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ATMEGA88 Datasheet(PDF) 32 Page - ATMEL Corporation |
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ATMEGA88 Datasheet(HTML) 32 Page - ATMEL Corporation |
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32 / 349 page  32 ATmega48/88/168 2545D–AVR–07/04 Figure 15. External Clock Drive Configuration When this clock source is selected, start-up times are determined by the SUT Fuses as shown in Table 16. When applying an external clock, it is required to avoid sudden changes in the applied clock frequency to ensure stable operation of the MCU. A variation in frequency of more than 2% from one clock cycle to the next can lead to unpredictable behavior. If changes of more than 2% is required, ensure that the MCU is kept in Reset during the changes. Note that the System Clock Prescaler can be used to implement run-time changes of the internal clock frequency while still ensuring stable operation. Refer to “System Clock Prescaler” on page 33 for details. Clock Output Buffer The device can output the system clock on the CLKO pin. To enable the output, the CKOUT Fuse has to be programmed. This mode is suitable when the chip clock is used to drive other circuits on the system. The clock also will be output during reset, and the normal operation of I/O pin will be overridden when the fuse is programmed. Any clock source, including the internal RC Oscillator, can be selected when the clock is output on CLKO. If the System Clock Prescaler is used, it is the divided system clock that is output. Timer/Counter Oscillator The device can operate its Timer/Counter2 from an external 32.768 kHz watch crystal or a external clock source. The Timer/Counter Oscillator Pins (TOSC1 and TOSC2) are shared with XTAL1 and XTAL2. This means that the Timer/Counter Oscillator can only be used when an internal RC Oscillator is selected as system clock source. See Figure 13 on page 26 for crystal connection. Applying an external clock source to TOSC1 requires EXTCLK in the ASSR Register written to logic one. See “Asynchronous operation of the Timer/Counter” on page 149 for further description on selecting external clock as input instead of a 32 kHz crystal. Table 16. Start-up Times for the External Clock Selection Power Conditions Start-up Time from Power- down and Power-save Additional Delay from Reset (V CC = 5.0V) SUT1..0 BOD enabled 6 CK 14CK 00 Fast rising power 6 CK 14CK + 4.1 ms 01 Slowly rising power 6 CK 14CK + 65 ms 10 Reserved 11 NC EXTERNAL CLOCK SIGNAL XTAL2 XTAL1 GND |
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