ATMega chip full datasheet - UCSD Department of Physics

ATmega48A/PA/88A/PA/168A/PA/328/P9.7 128kHz Internal OscillatorThe 128kHz internal Oscillator is a low power Oscillator providing a clock of 128kHz. The frequencyis nominal at 3V and 25°C. This clock may be select as the system clock byprogramming the CKSEL Fuses to “11” as shown in Table 9-13.Table 9-13.128kHz Internal Oscillator Operating ModesNominal Frequency (1)CKSEL3...0128kHz 0011Note: 1. Note that the 128kHz oscillator is a very low power clock source, and is not designed for highaccuracy.When this clock source is selected, start-up times are determined by the SUT Fuses as shown inTable 9-14.Table 9-14.Start-up Times for the 128kHz Internal OscillatorPower ConditionsStart-up Time from Powerdownand Power-saveAdditional Delay fromResetSUT1...0BOD enabled 6 CK 14CK (1) 00Fast rising power 6 CK 14CK + 4ms 01Slowly rising power 6 CK 14CK + 64ms 10Reserved 11Note: 1. If the RSTDISBL fuse is programmed, this start-up time will be increased to14CK + 4.1ms to ensure programming mode can be entered.9.8 External ClockTo drive the device from an external clock source, XTAL1 should be driven as shown in Figure9-4. To run the device on an external clock, the CKSEL Fuses must be programmed to “0000”(see Table 9-15).Table 9-15.Crystal Oscillator Clock FrequencyFrequency CKSEL3...00 - 20MHz 0000Figure 9-4.External Clock Drive ConfigurationPB7XTAL2EXTERNALCLOCKSIGNALXTAL1GNDWhen this clock source is selected, start-up times are determined by the SUT Fuses as shown inTable 9-16.8271D–AVR–05/1135