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I'm working on a project that uses a Atmega329 with a 2 digit lcd 7 segment display. It needs to keep track of days, using a small coin cell battery.

I originally planned on using the on board clock, as the accuracy doesn't have to be great, I'm fine with a couple hours each month off.

Doing some testing, I noticed that it is off by up to a few seconds every minute (this is used in varying temperatures). So I need an external clock source.

Should I use a higher frequency crystal and clock it down internally? Example: 16Mhz crystal, using a clock speed of 8 or 4 mhz to save battery life. Or should I use a 8Mhz crystal and keep it at that speed?

Would a ceramic oscillator have enough accuracy and temperature stability to use? I'll be going from 0c to 25c

Thanks

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  • \$\begingroup\$ The title of your question is about consumption, but content of your question is about accuracy. So something should be fixed. \$\endgroup\$
    – Chupacabras
    Commented Aug 14, 2017 at 22:59
  • \$\begingroup\$ @chupacabras Yup, fixed \$\endgroup\$
    – Kyle Hunter
    Commented Aug 15, 2017 at 1:12
  • \$\begingroup\$ "Example: 16Mhz crystal, using a clock speed of 8 or 4 mhz to save battery life." - In another question, someone here found that 16MHz + clock divider uses more power than using a lower speed crystal without divider. And intuitively it makes sense this way too. \$\endgroup\$
    – JimmyB
    Commented Aug 15, 2017 at 12:48
  • \$\begingroup\$ "I noticed that it is off by up to a few seconds every minute" - You should be able to calibrate the RC oscillator to better accuracy than that. \$\endgroup\$
    – JimmyB
    Commented Aug 15, 2017 at 12:50

1 Answer 1

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Use a 32.768kHz crystal or tuning fork. It can run asynchronously and you'll save as much power as you can while still having a clock. The system clock will still use the internal RC oscillator.

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  • \$\begingroup\$ OK that sounds like a plan. So the only time the external oscillator is really needed is when you need to increase clock speed or run at an odd frequency? \$\endgroup\$
    – Kyle Hunter
    Commented Aug 14, 2017 at 22:58
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    \$\begingroup\$ Yes. Higher frequency, or a specific frequency. The internal oscillator runs at "about" 8MHz. That's often not good enough for some applications, but crystals can do much, much better. \$\endgroup\$
    – uhours
    Commented Aug 14, 2017 at 23:05
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    \$\begingroup\$ To be more specific: the datasheet says "7.3 - 8.1 MHz". It's a pretty wide range. \$\endgroup\$
    – user39382
    Commented Aug 15, 2017 at 0:29
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    \$\begingroup\$ Add to this: while you can use the separately-clocked counter on the microcontroller as a real time clock, if you're running for a long time on battery sources it would be much more efficient to use a dedicated clock controller chip (e.g. a DS1307) and only power the microcontroller when you actually need it. DS1307s are available for pennies and using one could make your battery last much longer. \$\endgroup\$
    – Jules
    Commented Aug 15, 2017 at 7:58

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