I'm trying to figure out how much power I can expect my ATTiny84 to use in idle mode and I'm not sure which of the two graphs from the datasheet I should use, or why there are two in the first place. I intend to run it at a clock speed of 1MHz and off of a 3V battery. Also, if I choose to run it using an 8MHz crystal with CKDIV8 set (so the clock speed is 1MHz), should I read the graph at 8MHz or 1MHz?

enter image description here enter image description here

  • \$\begingroup\$ "I intend to run it at a clock speed of 1MHz" - with the internal RC oscillator? \$\endgroup\$ – Bruce Abbott May 24 at 4:26
  • \$\begingroup\$ No, with an external crystal. \$\endgroup\$ – Anubhav Srivastava May 24 at 5:06

The top image is when using an external clock source and does not include any current used by that external clock source

The bottom image is when using the internal RC clock source at 1MHz, using that circuitry draws some power,

If your using the internal clock source, you would be using the bottom graph.

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  • \$\begingroup\$ I see. I'm using an external crystal, so I should use the top graph. \$\endgroup\$ – Anubhav Srivastava May 24 at 5:43
  • \$\begingroup\$ The top graph + the energy used by the capacitive loading of the crystal circuit, I presume this is poorly documented in the datasheet because its hard for them to predict what loading capacitance your crystal will have, essentially some energy is lost each time the crystal flips, I believe this will not be a large variance from that top chart, or they would have documented it better \$\endgroup\$ – Reroute May 24 at 5:53
  • \$\begingroup\$ Thank you! Do you have an answer for the second question by the way (re: CKDIV8)? \$\endgroup\$ – Anubhav Srivastava May 24 at 6:11
  • \$\begingroup\$ you would read it as 8MHz, in both cases that 8MHz RC is running, the difference is switching all the electronics in the micro at 8x the rate draws more power. \$\endgroup\$ – Reroute May 24 at 6:23

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