Electronics > Beginners
Decoupling capacitor for ATtiny
Lee697:
I have a small circuit I'm testing - very basic.....
ATtiny85V, powered by a CR2032 (1, or 2 in parallel). It flashes a morse code sequence to a 3mm red LED, that's it.
LED draws approx 2mA when on, ATtiny draws 0.9mA in between.
I've just had it run for 240hr continuously to assess battery life, and it appeared to do well - surprisingly it was still functioning, seemingly normally, at 1.7V, although the LED was getting pretty dim.....
I wasn't using a decoupling cap in this circuit. I did note Vcc swinging about as the LED flashed.
In reading, a decoupling cap should be installed across Vcc/GND, so I'll do this - but what sort of cap, and what value? Battery life is a priority here - should I expect this to degrade battery life? (I'll test it anyway) Cheers!
Audioguru again:
When the battery becomes weak then its high internal resistance causes the LED to be dim. Try adding a 10uF electrolytic capacitor for it to charge when the LED is off and help power the LED when it should be on to make the LED brighter. Then of course the battery life will be reduced.
james_s:
a 100nf ceramic capacitor across the power pins as close to the IC as practical is always a good idea. The electrolytic mentioned may also be helpful when using small batteries. So long as the capacitor has a low leakage current it should not affect the battery life.
Lee697:
To test this.... is it mainly just a case of checking the voltage trace at Vcc with the scope, and trying different cap values to flatten it out?
T3sl4co1l:
Yes, in fact that procedure provides more confidence than "whack a 0.1 on it" does. :-+
It is very likely that 0.1 will be an adequate value, and best placed at the point of measurement, though. :)
Tim
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