UPSD3422_06 Datasheet, PDF (67/293 Pages) STMicroelectronics – Turbo Plus Series Fast Turbo 8032 MCU with USB and Programmable Logic

uPSD34xx
Oscillator and external components
16 Oscillator and external components
Note:
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 f_OSC, 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 V_CC or below, and logic high at 0.7V V_CC or above, up to 5.5V V_CC. 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.
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.
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