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ATMEGA1281V データシート(PDF) 44 Page - ATMEL Corporation |
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ATMEGA1281V データシート(HTML) 44 Page - ATMEL Corporation |
44 / 449 page 44 ATmega640/1280/1281/2560/2561 2549K–AVR–01/07 Low Frequency Crystal Oscillator The device can utilize a 32.768 kHz watch crystal as clock source by a dedicated Low Frequency Crystal Oscillator. The crystal should be connected as shown in Figure 21. When this Oscillator is selected, start-up times are determined by the SUT Fuses and CKSEL0 as shown in Table 16. Note: 1. These options should only be used if frequency stability at start-up is not important for the application. Calibrated Internal RC Oscillator By defaylt, the Internal RC Oscillator provides an approximate 8 MHz clock. Though volt- age and temperature dependent, this clock can be very accurately calibrated by the user. See Table 172 on page 384 and “Internal Oscillator Speed” on page 409 for more details. The device is shipped with the CKDIV8 Fuse programmed. See “System Clock Prescaler” on page 47 for more details. This clock may be selected as the system clock by programming the CKSEL Fuses as shown in Table 17. If selected, it will operate with no external components. During reset, hardware loads the pre-programmed calibration value into the OSCCAL Register and thereby automatically calibrates the RC Oscillator. The accuracy of this calibration is shown as Factory calibration in Table 172 on page 384. By changing the OSCCAL register from SW, see “OSCCAL – Oscillator Calibration Reg- ister” on page 48, it is possible to get a higher calibration accuracy than by using the factory calibration. The accuracy of this calibration is shown as User calibration in Table 172 on page 384. When this Oscillator is used as the chip clock, the Watchdog Oscillator will still be used for the Watchdog Timer and for the Reset Time-out. For more information on the pre- programmed calibration value, see the section “Calibration Byte” on page 345. Notes: 1. The device is shipped with this option selected. 2. The frequency ranges are preliminary values. Actual values are TBD. 3. If 8 MHz frequency exceeds the specification of the device (depends on V CC), the CKDIV8 Fuse can be programmed in order to divide the internal frequency by 8. When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in Table 18 on page 45. Table 16. Start-up Times for the Low Frequency Crystal Oscillator Clock Selection Power Conditions Start-up Time from Power-down and Power-save Additional Delay from Reset (VCC = 5.0V) CKSEL0 SUT1:0 BOD enabled 1K CK 14CK(1) 000 Fast rising power 1K CK 14CK + 4.1 ms(1) 001 Slowly rising power 1K CK 14CK + 65 ms(1) 010 Reserved 0 11 BOD enabled 32K CK 14CK 1 00 Fast rising power 32K CK 14CK + 4.1 ms 1 01 Slowly rising power 32K CK 14CK + 65 ms 1 10 Reserved 1 11 Table 17. Internal Calibrated RC Oscillator Operating Modes (1)(3) Frequency Range(2) (MHz) CKSEL3:0 7.3 - 8.1 0010 |
同様の部品番号 - ATMEGA1281V |
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同様の説明 - ATMEGA1281V |
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