atmega32u4-16mu ATMEL Corporation, atmega32u4-16mu Datasheet - Page 32

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

6.5.2
6.6
32
External Clock
ATmega32U4
Oscillator Control Register – RCCTRL
tion range is +/- 40% and linear (calibration step ~0.4%). With typical process at 25°C the code
should be 127 for 8 MHz. Input value of 0x00 gives the lowest frequency, and 0xFF the highest.
The temperature sensitivity is quite linear but as said previously depends on the process. To
determine its slope, the frequency must be measured at two temperatures. The temperature
sensor of the ATmega32U4 allows such an operation, that is detailed on
Calibration” on page
temperature measured.
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.
Bits 7..1 – Reserved
Do not set these bits. Bits should be read as ‘0’.
Bit 0– RCFREQ: RC Oscillator Frequency Select
When this bit is cleared (default value), the RC Oscillator output frequency is set to 8 MHz.
When the bit is set, the RC output frequency is 1 MHz. Note that the OSCCAL value has the
same effect on both 8 MHz and 1 MHz output modes (~0.4% / step).
The device can utilize a external clock source as shown in
external clock, the CKSEL Fuses must be programmed as shown in
Figure 6-3.
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
Table
Bit
Read/Write
Initial Value
6-9.
7
-
R
0
External Clock Drive Configuration
318. It is then possible to calibrate the oscillator frequency in function of the
6
-
R
0
EXTERNAL
SIGNAL
CLOCK
5
-
R
0
NC
4
-
R
0
3
-
R
0
XTAL2
XTAL1
GND
2
-
R
0
Figure
1
-
R
0
6-3. To run the device on an
Table
0
RCFREQ
R/W
0
Section 25.6.1 ”Sensor
6-1.
RCCTRL
7766A–AVR–03/08