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ATMEGA644_0702 Datasheet(PDF) 33 Page - ATMEL Corporation |
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ATMEGA644_0702 Datasheet(HTML) 33 Page - ATMEL Corporation |
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33 / 374 page  33 2593L–AVR–02/07 ATmega644 Notes: 1. These options should only be used when not operating close to the maximum frequency of the device, and only if frequency stability at start-up is not important for the application. These options are not suitable for crystals. 2. These options are intended for use with ceramic resonators and will ensure frequency stability at start-up. They can also be used with crystals when not operating close to the maximum fre- quency of the device, and if frequency stability at start-up is not important for the application. 7.5 Low Frequency Crystal Oscillator The device can utilize a 32.768 kHz watch crystal as clock source by a dedicated Low Fre- quency Crystal Oscillator. The crystal should be connected as described in ”Clock Source Connections” on page 30. To find suitable load capacitance for a 32kHz crysal, please consult the crystal datasheet. When this Oscillator is selected, start-up times are determined by the SUT Fuses and CKSEL0 as shown in Table 7-7. Note: 1. These options should only be used if frequency stability at start-up is not important for the application. Ceramic resonator, slowly rising power 1K CK 14CK + 65 ms(2) 100 Crystal Oscillator, BOD enabled 16K CK 14CK 1 01 Crystal Oscillator, fast rising power 16K CK 14CK + 4.1 ms 1 10 Crystal Oscillator, slowly rising power 16K CK 14CK + 65 ms 1 11 Table 7-6. Start-up Times for the Full Swing Crystal Oscillator Clock Selection Oscillator Source / Power Conditions Start-up Time from Power-down and Power-save Additional Delay from Reset (V CC = 5.0V) CKSEL0 SUT1..0 Table 7-7. 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 (V CC = 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 |
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