ST25R95 datasheet(49/61 Pages) STMICROELECTRONICS

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6.6

Oscillator characteristics

The external crystal used for this product is a 27.12 MHz crystal with an accuracy of ± 14 kHz.

Table 53. HFO 27.12 MHz oscillator characteristics

Symbol

Parameter

Conditions(1)(2) Min. Typ. Max. Unit

fXTAL

Oscillator frequency

27.12

MHz

Feedback resistor

MΩ

Recommended load capacitance versus equivalent serial resistance of the crystal

(RS) (3)

RS = 30 W

tSU(HFO) (4) Startup time

VPS stabilized

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

2. Based on characterization, not tested in production.

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.

4. tSU(HFO) is the startup time measured from the moment it is enabled (by software) to when 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.

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. Typical application with a 27.12 MHz crystal). 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

XOUT

XIN

fHFO

CL1

NFC device

27.12 MHz

crystal

CL2

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.

ST25R95

Oscillator characteristics

DS12807 - Rev 4

page 49/61

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