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C8051F300 Datasheet(PDF) 95 Page - List of Unclassifed Manufacturers |
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C8051F300 Datasheet(HTML) 95 Page - List of Unclassifed Manufacturers |
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95 / 174 page  C8051F300/1/2/3/4/5 Rev. 2.6 95 11.2. External Oscillator Drive Circuit The external oscillator circuit may drive an external crystal, ceramic resonator, capacitor, or RC network. A CMOS clock may also provide a clock input. For a crystal or ceramic resonator configuration, the crys- tal/resonator must be wired across the XTAL1 and XTAL2 pins as shown in Option 1 of Figure 11.1. A 10 M Ω resistor also must be wired across the XTAL2 and XTAL1 pins for the crystal/resonator configura- tion. In RC, capacitor, or CMOS clock configuration, the clock source should be wired to the XTAL2 pin as shown in Option 2, 3, or 4 of Figure 11.1. The type of external oscillator must be selected in the OSCXCN register, and the frequency control bits (XFCN) must be selected appropriately (see SFR Definition 11.3). Important Note on External Oscillator Usage: Port pins must be configured when using the external oscillator circuit. When the external oscillator drive circuit is enabled in crystal/resonator mode, Port pins P0.2 and P0.3 are occupied as XTAL1 and XTAL2 respectively. When the external oscillator drive circuit is enabled in capacitor, RC, or CMOS clock mode, Port pin P0.3 is occupied as XTAL2. The Port I/O Cross- bar should be configured to skip the occupied Port pins; see Section “12.1. Priority Crossbar Decoder” on page 100 for Crossbar configuration. Additionally, when using the external oscillator circuit in crys- tal/resonator, capacitor, or RC mode, the associated Port pins should be configured as analog inputs. In CMOS clock mode, the associated pin should be configured as a digital input. See Section “12.2. Port I/O Initialization” on page 102 for details on Port input mode selection. 11.3. System Clock Selection The CLKSL bit in register OSCICN selects which oscillator is used as the system clock. CLKSL must be set to ‘1’ for the system clock to run from the external oscillator; however the external oscillator may still clock peripherals (timers, PCA) when the internal oscillator is selected as the system clock. The system clock may be switched on-the-fly between the internal and external oscillator, so long as the selected oscil- lator is enabled and has settled. The internal oscillator requires little start-up time and may be enabled and selected as the system clock in the same write to OSCICN. External crystals and ceramic resonators typi- cally require a start-up time before they are settled and ready for use as the system clock. The Crystal Valid Flag (XTLVLD in register OSCXCN) is set to ‘1’ by hardware when the external oscillator is settled. To avoid reading a false XTLVLD, in crystal mode software should delay at least 1 ms between enabling the external oscillator and checking XTLVLD. RC and C modes typically require no start-up time. Table 11.1. Internal Oscillator Electrical Characteristics –40 to +85 °C unless otherwise specified Parameter Conditions Min Typ Max Units Calibrated Internal Oscillator Frequency C8051F300/1 devices –40 to +85 °C 24 24.5 25 MHz C8051F300/1 devices 0 to +70 °C 24.3 24.7 25 MHz Uncalibrated Internal Oscillator Frequency C8051F302/3/4/5 devices 16 20 24 MHz Internal Oscillator Supply Current (from VDD) OSCICN.2 = 1 450 µA |
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