UPSD3354DV-40U6 Datasheet, PDF (74/272 Pages) STMicroelectronics – fast 8032 MCU with programmable logic

Oscillator and external components
16 Oscillator and external components
UPSD33xx
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 f_OSC, 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
Obsolete ^{Product(s) - Obsolete Product(s) Note:}
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 V_CC or below, and logic high at 0.7 V_CC or above, up to 5.5 V_CC. 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.
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.
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Doc ID 9685 Rev 7