Why not put the crystal oscillator inside the MCU itself, rather than using an RC internal oscillator? I could think of the following possibilities but not sure if they are valid:
- MCU manufacturer wants to keep a low price of their product. A crystal will be more expensive than RC oscillator.
- Designers might want to skip the crystal in a majority of the designs.
- Designers need the flexibility to choose the right crystal for the same MCU depending upon their application.
- Packing a crystal inside the MCU is not feasible.
I personally am unable to relate with 2 and 3. I typically use crystals in all circuits and just follow what they recommend in the application note. It will be great if they take the headache of layout, correct capacitor calculation, etc. away from me. It would save PCB space, and probably a packaged microcontroller would be cheaper than buying a microcontroller and crystal separately.
Are there other valid reasons for not putting the crystal inside the microcontroller?
I feel that having a precise internal crystal oscillator would be cost-effective and convenient to design, in general. At least for micrcontrollers that most probably need a crystal for functioning. Examples - any RF chip (wifi, BLE, zigbee.)
I recently came across what they call SIP modules for some wireless chips (EFR family from Silicon Labs and CC2652R from TI), where they make a 7 mm X 7 mm chip-like design containing a crystal, RF circuit etc. Designers just need to connect the antenna and other basic components.
I am wondering why not do it with general microcontrollers as well.