ATMega chip full datasheet - UCSD Department of Physics

ATmega48A/PA/88A/PA/168A/PA/328/PTable 9-10.CKSEL3...0Start-up Times for the Low-frequency Crystal Oscillator Clock SelectionStart-up Time fromPower-down and Power-save0100 (1) 1K CKRecommended Usage0101 32K CK Stable frequency at start-upNote: 1. This option should only be used if frequency stability at start-up is not important for theapplication9.6 Calibrated Internal RC OscillatorBy default, the Internal RC Oscillator provides an approximate 8.0MHz clock. Though voltageand temperature dependent, this clock can be very accurately calibrated by the user. See Table29-10 on page 323 for more details. The device is shipped with the CKDIV8 Fuse programmed.See ”System Clock Prescaler” on page 36 for more details.This clock may be selected as the system clock by programming the CKSEL Fuses as shown inTable 9-11. If selected, it will operate with no external components. During reset, hardware loadsthe pre-programmed calibration value into the OSCCAL Register and thereby automatically calibratesthe RC Oscillator. The accuracy of this calibration is shown as Factory calibration in Table29-10 on page 323.By changing the OSCCAL register from SW, see ”OSCCAL – Oscillator Calibration Register” onpage 38, 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 29-10 on page 323.When this Oscillator is used as the chip clock, the Watchdog Oscillator will still be used for theWatchdog Timer and for the Reset Time-out. For more information on the pre-programmed calibrationvalue, see the section ”Calibration Byte” on page 302.Table 9-11.Notes: 1. The device is shipped with this option selected.2. If 8MHz frequency exceeds the specification of the device (depends on V CC ), the CKDIV8Fuse 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 inTable 9-12.Table 9-12.Power ConditionsInternal Calibrated RC Oscillator Operating ModesFrequency Range (2) (MHz) CKSEL3...07.3 - 8.1 0010 (1)Start-up times for the internal calibrated RC Oscillator clock selectionStart-up Time from Powerdownand Power-saveAdditional Delay fromReset (V CC = 5.0V)Note: 1. If the RSTDISBL fuse is programmed, this start-up time will be increased to14CK + 4.1ms to ensure programming mode can be entered.2. The device is shipped with this option selected.SUT1...0BOD enabled 6 CK 14CK (1) 00Fast rising power 6 CK 14CK + 4.1ms 01Slowly rising power 6 CK 14CK + 65ms (2) 10Reserved 118271D–AVR–05/1134