@MarkT and @dhenry:
Very valuable and interesting-to-read points. I hadn't considered the fact that the peripherals also consume additional current while the uC is awake longer.
So, it appears I can get good savings by using the 8Mhz and just going back to sleep quicker, and thus also without risking errors (esp. for SdCard's worst-case instruction count).
I am also curious about a couple of other things now:
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(1) What kinds of applications are low clock speeds (e.g., 1 Mhz or lower) advantageous for? Or is the datasheet listing them just for theoretical reference?
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(2) What exactly is counted as "Idle" (versus Active) when the microcontroller is running my code? The table you see above also infers that there is a sizable reduction in the supply current in the Idle condition vs. Active condition.