Why are there so many different integrated oscillators on this product? When would you typically use each of them?
The C8051F9xx devices have four different oscillators that may be used to derive the system clock. The precision internal oscillator allows for increased integration and external component elimination. This 24.5 MHz oscillator eliminates the need for an external crystal oscillator or resonator. With a spec of ±2 % across all voltage and temperature conditions, it is accurate enough to generate the communication clock for the standard UART baud rates. It consumes approximately 300 μA of current. A spread spectrum mode is provided to minimize electromagnetic interference (EMI).
For applications that do not require ±2 % accuracy, the low-power internal oscillator may be used. It is a 20 MHz oscillator with ±10 % accuracy across all voltage and temperature conditions, and it consumes less than 100 μA. The C8051F9xx external oscillator provides four modes of operation—external crystal, capacitor, RC network, or external CMOS clock source—that allow the user to set a clock frequency between 20 kHz and 25 MHz.
Finally, the smaRTClock peripheral includes a very low power 32.768 KHz crystal oscillator that can be used both as the clocking source for the real time clock and as the system clock. The smaRTClock oscillator has a self-oscillate mode (with provision for internal digital calibration of the oscillation frequency) for low-cost timekeeping applications that do not require the accuracy of a crystal.