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ATMEGA8515-16AU Datasheet(PDF) 38 Page - ATMEL Corporation |
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ATMEGA8515-16AU Datasheet(HTML) 38 Page - ATMEL Corporation |
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38 / 254 page  38 ATmega8515(L) 2512G–AVR–03/05 Calibrated Internal RC Oscillator The calibrated internal RC Oscillator provides a fixed 1.0, 2.0, 4.0, or 8.0 MHz clock. All frequencies are nominal values at 5V and 25 °C. This clock may be selected as the sys- tem clock by programming the CKSEL Fuses as shown in Table 12. If selected, it will operate with no external components. The CKOPT Fuse should always be unpro- grammed when using this clock option. During reset, hardware loads the calibration byte into the OSCCAL Register 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 run-time calibration methods as described in application notes available at www.atmel.com/avr it is possible to achieve ± 1% accu- racy 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 179. 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 13. XTAL1 and XTAL2 should be left unconnected (NC). Note: 1. The device is shipped with this option selected. Oscillator Calibration Register – OSCCAL • Bits 7..0 – CAL7..0: Oscillator Calibration Value Writing the calibration byte to this address will trim the internal Oscillator to remove pro- cess variations from the Oscillator frequency. During Reset, the 1 MHz calibrated 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 calibration 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 Table 12. 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 13. Start-up Times for the Internal Calibrated RC Oscillator Clock Selection SUT1..0 Start-up Time from Power-down Additional Delay from Reset (V CC = 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 7654 321 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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