and get 1 pps to drive the Lavet stepper driving the seconds hand.
Watch crystals are also commonly used by MCUs for their RTC (real time, very low power clock), but never used to produce the main clock of a SoC or something like that. Mostly because that'd need an insanely high multiplication through a PLL (higher frequency multiplication ~ higher phase noise). Base clock crystals are typically 20-50 MHz.
Quartz crystals used to be hermetically sealed.
- deleted 8y ago[deleted]
- IshKebab 8y agoI agree it is unlikely to be the RTC clock at issue unless it runs a watchdog of done form. But given that they are using MEMS for the 32kHz clock it seems likely they are using it for other clocks too.
- blattimwind 8y ago> I agree it is unlikely to be the RTC clock at issue unless it runs a watchdog of done form. Fairly standard technique to run the watchdog off the RTC clock, because that might still work if the main clock is wonky.
- analog31 8y agoIndeed, and historically, a slower RTC clock was also preferred because it would draw a lot less current during sleep mode. And it allows the rest of the system to throttle the main system clock up and down for power savings. I said "historically" because I don't know is if these are still important issues on a modern system relative to all of the other things that are probably using the main system clock during sleep mode, and the more generous battery.
- dbcurtis 8y agoClock throttling is still a thing. Many ARM Cortex-Mx parts, when in deep sleep, draw less current than the self-discharge rate of a standard alkaline battery sitting on a store shelf waiting to be purchased. It is common to leave the RTC running, stash important data in the small RAM that is backed up by a super-cap, and go into a “wait for interrupt” deep sleep mode.
- hazeii 8y agoIn the embedded world this is very standard; timed wake-up from sleep is driven by the 32.768KHz clock, the code then wakes up faster clocks/peripherals as required by the application.
- DrStalker 8y ago> Quartz crystals used to be hermetically sealed. Are they sealed against helium though? It can get through a lot of materials that more common gases can't.
- Triesault 8y agoHermetically sealed by definition is airtight, i.e. "excludes the passage of air, oxygen, or other gases". [1] With that said, the diffusion rate depends on both the properties of both the seal and the gas. [1] https://en.wikipedia.org/wiki/Hermetic_seal https://en.wikipedia.org/wiki/Hermetic_seal
- garmaine 8y agoVery few air seals protect against helium, the smallest gas. Most materials, even metals, are like sponges to helium. It'll seep through slowly.
- hanniabu 8y agoCase in point, helium filled balloons that deflate over a couple days.
- yetihehe 8y agoAir filled balloons also deflate over a couple days.
- namibj 8y agoCompare with aluminium coated foil. They don't deflate that fast.
- Keloo 8y agoOr intestines of cattle. https://en.wikipedia.org/wiki/Zeppelin https://en.wikipedia.org/wiki/Zeppelin Early Zeppelins used rubberised cotton for the gasbags, but most later craft used goldbeater's skin, made from the intestines of cattle
- zik 8y agoNit-pick - you mean to divide 32.768kHz by 2^15, not 2^16 to get 1 pps.
- tejuis 8y agoMore nit-picking... according to wikipedia Lavet requires two states for one step, so 2^16 looks good based on this.
- tejuis 8y agoDamn... I mean 2^14 looks good. :)
- davrosthedalek 8y agoIf you divide by 2^15, you have 1pps, but two states per second (high/low). That might be enough.
- tejuis 8y agoWell, division was to be made with a binary counter. They typically work with rising edge clock, and you get a smaller circuit if you have 14 bits compared to 15 bits. I agree that 2^15 can be made to work by using both edges of clock signal.