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Is the two-clock start-up technique common these days?

Is the two-clock start-up technique common these days?

Came across a low power technique for microcontrollers.  A crystal may take few hundred milliseconds to start up.  The MCU is consuming power, and not executing instructions during that time.  Imagine a situation where an MCU is in cyclic sleep, and it needs to do only 20ms worth of work every second¹.  It would have to spend more time waiting for the crystal to start up than doing the work.

![crystal xtal start up, oscilloscope screenshot](https://electrical.codidact.com/uploads/4kdtz57u9by6adt8ppdmi9t8pkdk)

An internal RC is quicker to start up: down to single-digit microseconds.  But an internal RC is less accurate.

![internal RC oscillator start up, oscilloscope screenshot](https://electrical.codidact.com/uploads/rczr0zoxz4sejuoj1kwf96jvx2ty)

The proposed technique is two-clock start-up.  The MCU wakes up and starts the internal RC.  If the MCU determines that it will be doing something that requires a precise clock², then it starts the crystal and waits the necessary time for it to come up.  Otherwise, the MCU continues on the internal RC, and goes back to sleep.

I came across this technique in the book *A Baker’s dozen* by Bonnie Baker (2005).  There’s also this [EE Times article by Bonnie Baker (2003)](https://www.eetimes.com/battery-apps-benefit-from-mcu-clocking-mode-control-and-low-power-peripherals).  There’s a [firmware doc for 8-bit PICs](https://developerhelp.microchip.com/xwiki/bin/view/products/mcu-mpu/8bit-pic/oscillator-options/two-speed-clock-start-up).  Bonnie Baker worked for Microchip during the 8-bit PIC era, so all these sources have the common origin.

¹  Something like checking the “on” button (which nobody presses for days), and maybe monitoring state of charge.  

²  That would happen only once in a while, and not every cyclic sleep epoch.  For example, it could be communication.