Some processors have a low frequency mode for using in low power mode.

For example MSP430 microcontroller can work in both

  • Normal clock frequency 16M Hz
  • Low clock frequency 32 kHz

I am wondering if in general, sleep mode or low power mode for processors is about reducing the frequency to save power or is it a concept beyond that? What happens in low power mode/sleep mode to save power (on processor)?

  • \$\begingroup\$ The MSP430, in particular, is quite good powering up to full speed after being activated. You can operate from the VLO or an external 32 kHz xtal with almost no power at all and be up and running full speed in one to two microseconds. You can go from LPM4 to full up speed in about six microseconds, using a comparator interrupt for example. It's almost always better with the MSP430 to burst-run at full speed and then shut down, than to tick along at a slow speed. This is not the case with PIC "nanopower" devices, though. So what choices you make vary. We need a lot more info to provide advice. \$\endgroup\$ – jonk Jun 29 '19 at 5:04

The majority of current draw in CMOS parts occurs when signal switch state. So, to conserve power you want to minimize signals switching states. Clock nets are large and therefore have a lot of capacitance, so you want to disable clocks to circuits that aren't being used.

You might disable all clocks except the timers so a timer interrupt can wake-up the processor (from a low-power mode). Or, you could use an external signal to wake-up the processor.

Table 2.2 in this processor user's guide lists some of the power-down modes. http://www.ti.com/lit/ug/slau144j/slau144j.pdf


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