UPSD3422 datasheet(67/293 Pages) STMICROELECTRONICS

67 / 293 page

uPSD34xx

Oscillator and external components

67/293

Oscillator and external components

The oscillator circuit of uPSD34xx 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 uPSD34xx 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

3MHz if the USB is used. The minimum is 8MHz if I2C is used. The maximum is 40MHz in all

cases. This frequency is fOSC, which can be divided internally as described in Section 14:

MCU clock generation on page 59.

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

uPSD34xx 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 0.7V VCC or above, up to 5.5V VCC. The XTAL1 input is

5V tolerant.

Most of the quartz crystals in the range of 25MHz to 40MHz 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 15 on page 68. 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 15 on page 68 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.

UPSD3422 Datasheet(PDF) 67 Page - STMicroelectronics