ST10F271 datasheet(150/173 Pages) STMICROELECTRONICS

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Electrical characteristics

ST10F271

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The given values of CA do not include the stray capacitance of the package and of the

printed circuit board: the negative resistance values are calculated assuming additional 5pF

to the values in the table. The crystal shunt capacitance (C0) and the package capacitance

between XTAL3 and XTAL4 pins is globally assumed equal to 4pF. The external resistance

between XTAL3 and XTAL4 is not necessary, since already present on the silicon.

Warning:

Direct driving on XTAL3 pin is not supported. Always use a

32kHz crystal oscillator.

24.8.13

External clock drive XTAL1

When Direct Drive configuration is selected during reset, it is possible to drive the CPU clock

directly from the XTAL1 pin, without particular restrictions on the maximum frequency, since

the on-chip oscillator amplifier is bypassed. The speed limit is imposed by internal logic that

targets a maximum CPU frequency of 64MHz.

In all other clock configurations (Direct Drive with Prescaler or PLL usage) the on-chip

oscillator amplifier is not bypassed, so it determines the input clock speed limit. Then, when

the on-chip oscillator is enabled it is forbidden to use any external clock source different

from crystal or ceramic resonator.

The minimum value for the XTAL1 signal period shall be considered as the theoretical minimum. The real

minimum value depends on the duty cycle of the input clock signal.

4-8 MHz is the input frequency range when using an external clock source. 64 MHz can be applied with an

external clock source only when Direct Drive mode is selected: in this case, the oscillator amplifier is

bypassed so it does not limit the input frequency.

The input clock signal must reach the defined levels VIL2 and VIH2.

Table 74.

Minimum values of negative resistance (module) for 32kHz oscillator

CA = 6pF

CA = 12pF CA = 15pF CA = 18pF CA = 22pF CA = 27pF CA = 33pF

32kHz

---

150 k

Ω

120 k

Ω

90 kW

Table 75.

External clock drive

Parameter

Symbol

Direct drive

fCPU = fXTAL

Direct drive with

prescaler

fCPU = fXTAL / 2

PLL usage

fCPU = fXTAL x F

Unit

min.

max.

min.

max.

min.

max.

XTAL1 period 1, 2 tOSC SR

15.625

–

83.3

250

83.3

250

High time 3

–3

–6

–

Low time 3

6–3

–6

–

Rise time 3

–2–

2–

Fall time 3

–2–

2–

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