ATmega16U4 datasheet(33/408 Pages) ATMEL

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7766D–AVR–01/09

ATmega16U4/ATmega32U4

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 ATmega16U4/ATmega32U4 allows such an operation, that is detailed on Section

24.6.1 ”Sensor Calibration” on page 301. It is then possible to calibrate the oscillator frequency

in function of the 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.

6.5.2

Oscillator Control Register – RCCTRL

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).

6.6

External Clock

The device can utilize a external clock source as shown in Figure 6-3. To run the device on an

external clock, the CKSEL Fuses must be programmed as shown in Table 6-1.

Figure 6-3.

External Clock Drive Configuration

When this clock source is selected, start-up times are determined by the SUT Fuses as shown in

Table 6-9.

Bit

7654

----

RCFREQ

RCCTRL

Read/Write

RRRR

R/W

Initial Value

0000

EXTERNAL

CLOCK

SIGNAL

XTAL2

XTAL1

GND

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