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ST72E311 Datasheet(PDF) 17 Page - STMicroelectronics |
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ST72E311 Datasheet(HTML) 17 Page - STMicroelectronics |
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17 / 101 page  17/101 ST72E311 ST72T311 3 CLOCKS, RESET, INTERRUPTS & POWER SAVING MODES 3.1 CLOCK SYSTEM 3.1.1 General Description The MCU accepts either a crystal or ceramic reso- nator, or an external clock signal to drive the inter- nal oscillator. The internal clock (fCPU) is derived from the external oscillator frequency (fOSC). The external Oscillator clock is first divided by 2, and an additional division factor of 2, 4, 8, or 16 can be applied, in Slow Mode, to reduce the frequency of the fCPU; this clock signal is also routed to the on- chip peripherals. The CPU clock signal consists of a square wave with a duty cycle of 50%. The internal oscillator is designed to operate with an AT-cut parallel resonant quartz crystal resona- tor in the frequency range specified for fosc. The circuit shown in Figure 11 is recommended when using a crystal, and Table 5 lists the recommend- ed capacitance and feedback resistance values. The crystal and associated components should be mounted as close as possible to the input pins in order to minimize output distortion and start-up stabilisation time. Use of an external CMOS oscillator is recom- mended when crystals outside the specified fre- quency ranges are to be used. 3.1.2 External Clock An external clock may be applied to the OSCIN in- put with the OSCOUT pin not connected, as shown on Figure 10. Table 5 Recommended Values for 16 MHz Crystal Resonator (C0 < 7pF) RSMAX: Parasitic series resistance of the quartz crystal (upper limit). C0: Parasitic shunt capacitance of the quartz crys- tal (upper limit 7pF). COSCOUT, COSCIN: Maximum total capacitance on pins OSCIN and OSCOUT (the value includes the external capacitance tied to the pin plus the para- sitic capacitance of the board and of the device). Figure 10. External Clock Source Connections Figure 11. Crystal/Ceramic Resonator Figure 12. Clock Prescaler Block Diagram RSMAX 40 Ω 60 Ω 150 Ω COSCIN 56pF 47pF 22pF COSCOUT 56pF 47pF 22pF OSCIN OSCOUT EXTERNAL CLOCK NC OSCIN OSCOUT COSCIN COSCOUT OSCIN OSCOUT COSCIN COSCOUT %2 % 2, 4, 8, 16 fCPU to CPU and Peripherals 17 |
Similar Part No. - ST72E311_01 |
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Similar Description - ST72E311_01 |
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