ST95HF datasheet(59/93 Pages) STMICROELECTRONICS

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ST95HF

Electrical characteristics

6.6

Oscillator characteristics

The external crystal used for this product is a 27.12 MHz crystal with an accuracy of

± 14 kHz.

For CL1 and CL2, it is recommended to use high-quality external ceramic capacitors in the

10 pF to 20 pF range (typ.), designed for high-frequency applications, and selected to match

the requirements of the crystal or resonator (see Figure 14). CL1 and CL2 are usually the

same size. The crystal manufacturer typically specifies a load capacitance which is the

series combination of CL1 and CL2.

Figure 14. Typical application with a 27.12 MHz crystal

Note:

For CL1 and CL2 it is recommended to use high-quality ceramic capacitors in the 10 to 20 pF

range, selected to match the requirements of the crystal or resonator. CL1 and CL2, have

usually the same size. The crystal manufacturer typically specifies a load capacitance which

is the series combination of CL1 and CL2.

Load capacitance CL has the following formula: CL = CL1 x CL2 / (CL1 + CL2) + Cstray, where

Cstray (typically between 2 and 7 pF) is the pin capacitance and board or trace PCB-related

capacitance.

Table 53. HFO 27.12 MHz oscillator characteristics(1) (2)

1. Resonator characteristics given by the crystal/ceramic resonator manufacturer.

2. Based on characterization, not tested in production.

Symbol

Parameter

Conditions

Min.

Typ.

Max.

Unit

fXTAL

Oscillator frequency

27.12

MHz

Feedback resistor

Recommended load capacitance

versus equivalent serial resistance of

the crystal (RS)(3)

3. The relatively low value of the RF resistor offers a good protection against issues resulting from use in a

humid environment, due to the induced leakage and the bias condition change. However, it is

recommended to take this point into account if the Host is used in tough humidity conditions.

RS = 30 Ω

-6-

tSU(HFO)(4)

4. tSU(HFO) is the startup time measured from the moment it is enabled (by software) to a stabilized 27.12 MHz

oscillation is reached. This value is measured for a standard crystal resonator and it can vary significantly

with the crystal manufacturer.

Startup time

VPS is stabilized

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