UPSD3312D datasheet(74/272 Pages) STMICROELECTRONICS

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Oscillator and external components

UPSD33xx

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Doc ID 9685 Rev 7

Oscillator and external components

The oscillator circuit of UPSD33xx devices is a single stage, inverting amplifier in a Pierce

oscillator configuration. The internal circuitry between pins XTAL1 and XTAL2 is basically an

inverter biased to the transfer point. Either an external quartz crystal or ceramic resonator

can be used as the feedback element to complete the oscillator circuit. Both are operated in

parallel resonance. Ceramic resonators are lower cost, but typically have a wider frequency

tolerance than quartz crystals. Alternatively, an external clock source from an oscillator or

other active device may drive the UPSD33xx oscillator circuit input directly, instead of using

a crystal or resonator.

The minimum frequency of the quartz crystal, ceramic resonator, or external clock source is

1 MHz if the I2C interface is not used. The minimum is 8 MHz if I2C is used. The maximum is

40 MHz in all cases. This frequency is fOSC, which can be divided internally as described in

Section 14: MCU clock generation on page 68.

The pin XTAL1 is the high gain amplifier input, and XTAL2 is the output. To drive the

UPSD33xx device externally from an oscillator or other active device, XTAL1 is driven and

XTAL2 is left open-circuit. This external source should drive a logic low at the voltage level of

0.3 VCC or below, and logic high at 0.7 VCC or above, up to 5.5 VCC. The XTAL1 input is 5 V

tolerant.

Most of the quartz crystals in the range of 25 MHz to 40 MHz operate in the third overtone

frequency mode. An external LC tank circuit at the XTAL2 output of the oscillator circuit is

needed to achieve the third overtone frequency, as shown in Figure 14 on page 75. Without

this LC circuit, the crystal will oscillate at a fundamental frequency mode that is about 1/3 of

the desired overtone frequency.

Note:

In Figure 14 on page 75 crystals which are specified to operate in fundamental mode (not

overtone mode) do not need the LC circuit components. Since quartz crystals and ceramic

resonators have their own characteristics based on their manufacturer, it is wise to also

consult the manufacturer’s recommended values for external components.

UPSD3312D Datasheet(PDF) 74 Page - STMicroelectronics