# AtTiny85 128khz clock and power consumption

I am wondering about the Tiny85 power consumption and methods.
I cannot find answers for the following questions in the datasheet, maybe I will find it here..

The 128Khz internal watchdog clock should use less power than the 8Mhz clock [choosing 128khz will not use the 8Mhz clock, which save power].
I am assuming 8Mhz with prescaler of 64 (gives 128khz) will be waste of power, right? I can just use the 128khz internal oscillator?

Theoretically, can I use the clock prescaler even with the 128Khz clock (CLKPR register)? If yes, I can set it to /256 = 500hz clock, which should save me some more power. In the real life I cannot do it, I need ADC, which the minimum prescaler is 2, so 128Khz will give me 64kHz at max, while the range of the ADC is 50-200khz which fit well. Am I right?

Another aspect is the output PORTs. Setting all of the output ports to be input should save some more. but Should I set them to 1 or 0 (in terms of pull up resistance) to save more power?

And last thing I need to clarify - Before powering down, I am disabling ADC in order to reduce consumption. I am setting ADEN to 0 on ADCSRA.
I read about PRR register on the datasheet and I saw I can shut TIMER0, TIMER1, USI, ADC.
Will it change the power down consumption?

Thanks.

• I'm sorry, but this kind of annoys me every time: The unit of frequency, Hertz, is abbreviated Hz, not hz. The magnitude symbol for 1000, kilo, is abbreviated k, not K. For a million, mega, the abbreviation is M. So, 128,000 oscillations per second are 128 kHz (there's a space between number and unit, and note the small k and the capital H in kHz), and eight million oscillations per second are 8 MHz. Oct 31, 2016 at 16:26
• You made it worse!!! It's kHz, not khz! Oct 31, 2016 at 16:32
• Too many questions for one question. Maybe narrow down to a specific ask? Oct 31, 2016 at 16:52
• Long answer short: yes, most of the things you propose will probably save some power. Maybe a little, maybe a lot. Try it and measure :) Oct 31, 2016 at 17:01
• It is in the datasheet. Section 24.1,2,3,4 Oct 31, 2016 at 23:34