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ATMEGA8-16PU 데이터시트(PDF) 28 Page - ATMEL Corporation |
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ATMEGA8-16PU 데이터시트(HTML) 28 Page - ATMEL Corporation |
28 / 308 page 28 2486T–AVR–05/08 ATmega8(L) 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. Low-frequency Crystal Oscillator To use a 32.768 kHz watch crystal as the clock source for the device, the Low-frequency Crystal Oscillator must be selected by setting the CKSEL Fuses to “1001”. The crystal should be con- nected as shown in Figure 11. By programming the CKOPT Fuse, the user can enable internal capacitors on XTAL1 and XTAL2, thereby removing the need for external capacitors. The inter- nal capacitors have a nominal value of 36 pF. When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in Table 6. Note: 1. These options should only be used if frequency stability at start-up is not important for the application. External RC Oscillator For timing insensitive applications, the external RC configuration shown in Figure 12 can be used. The frequency is roughly estimated by the equation f = 1/(3RC). C should be at least 22 Table 5. Start-up Times for the Crystal Oscillator Clock Selection CKSEL0 SUT1..0 Start-up Time from Power-down and Power-save Additional Delay from Reset (VCC = 5.0V) Recommended Usage 0 00 258 CK(1) 4.1 ms Ceramic resonator, fast rising power 0 01 258 CK(1) 65 ms Ceramic resonator, slowly rising power 010 1K CK(2) – Ceramic resonator, BOD enabled 011 1K CK(2) 4.1 ms Ceramic resonator, fast rising power 100 1K CK(2) 65 ms Ceramic resonator, slowly rising power 101 16K CK – Crystal Oscillator, BOD enabled 1 10 16K CK 4.1 ms Crystal Oscillator, fast rising power 1 11 16K CK 65 ms Crystal Oscillator, slowly rising power Table 6. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection SUT1..0 Start-up Time from Power-down and Power-save Additional Delay from Reset (V CC = 5.0V) Recommended Usage 00 1K CK(1) 4.1 ms Fast rising power or BOD enabled 01 1K CK(1) 65 ms Slowly rising power 10 32K CK 65 ms Stable frequency at start-up 11 Reserved |
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