STR750FV0 datasheet(47/84 Pages) STMICROELECTRONICS

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Electrical parameters

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OSC32K crystal / ceramic resonator oscillator

The STR7 RTC clock can be supplied with a 32.768 kHz Crystal/Ceramic resonator

oscillator. All the information given in this paragraph are based on product characterisation

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/ceramic resonator

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

Figure 22.

Typical application with a 32.768 kHz crystal or ceramic resonator

PLL characteristics

PLL Jitter Terminology

●

Self-referred single period jitter (period jitter)

Period Jitter is defined as the difference of the maximum period (Tmax) and minimum

period (Tmin) at the output of the PLL where Tmax is the maximum time difference

between 2 consecutive clock rising edges and Tmin is the minimum time difference

between 2 consecutive clock rising edges.

See Figure 23

●

Self-referred long term jitter (N period jitter)

Self-referred long term Jitter is defined as the difference of the maximum period (Tmax)

and minimum period (Tmin) at the output of the PLL where Tmax is the maximum time

Table 23.

OSC32K crystal / ceramic resonator oscillator

Symbol

Parameter

Conditions

Min

Typ

Max

Unit

fOSC32K

Oscillator Frequency

32.768

kHz

Feedback resistor

VDD_IO=3.3 V or 5.0 V

270

310

370

Ω

CL1

CL2

Recommended load capacitance

versus equivalent serial resistance of

the crystal or ceramic resonator (RS)

(1)

RS=40KΩ

12.5

XT2 driving current

VDD_IO=3.3 V or 5.0 V

VIN=VSS

µA

tSU(OSC32K)

(2)

Startup time

VDD_IO is stabilized

2.5

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

tSU(OSC32K) is the start-up time measured from the moment it is enabled (by software) to a stabilized 32 kHz oscillation is

reached. This value is measured for a standard crystal resonator and it can vary significantly with the crystal/ceramic

resonator manufacturer

XRTC2

XRTC1

fOSC32K

CL1

CL2

STR750

32 kHz

WHEN RESONATOR WITH

INTEGRATED CAPACITORS

RESONATOR

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