ST72340 datasheet(161/191 Pages) STMICROELECTRONICS

161 / 191 page

ST72340, ST72344, ST72345

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CLOCK AND TIMING CHARACTERISTICS (Cont’d)

13.7.4 Crystal and Ceramic Resonator Oscillators

The ST7 internal clock can be supplied with four

different Crystal/Ceramic resonator oscillators. All

the information given in this paragraph is based on

characterization results with specified typical ex-

ternal components. In the application, the resona-

tor and the load capacitors have to be placed as

close as possible to the oscillator pins in order to

minimize output distortion and start-up stabiliza-

tion time. Refer to the crystal/ceramic resonator

manufacturer for more details (frequency, pack-

age, accuracy...).

Notes:

1. The oscillator selection can be optimized in terms of supply current using an high quality resonator with small RS value.

Refer to crystal/ceramic resonator manufacturer for more details.

2. Data based on characterisation results, not tested in production. 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 con-

dition change. However, it is recommended to take this point into account if the µC is used in tough humidity conditions.

3. For CL1 and CL2 it is recommended to use high-quality ceramic capacitors in the 5-pF to 25-pF range (typ.) designed

for high-frequency applications and selected to match the requirements of the crystal or resonator. CL1 and CL2, are usu-

ally the same size. The crystal manufacturer typically specifies a load capacitance which is the series combination of CL1

and CL2. PCB and MCU pin capacitance must be included when sizing CL1 and CL2 (10 pF can be used as a rough esti-

mate of the combined pin and board capacitance).

Figure 94. Typical Application with a Crystal or Ceramic Resonator

Symbol

Parameter

Conditions

Min

Max

Unit

fOSC

Oscillator Frequency 1)

116

MHz

Feedback resistor2)

Ω

CL1

CL2

Recommended load capacitance ver-

sus equivalent serial resistance of the

crystal or ceramic resonator (RS)

fOSC= 1 to 2 MHz

fOSC= 2 to 4 MHz

fOSC= 4 to 8 MHz

fOSC= 8 to 16 MHz

Symbol

Parameter

Conditions

Typ

Max

Unit

OSC2 driving current

VDD=5V:

fOSC= 2MHz, C0 = 6pF, Cl1 = Cl2 = 68pF

fOSC= 4MHz, C0 = 6pF, Cl1 = Cl2 = 68pF

fOSC= 8MHz, C0 = 6pF, Cl1 = Cl2 = 40pF

fOSC= 16MHz, C0 = 7pF, Cl1 = Cl2 = 20pF

426

425

456

660

µA

OSC2

OSC1

fOSC

CL1

CL2

ST72XXX

RESONATOR

WHEN RESONATOR WITH

INTEGRATED CAPACITORS

VDD/2

Ref

POWER DOWN

LOGIC

FEEDBACK

LOOP

LINEAR

AMPLIFIER

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