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ST72262G2 Datasheet(PDF) 21 Page - STMicroelectronics |
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ST72262G2 Datasheet(HTML) 21 Page - STMicroelectronics |
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21 / 171 page  ST72260G, ST72262G, ST72264G 21/171 6.2 MULTI-OSCILLATOR (MO) The main clock of the ST7 can be generated by four different source types coming from the multi- oscillator block: s an external source s 5 crystal or ceramic resonator oscillators s an internal high frequency RC oscillator Each oscillator is optimized for a given frequency range in terms of consumption and is selectable through the option byte. The associated hardware configurations are shown in Table 3. Refer to the electrical characteristics section for more details. Caution: The OSC1 and/or OSC2 pins must not be left unconnected. For the purposes of Failure Mode and Effects Analysis, it should be noted that if the OSC1 and/or OSC2 pins are left unconnect- ed, the ST7 main oscillator may start and, in this configuration, could generate an fOSC clock fre- quency in excess of the allowed maximum (>16MHz.), putting the ST7 in an unsafe/unde- fined state. The product behaviour must therefore be considered undefined when the OSC pins are left unconnected. External Clock Source In this external clock mode, a clock signal (square, sinus or triangle) with ~50% duty cycle has to drive the OSC1 pin while the OSC2 pin is tied to ground. Crystal/Ceramic Oscillators This family of oscillators has the advantage of pro- ducing a very accurate rate on the main clock of the ST7. The selection within a list of 5 oscillators with different frequency ranges has to be done by option byte in order to reduce consumption (refer to Section 15.1 on page 157 for more details on the frequency ranges). In this mode of the multi- oscillator, the resonator and the load capacitors have to be placed as close as possible to the oscil- lator pins in order to minimize output distortion and start-up stabilization time. The loading capaci- tance values must be adjusted according to the selected oscillator. These oscillators are not stopped during the RESET phase to avoid losing time in the oscillator start-up phase. Internal RC Oscillator This oscillator allows a low cost solution for the main clock of the ST7 using only an internal resis- tor and capacitor. Internal RC oscillator mode has the drawback of a lower frequency accuracy and should not be used in applications that require ac- curate timing. In this mode, the two oscillator pins have to be tied to ground. Table 3. ST7 Clock Sources Hardware Configuration OSC1 OSC2 EXTERNAL ST7 SOURCE OSC1 OSC2 LOAD CAPACITORS ST7 CL2 CL1 OSC1 OSC2 ST7 |
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