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The system clock speed can be selected via software using the Internal Oscillator Frequency Select bits IRCF<2:0> of the OSCCON register. The system clock can be selected between external or internal clock sources via the System Clock Selection (SCS) bit of the OSCCON register when read mcu at89c51rc2 bin.

The INTOSC and INTOSCIO modes configure the internal oscillators as the system clock source when the device is programmed using the oscillator selection or the FOSC<2:0> bits in the Configuration Word.

In INTOSC mode, OSC1/CLKIN is available for general purpose I/O. OSC2/CLKOUT outputs the selected internal oscillator frequency divided by 4. The CLKOUT signal may be used to provide a clock for external circuitry, synchronization, calibration, test or other application requirements.

In INTOSCIO mode, OSC1/CLKIN and OSC2/CLKOUT are available for general purpose I/O. The High-Frequency Internal Oscillator (HFINTOSC) is a factory calibrated 8 MHz internal clock source. The frequency of the HFINTOSC can be altered via software using the OSCTUNE register (Register 3-2) after extract at89c5115 microcontroller code.

The output of the HFINTOSC connects to a postscaler and multiplexer (see Figure 3-1). One of seven frequencies can be selected via software using the IRCF<2:0> bits of the OSCCON register. See Section 3.5.4 “Frequency Select Bits (IRCF)” for more information.

The HFINTOSC is enabled by selecting any frequency between 8 MHz and 125 kHz by setting the IRCF<2:0> bits of the OSCCON register ≠ 000. Then, set the System Clock Source (SCS) bit of the OSCCON register to ‘1’ or enable Two-Speed Start-up by setting the IESO bit in the Configuration Word register (CONFIG) to ‘1’.

The HF Internal Oscillator (HTS) bit of the OSCCON register indicates whether the HFINTOSC is stable or not.