How can I optimize my code for a low-power microcontroller to increase its battery life?

I totally get it - you want to squeeze as much life out of your battery as possible. One thing you can try is to disable any features or peripherals on your microcontroller that you're not using. For example, if you're not using the USB or SPI interfaces, make sure to disable them to save power.

Another thing that can help is to use the microcontroller's sleep modes. Most microcontrollers have a low-power sleep mode that you can put the chip into when it's not actively doing anything. You can use the chip's built-in timers or interrupts to wake it up when something needs to happen. For example, if you have a timer that triggers an interrupt every 10 seconds, you can put the chip to sleep until the interrupt fires and then wake it back up. This can save a lot of power.

Regarding clock stretching and voltage scaling, these are usually done at the hardware level and not directly in your code. However, you can use the microcontroller's registers to adjust the clock speed and voltage if your chip supports it. For example, on the Arduino boards, you can use the BrownOutDetector and PowerManagement libraries to adjust the voltage and clock speed. The specifics will depend on your microcontroller, so check the datasheet or look for example code online.

As for libraries, the LowPower library for Arduino is a good place to start. It provides a lot of useful functions for putting the chip to sleep and waking it back up. You can also check out the PowerManagement library, which provides functions for adjusting the voltage and clock speed. Finally, make sure to use the sleep function to put the chip to sleep when it's not actively doing anything.