AT90USB1286-16MU ATMEL [ATMEL Corporation], AT90USB1286-16MU Datasheet - Page 45

• Manufacturer Part Number: AT90USB1286-16MU
• Description: Microcontroller with 64/128K Bytes of ISP Flash and USB Controller
• Manufacturer: ATMEL [ATMEL Corporation]

6.5.1
6.6
7593A–AVR–02/06
128 kHz Internal Oscillator
Oscillator Calibration Register – OSCCAL
Table 6-8.
Note:
• 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 automat-
ically written to this register during chip reset, giving an oscillator frequency of 8.0 MHz at 25°C.
The application software can write this register to change the oscillator frequency. The oscillator
can be calibrated to any frequency in the range 7.3 - 8.1 MHz within ±1% accuracy. Calibration
outside that range is not guaranteed.
Note that this oscillator is used to time EEPROM and Flash write accesses, and these write
times will be affected accordingly. If the EEPROM or Flash are written, do not calibrate to more
than 8.8 MHz. Otherwise, the EEPROM or Flash write may fail.
The CAL7 bit determines the range of operation for the oscillator. Setting this bit to 0 gives the
lowest frequency range, setting this bit to 1 gives the highest frequency range. The two fre-
quency ranges are overlapping, in other words a setting of OSCCAL = 0x7F gives a higher
frequency than OSCCAL = 0x80.
The CAL6..0 bits are used to tune the frequency within the selected range. A setting of 0x00
gives the lowest frequency in that range, and a setting of 0x7F gives the highest frequency in the
range. Incrementing CAL6..0 by 1 will give a frequency increment of less than 2% in the fre-
quency range 7.3 - 8.1 MHz.
The 128 kHz internal Oscillator is a low power Oscillator providing a clock of 128 kHz. The fre-
quency is nominal at 3V and 25°C. This clock may be select as the system clock by
programming the CKSEL Fuses to “11” as shown in
Table 6-9.
Note:
Bit
Read/Write
Initial Value
Power Conditions
BOD enabled
Fast rising power
Slowly rising power
1. The device is shipped with this option selected.
1. The frequency is preliminary value. Actual value is TBD.
7
CAL7
R/W
Device Specific Calibration Value
Start-up times for the internal calibrated RC Oscillator clock selection
128 kHz Internal Oscillator Operating Modes
Nominal Frequency
6
CAL6
R/W
128 kHz
Start-up Time from Power-
down and Power-save
5
CAL5
R/W
Reserved
6 CK
6 CK
6 CK
4
CAL4
R/W
3
CAL3
R/W
Table
2
CAL2
R/W
Additional Delay from
Reset (V
14CK + 65 ms
14CK + 4.1 ms
6-9.
14CK
1
CAL1
R/W
CC
AT90USB64/128
CKSEL3..0
= 5.0V)
0011
(1)
0
CAL0
R/W
SUT1..0
OSCCAL
00
01
10
11
45