Datasheet

45
7593L–AVR–09/12
AT90USB64/128
Note: 1. These options should only be used if frequency stability at start-up is not important for the
application.
7.5 Calibrated internal RC oscillator
The calibrated internal RC oscillator by default provides a 8.0MHz clock. The frequency is nomi-
nal value at 3V and 25°C. The device is shipped with the CKDIV8 Fuse programmed. See
“System clock prescaler” on page 47 for more details. This clock may be selected as the system
clock by programming the CKSEL Fuses as shown in Table 7-7. If selected, it will operate with
no external components. During reset, hardware loads the calibration byte into the OSCCAL
Register and thereby automatically calibrates the RC oscillator. At 3V and 25°C, this calibration
gives a frequency of 8MHz ±10%. The oscillator can be calibrated to any frequency in the range
7.3 - 8.1MHz within ±10% accuracy, by changing the OSCCAL register. 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 Section
“Calibration byte” on page 362
Notes: 1. The device is shipped with this option selected.
2. The frequency ranges are preliminary values. Actual values are TBD.
3. If 8MHz frequency exceeds the specification of the device (depends on V
CC
), the CKDIV8
Fuse can be programmed in order to divide the internal frequency by 8.
When this oscillator is selected, start-up times are determined by the SUT Fuses as show n in
Table 7-5 on page 44.
Note: 1. The device is shipped with this option selected.
7.5.1 OSCCAL – Oscillator Calibration Register
• Bits 7..0 – CAL7..0: Oscillator calibration value
The Oscillator Calibration Register is used to trim the calibrated internal RC oscillator to remove
process variations from the oscillator frequency. The factory-calibrated value is automatically
written to this register during chip reset, giv ing an oscillator frequency of 8.0MHz at 25°C. The
application software can write this register to change the oscillator frequency. The oscillator can
Table 7-7. Internal calibrated RC oscillator operating modes
(1)(3)
.
Frequency range
(2)
[MHz] CKSEL3..0
7.3 - 8.1 0010
Table 7-8. Start-up times for the internal calibrated RC oscillator 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 6CK 14CK 00
Fast rising power 6CK 14CK + 4.1ms 01
Slowly rising power 6CK 14CK + 65ms
(1)
10
Reserved 11
Bit 76543210
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