ST10F273E datasheet(146/179 Pages) STMICROELECTRONICS

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Electrical characteristics

ST10F273E

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24.8.3

Clock generation modes

Table 69 associates the combinations of these three bits with the respective clock

generation mode.

24.8.4

Prescaler operation

When pins P0.15-13 (P0H.7-5) equal ’001’ during reset, the CPU clock is derived from the

internal oscillator (input clock signal) by a 2:1 prescaler.

The frequency of fCPU is half the frequency of fXTAL and the high and low time of fCPU (i.e.

the duration of an individual TCL) is defined by the period of the input clock fXTAL.

The timings listed in the AC Characteristics that refer to TCL therefore can be calculated

using the period of fXTAL for any TCL.

Note that if the bit OWDDIS in SYSCON register is cleared, the PLL runs on its free-running

frequency and delivers the clock signal for the Oscillator Watchdog. If bit OWDDIS is set,

then the PLL is switched off.

Table 69.

On-chip clock generator selections

P0.15-13

(P0H.7-5)

CPU frequency

fCPU = fXTAL x F

External clock input range (1) (2)

The external clock input range refers to a CPU clock range of 1...64 MHz. Besides, the PLL usage is limited

to 4 to 12 MHz input frequency range. All configurations need a crystal (or ceramic resonator) to generate

the CPU clock through the internal oscillator amplifier (apart from Direct Drive): vice versa, the clock can be

forced through an external clock source only in Direct Drive mode (on-chip oscillator amplifier disabled, so

no crystal or resonator can be used).

The limits on input frequency are 4 to 12 MHz since the usage of the internal oscillator amplifier is required.

Also when the PLL is not used and the CPU clock corresponds to FXTAL/2, an external crystal or resonator

shall be used: it is not possible to force any clock though an external clock source.

Notes

Main OSC (MHz)

111

FXTAL x 4

4 to 8

Default configuration

110

FXTAL x 3

5.3 to 10.6

101

FXTAL x 8

4 to 8

100

FXTAL x 5

6.4 to 12

011

FXTAL x 1

1 to 64

Direct drive

(oscillator bypassed) (3)

The maximum depends on the duty cycle of the external clock signal: When 64 MHz is used, 50% duty

cycle shall be granted (low phase = high phase = 7.8ns); when 32 MHz is selected, a 25% duty cycle can

be accepted (minimum phase, high or low, again equal to 7.8ns).

010

FXTAL x 10

4 to 6.4

001

FXTAL / 2

4 to 12

CPU clock via prescaler(2)

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