STR71XF_07 datasheet(43/77 Pages) STMICROELECTRONICS

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STR71xF

Electrical parameters

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OSC32K crystal / Ceramic resonator oscillator

The STR7 RTC clock can be supplied with a 32kHz Crystal/Ceramic resonator oscillators.

All the information given in this paragraph are based on characterization results with

specified typical external components. In the application, the resonator 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 stabilization time. Refer to the crystal resonator manufacturer

for more details (frequency, package, accuracy...).

Notes:

The oscillator selection can be optimized in terms of supply current using an high quality resonator with

small RS value for example MSIV-TIN32.768kHz. Refer to crystal manufacturer for more details

tSU(OSC32KHZ) is the start-up time measured from the moment it is enabled (by software) to a stabilized

32kHz oscillation is reached. This value is measured for a standard crystal resonator and it can vary

significantly with the crystal manufacturer

Figure 15.

Typical Application with a 32kHz Crystal

Table 19.

32K Oscillator characteristics (fOSC32K= 32.768kHz)

Symbol

Parameter

Conditions

Typ

Unit

Feedback resistor

2.7

Ω

CL1

CL2

Recommended load capacitance

versus equivalent serial resistance

of the crystal (RS)

RS=40KΩ

12.5

RTCXT2 driving current

V33=3.3V

VIN=VSS

3.2

µA

Oscillator Transconductance

µA/V

tSU(OSC32KHZ)

startup time

V33 is stabilized

RTCXT2

RTCXT1

fOSC32K

CL1

CL2

STR710

32KHz

WHEN RESONATOR WITH

INTEGRATED CAPACITORS

RESONATOR

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