STR750F datasheet(45/73 Pages) STMICROELECTRONICS

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STR750F

Electrical parameters

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4/8 MHz Crystal / Ceramic Resonator Oscillator (XT1/XT2)

The STR750 system clock or the input of the PLL can be supplied by a OSC4M which is a 4

MHz clock generated from a 4 MHz or 8 MHz crystal or ceramic resonator. If using an 8 MHz

oscillator, software set the XTDIV bit to enable a divider by 2 and generate a 4 MHz OSC4M

clock. 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 21.

Typical application with a 4 or 8 MHz crystal or ceramic resonator

Table 21.

4/8 MHz crystal / ceramic resonator oscillator (XT1/XT2)(1)

Symbol

Parameter

Conditions

Min

Typ

Max

Unit

fOSC4M

Oscillator frequency

4 MHz Crystal/Resonator Oscillator

connected on XT1/XT2 XTDIV=0

8 MHz Crystal/Resonator Oscillator

connected on XT1/XT2 XTDIV=1

4MHz

Feedback resistor

200

240

270

Ω

CL1

(2)

CL2

Recommended load

capacitance versus equivalent

serial resistance of the crystal or

ceramic resonator (RS)

(3)

RS=200Ω

XT2 driving current

VDD_IO=3.3 V or 5.0 V

425

µA

tSU(OSC4M)

(4)

Startup time at VDD_IO power-up

Resonator characteristics given by the crystal/ceramic resonator manufacturer.

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 usually 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 estimate of the

combined pin and board capacitance).

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 MCU is used in tough humidity conditions.

tSU(OSC4M) is the typical start-up time measured from the moment VDD_IO is powered (with a quick VDD_IO ramp-up from 0

to 3.3V (<50

µs) to a stabilized 4MHz oscillation is reached. This value is measured for a standard crystal resonator and it

can vary significantly with the crystal/ceramic resonator manufacturer.

XT2

XT1

fOSC4M

CL1

CL2

STR75X

RESONATOR

WHEN RESONATOR WITH

INTEGRATED CAPACITORS

VDD/2

Ref

FEEDBACK

LOOP

LINEAR

AMPLIFIER

XTDIV

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