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전자부품 데이터시트 검색엔진 |
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전자부품 데이터시트 검색엔진 |
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ATMEGA128 데이터시트(PDF) 42 Page - ATMEL Corporation |
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ATMEGA128 데이터시트(HTML) 42 Page - ATMEL Corporation |
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42 / 386 page  42 2467R–AVR–06/08 ATmega128 Calibrated Internal RC Oscillator The Calibrated Internal RC Oscillator provides a fixed 1.0, 2.0, 4.0, or 8.0 MHz clock. All fre- quencies are nominal values at 5V and 25 °C. This clock may be selected as the system clock by programming the CKSEL fuses as shown in Table 13. If selected, it will operate with no external components. The CKOPT fuse should always be unprogrammed when using this clock option. During Reset, hardware loads the calibration byte for the 1MHz oscillator into the OSCCAL Reg- ister and thereby automatically calibrates the RC Oscillator. At 5V, 25 °C and 1.0 MHz Oscillator frequency selected, this calibration gives a frequency within ± 3% of the nominal frequency. Using calibration methods as described in application notes available at www.atmel.com/avr it is possible to achieve ± 1% accuracy at any given V CC and Temperature. When this Oscillator is used as the chip clock, the Watchdog Oscillator will still be used for the Watchdog Timer and for the Reset Time-out. For more information on the pre-programmed calibration value, see the sec- tion “Calibration Byte” on page 289. Note: 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 in Table 14. XTAL1 and XTAL2 should be left unconnected (NC). Notes: 1. The device is shipped with this option selected. Oscillator Calibration Register – OSCCAL Note: OSCCAL Register is not available in ATmega103 compatibility mode. • Bits 7..0 – CAL7..0: Oscillator Calibration Value Writing the calibration byte to this address will trim the Internal Oscillator to remove process vari- ations from the Oscillator frequency. During Reset, the 1 MHz calibration value which is located in the signature row high byte (address 0x00) is automatically loaded into the OSCCAL Register. If the internal RC is used at other frequencies, the calibration values must be loaded manually. This can be done by first reading the signature row by a programmer, and then store the calibra- tion values in the Flash or EEPROM. Then the value can be read by software and loaded into the OSCCAL Register. When OSCCAL is zero, the lowest available frequency is chosen. Writing non-zero values to this register will increase the frequency of the Internal Oscillator. Writing $FF Table 13. Internal Calibrated RC Oscillator Operating Modes CKSEL3..0 Nominal Frequency (MHz) 0001(1) 1.0 0010 2.0 0011 4.0 0100 8.0 Table 14. Start-up Times for the Internal Calibrated RC Oscillator Clock Selection SUT1..0 Start-up Time from Power- down and Power-save Additional Delay from Reset (VCC = 5.0V) Recommended Usage 00 6 CK – BOD enabled 01 6 CK 4.1 ms Fast rising power 10(1) 6 CK 65 ms Slowly rising power 11 Reserved Bit 765 4321 0 CAL7 CAL6 CAL5 CAL4 CAL3 CAL2 CAL1 CAL0 OSCCAL Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value Device Specific Calibration Value |
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