There are numerous ways in which one might want to use multiple crystals on a single MCU. One of these is to match the rate of an externally driven process. Another might be to synchronize with such a process being driven externally.

While it's true that some tasks won't be in any sense impaired by the multi-second reconfiguration and acquisition-time delays imposed by the PLL that's in the synthesizer, others will be negatively impacted, and, many others may not work at all because of the delay.

If you have two different rates at which you need the MCU to operate, there's usually a driving reason, one of which might be a different rate for, for example, a communication task, or a situation like what's in front of me right now, where I have a process that operates at 20.000 MHz and another that uses 19.6608 MHz (sound familiar? it's an exact harmonic of the common baud rates). When I'm processing data at the 20 MHz rate, it's very handy to be able to remain in sync with that task, while serial communication is easy using the 196608 MHz clock.

How long does it take the 'F12x to switch from one reference frequency to the other using the PLL?

RE