6. Switched-Capacitor Power Electronics Circuits, Adrian Ioinovici. 7. “State-of-the-Art Monolithic Switched-Capacitor Voltage Regulators for Ultra-Low Power Internet of Things,” Kang Wei ...

Or, using a fifth- or seventh-order switched-capacitor filter with a square-wave output ties up two I/O pins on the MCU. One output is used for the filter input and one for the filter clock.