When you use the PSOC-internal RTC for keeping time, you need to ensure that the PSoC runs continously. You can do this by adding a backup battery - but then you should make sure the PSoC runs in sleep mode to ensure long battery life.
Another option is to use an external RTC, and back this up with a battery (or maybe only a large capacitor). For example the PCF2129A from NXP draw only 650nA in backup mode, and is internally temperature-compensated. It should last several weeks from a 1000µF capacitor. This might be a better option since it is more accurate, and doesnÄt need power management in the PSoC.
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Obviously two capacitors are better than one (broad smile)
http://www.cypress.com/?id=4&rID=51941 -> Refer to this KB article. If you want to calculate the value of Capacitor you want to use to run RTC even when the power supply if OFF, you need to know the following parameters:
1. What is the duration of time for which main power supply will be OFF?
2. What is the power consumption of the device during active & during sleep modes?
3. How much time does it take for you to detect the failure of main supply and move onto low power mode?
4. Based on the above points, you can calculate the average current you need to provide the device for continuous operation. Also find out the peak current required. Calculate the value of Capacitor which can provide this peak current and has the enough amount of energy stored in it.
But be aware that when using a super cap, its leakage current might be higher that the current needed by the RTC (several microamps are not uncommon for larger caps). Also, there caps often have a rather high impedance and can be used only with small currents (I have one at home which is rated only for 10 microamps and has an internal resistance of 1k).
There are super caps with mOhm ESR, and low uA self discharge.
Some reference material -
So in short Q = CV, I = C * dV/dT, use this equation to get a size of C needed. I is sum
of PSOC sleep and capacitor self discharge. Hot values generally worst case. dT is how long
you want cap to sustain PSOC in sleep. C is a f(V) and a f(T), so use min value in your
calculations as worst case on the bacvkup side. Max value for when you are charging C.
dV is fully charged - PSOC min operating V in sleep.
The other issue is calculating how long it will take in your circuit to charge the super cap.
Panasonic, NEC-Tokin, Cooper, Maxwell vendors.
Oh, I know that they exist. It's just something you need to check beforehand.
But OTOH, it might be simpler to just connect a CR2032 or so to the battery input of the RTC - no hassle with charging and it will last several years.