atmega32u4-16mu ATMEL Corporation, atmega32u4-16mu Datasheet - Page 31

• Manufacturer Part Number: atmega32u4-16mu
• Description: Atmega32u4 8-bit Avr Microcontroller With 32k Bytes Of Isp Flash And Usb Controller
• Manufacturer: ATMEL Corporation

6.5
6.5.1
7766A–AVR–03/08
Calibrated Internal RC Oscillator
Oscillator Calibration Register – OSCCAL
Note:
The calibrated internal RC Oscillator by default provides a 8.0 MHz clock. This frequency is
nominal value at 3V and 25°C. The device is shipped with the CKDIV8 Fuse programmed. See
“System Clock Prescaler” on page 36
clock by programming the CKSEL Fuses as shown in
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 8 MHz ± 1%. The oscillator can be calibrated to any frequency in the range
7.3 - 8.1 MHz within ±1% 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 the sec-
tion
Table 6-7.
Notes:
When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in
Table 6-5 on page
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 calibra-
Bit
Read/Write
Initial Value
Power Conditions
BOD enabled
Fast rising power
Slowly rising power
“Calibration Byte” on page 368
1. These options should only be used if frequency stability at start-up is not important for the
1. The device is shipped with this option selected.
2. The frequency ranges are preliminary values. Actual values are TBD.
3. If 8 MHz frequency exceeds the specification of the device (depends on V
1. The device is shipped with this option selected.
application.
Fuse can be programmed in order to divide the internal frequency by 8.
7
CAL7
R/W
Device Specific Calibration Value
Frequency Range
Internal Calibrated RC Oscillator Operating Modes
Start-up times for the internal calibrated RC Oscillator clock selection
30.
6
CAL6
R/W
7.3 - 8.1
Start-up Time from Power-
down and Power-save
5
CAL5
R/W
(2)
(MHz)
Reserved
6 CK
6 CK
6 CK
for more details. This clock may be selected as the system
4
CAL4
R/W
3
CAL3
R/W
2
CAL2
R/W
Table
Additional Delay from
Reset (V
14CK + 65 ms
14CK + 4.1 ms
6-7. If selected, it will operate with
14CK
1
CAL1
R/W
CC
(1)(3)
CKSEL[3:0]
= 5.0V)
0010
ATmega32U4
(1)
0
CAL0
R/W
CC
SUT1..0
OSCCAL
), the CKDIV8
00
01
10
11
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