ATmega32A Datasheet - Sunrom Technologies

ATmega32A8.7 Calibrated Internal RC OscillatorThe Calibrated Internal RC Oscillator provides a fixed 1.0, 2.0, 4.0, or 8.0 MHz clock. All frequenciesare nominal values at 5V and 25°C. This clock may be selected as the system clock byprogramming the CKSEL fuses as shown in Table 8-8. If selected, it will operate with no externalcomponents. The CKOPT Fuse should always be unprogrammed when using this clock option.During Reset, hardware loads the calibration byte for the 1MHz into the OSCCAL Register andthereby automatically calibrates the RC Oscillator. At 5V, 25°C and 1.0MHz Oscillator frequencyselected, this calibration gives a frequency within ±3% of the nominal frequency. Using calibrationmethods as described in application notes available at www.atmel.com/avr it is possible toachieve ±1% accuracy at any given V CC and Temperature. When this Oscillator is used as theChip Clock, the Watchdog Oscillator will still be used for the Watchdog Timer and for the resettime-out. For more information on the pre-programmed calibration value, see the section “CalibrationByte” on page 269.Table 8-8.Internal Calibrated RC Oscillator Operating ModesCKSEL3:0Nominal Frequency (MHz)0001 (1) 1.00010 2.00011 4.00100 8.0Note: 1. The device is shipped with this option selected.When this Oscillator is selected, start-up times are determined by the SUT fuses as shown inTable 8-9. XTAL1 and XTAL2 should be left unconnected (NC).Table 8-9.Start-up Times for the Internal Calibrated RC Oscillator Clock SelectionSUT1:0Start-up Time fromPower-down andPower-saveAdditional Delayfrom Reset(V CC = 5.0V) Recommended Usage00 6 CK – BOD enabled01 6 CK 4.1ms Fast rising power10 (1) 6 CK 65ms Slowly rising power11 ReservedNote: 1. The device is shipped with this option selected.8.8 External ClockTo drive the device from an external clock source, XTAL1 should be driven as shown in Figure8-4 on page 31. To run the device on an external clock, the CKSEL fuses must be programmedto “0000”. By programming the CKOPT Fuse, the user can enable an internal 36pF capacitorbetween XTAL1 and GND.8155C–AVR–02/1130