RX95HF Datasheet, PDF (33/44 Pages) STMicroelectronics – Near field communication transceiver for tag emulation

RX95HF
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.
Symbol
Table 37. HFO 27.12 MHz oscillator characteristics^{(1) (2)}
Parameter
Conditions Min. Typ. Max. Unit
f_XTAL Oscillator frequency
R_F
Feedback resistor
C
t_SU(HFO)(4)
Recommended load capacitance
versus equivalent serial resistance of
the crystal (R_S)⁽³⁾
Startup time
R_S = 30
V_PS is stabilized
27.12
2
MHz
M
6
pF
6 10 ms
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 R_F 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. t_SU(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.
For C_L1 and C_L2, 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 12). C_L1 and C_L2 are usually the
same size. The crystal manufacturer typically specifies a load capacitance which is the
series combination of C_L1 and C_L2.
Figure 12. Typical application with a 27.12 MHz crystal
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For C_L1 and C_L2 it is recommended to use high-quality ceramic capacitors in the 10 pF to
20 pF range selected to match the requirements of the crystal or resonator. C_L1 and C_L2, are
usually the same size. The crystal manufacturer typically specifies a load capacitance which
is the series combination of C_L1 and C_L2.
Load capacitance C_L has the following formula: C_L = C_L1 x C_L2 / (C_L1 + C_L2) + C_stray where
C_stray is the pin capacitance and board or trace PCB-related capacitance. Typically, it is
between 2 pF and 7 pF.
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