VS1005 RTC Frequency tolerance

Discussion about writing software for VS1005 and the VSOS Operating System. Also posts about VS1005-related hardware design and device drivers should be posted here.
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DavidMc
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VS1005 RTC Frequency tolerance

Post by DavidMc » Tue 2019-03-26 19:57

Hello!

In the VS1005 datasheet, page 10, Oscillator Specification table, it lists the RTC Frequency Tolerance as "±100ppm".

Questions:
Is this ±100ppm the range in which the VS1005 will work with a supplied clock (crystal, oscillator)? (i.e., if the supplied 32.768kHz clock is beyond ±100ppm, VS1005 will not work well or not meet spec)

or

Is this the tolerance of VS1005's internal frequency independent of the supplied clock? (i.e., a supplied clock with ±20ppm tolerance would then become ±120ppm)

Thanks!
David

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Panu
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Re: VS1005 RTC Frequency tolerance

Post by Panu » Thu 2019-03-28 11:15

Hi!

Ha! What an intriguing question! But out of your choices, #1 is closer to the truth. The RTC oscillator itself has a wide operating range, but the 100 ppm requirement for the crystal is just to get meaningful results when interpreting the frequency as a basis for calculating either time or lengths of time periods. If you put a 50 kHz crystal there, the VS1005 will still work but time calcuation software will give confusing results.
(would) a supplied clock with ±20ppm tolerance would then become ±120ppm?
No.

-Panu
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DavidMc
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Re: VS1005 RTC Frequency tolerance

Post by DavidMc » Mon 2019-04-01 17:35

Thank you Panu.

I was thinking the same, but had to ask to be sure I wasn't overlooking something.
Thanks!
DMc

DavidMc
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Re: VS1005 RTC Frequency tolerance

Post by DavidMc » Tue 2019-04-02 19:22

Hello Panu,

A new, related question.

We need to consider a more accurate RTC clock. I'm wondering if it is possible to switch between two hardware 32.768kHz clocks (one is low accuracy, low power; other is high accuracy, high power) depending on whether the product is turned Off and running from limited battery, or turned ON and running from ample power supply.

Example procedure:
1) Vhigh power is turned Off, but RTC power is always ON via always-ready battery
2) Crystal is connected to VS1005
3) Vhigh power is turned ON
4) Crystal is disconnected from VS1005
5) XO (i.e., active component) is connected to VS1005 in place of the crystal
6) Go to step 1

It is assumed that the time between steps 1-2 and 3-5 is very small - probably less than a millisecond; while the time between steps 2-3 and 5-6 could be seconds/minutes/hours.

Setting aside, for a moment, capacitance and drive-level issues associated with inserting a silicon switch between the crystal and VS1005, will VS1005 be ok with a momentary pause or a non-periodic (but brief) glitch(es) during start-up and power-down?

This sequence here viewtopic.php?f=13&t=2349#p12443 suggests there is likely enough delay during start-up to transition hardware clocks.

David

Hannu
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Re: VS1005 RTC Frequency tolerance

Post by Hannu » Wed 2019-04-03 8:51

DavidMc wrote:
Tue 2019-04-02 19:22
Hello Panu,

A new, related question.

We need to consider a more accurate RTC clock. I'm wondering if it is possible to switch between two hardware 32.768kHz clocks (one is low accuracy, low power; other is high accuracy, high power) depending on whether the product is turned Off and running from limited battery, or turned ON and running from ample power supply.

Example procedure:
1) Vhigh power is turned Off, but RTC power is always ON via always-ready battery
2) Crystal is connected to VS1005
This is normal status. The RTC circuit keeps running and time is kept.
3) Vhigh power is turned ON
4) Crystal is disconnected from VS1005
5) XO (i.e., active component) is connected to VS1005 in place of the crystal
This is tricky part. How to switch the clock source? If the switching element is semiconductor, powering must thought carefully. If the switching is done with relay, it is much easier as it can take the power from the infinite power supply.
6) Go to step 1

It is assumed that the time between steps 1-2 and 3-5 is very small - probably less than a millisecond; while the time between steps 2-3 and 5-6 could be seconds/minutes/hours.

Setting aside, for a moment, capacitance and drive-level issues associated with inserting a silicon switch between the crystal and VS1005, will VS1005 be ok with a momentary pause or a non-periodic (but brief) glitch(es) during start-up and power-down?
The switching will probably genera