UPSD33XX Datasheet, PDF(51/231 Page) STMicroelectronics – Fast 8032 MCU with Programmable Logic

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UPSD33XX Datasheet, PDF (51/231 Pages) STMicroelectronics – Fast 8032 MCU with Programmable Logic

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uPSD33xx

OSCILLATOR AND EXTERNAL COMPONENTS

The oscillator circuit of uPSD33xx devices is a sin-

gle 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 crys-

tal or ceramic resonator can be used as the feed-

back element to complete the oscillator circuit.

Both are operated in parallel resonance. Ceramic

resonators are lower cost, but typically have a wid-

er frequency tolerance than quartz crystals. Alter-

natively, 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, ce-

ramic resonator, or external clock source is 1MHz

if the I2C interface is not used. The minimum is

8MHz if I2C is used. The maximum is 40MHz in all

cases. This frequency is fOSC, which can be divid-

ed internally as described in MCU CLOCK

GENERATION, page 46.

The pin XTAL1 is the high gain amplifier input, and

XTAL2 is the output. To drive the uPSD33xx de-

vice 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., page 52. 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., page 52 crystals which are

specified to operate in fundamental mode (not

overtone mode) do not need the LC circuit compo-

nents. Since quartz crystals and ceramic resona-

tors have their own characteristics based on their

manufacturer, it is wise to also consult the manu-

facturerâs recommended values for external com-

ponents.

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