I was at a Motorola seminar (no flames please :-) about the HCS08 low power
chips. We did a lab experiment that proved conclusively the key to power
savings is to sleep as much as possible and run as fast as you can when you
aren't sleeping. The power savings were significant.

-Denny


> Power increases linearly with switching speed, so 20 MHz will
> draw ~5 times as much current as 4 MHz, but will also do the
> job 5 times faster.  I'd bet that the overall energy consumption
> wouldn't differ too greatly, with the 4 MHz having a tiny lead
> due to lead and source impedance losses of the higher current
> (the whole I^2*R thing).
>
> The important thing to consider is peripheral operations.  At
> 20 MHz, your peripherals (external ADCs or memories and such)
> will be on for a much shorter time, because you'll be able to
> read/write to/from them much faster (assuming you're using
> SPI; I2C probably maxes out well before 5 or even 1 MHz).
>
> This maybe a gross over simplification.  Other factors to consider
> are data transfer out of the device (USART), EMI and sampling
> rates.  Finally, consider using a PIC18F series that can have the
> clock switched between a relatively slower internal RC and a
> faster external oscillator for USART transmission.
>
> Mike H.
>
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