2 Clock management

AN2168

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2

Clock management

Internal operation of CPU and peripherals are controlled by the same main clock named Fcpu.

It is provided by the on-chip clock generator. The RTC module can be controlled either by the

ST10F27x main clock or by the auxiliary 32 kHz oscillator.

2.1

Main clock control unit

The ST10F27x core and peripherals can operate with a frequency up to 64 MHz.

The ST10F27x main clock can be:

●

generated using the ST10F27x internal oscillator amplifier. In this case, the input signal for

the internal oscillator is generated by an external crystal or resonator with a frequency

between 4MHz and 12 MHz, and is connected between XTAL1 and XTAL2: XTAL1 is the

input and XTAL2 is the output. This configuration is available when using on chip PLL or

the prescaler operation.

●

forced by an external clock when the direct drive option is selected. In this case, the

internal oscillator is disabled and neither the external crystal nor resonator is needed. The

external clock is input on the XTAL1 pin of the ST10F27x and the XTAL2 pin must be left

unconnected.

The clock generation mode is selected during reset according to port 0 lines: P0H.5, P0H.6,

P0H.7. Each mode is described below.

2.1.1

Direct Drive operation

When pins P0H.7 = 0, P0H.6 = 1, P0H.5 = 1 during reset, the CPU clock is directly driven from

the external generator connected to XTAL1 pin. In this case, neither the crystal nor the

resonator is used.

Figure 1.

ST10F27x Direct Drive

2.1.2

Prescaler operation

When pins P0H.7 = 0, P0H.6 = 0, P0H.5 = 1 during reset, the CPU clock is derived from the

internal oscillator amplifier. The input clock signal is generated by a crystal or a resonator with a

frequency between 4MHz and 8 Mhz. The CPU frequency is equal to the input signal on pin

XTAL1 divided by 2

External Clock

XTAL1

XTAL2

Unconnected

1 .. 64 MHz

P0H.5 = 1

VDD

P0H.6 = 1

VDD

P0H.7 = 0

VSS

8 KOhms