ATmega32U2 datasheet(29/314 Pages) ATMEL

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7799B–AVR–07/09

ATmega8U2/16U2/32U2

8.3

Clock Sources

The device has the following clock source options, selectable by Flash Fuse bits as shown

below. The clock from the selected source is input to the AVR clock generator, and routed to the

appropriate modules.

Note:

1. For all fuses “1” means unprogrammed while “0” means programmed.

8.3.1

Default Clock Source

The device is shipped with internal RC oscillator at 8.0 MHz and with the fuse CKDIV8 pro-

grammed, resulting in 1.0 MHz system clock. The startup time is set to maximum and time-out

period enabled. (CKSEL = "0010", SUT = "10", CKDIV8 = "0"). The default setting ensures that

all users can make their desired clock source setting using any available programming interface.

8.3.2

Clock Startup Sequence

Any clock source needs a sufficient V

CC to start oscillating and a minimum number of oscillating

cycles before it can be considered stable.

To ensure sufficient V

CC, the device issues an internal reset with a time-out delay (tTOUT) after

the device reset is released by all other reset sources. “On-chip Debug System” on page 45

describes the start conditions for the internal reset. The delay (t

TOUT) is timed from the Watchdog

Oscillator and the number of cycles in the delay is set by the SUTx and CKSELx fuse bits. The

selectable delays are shown in Table 8-2. The frequency of the Watchdog Oscillator is voltage

dependent as shown in “Typical Characteristics” on page 273.

Main purpose of the delay is to keep the AVR in reset until it is supplied with minimum Vcc. The

delay will not monitor the actual voltage and it will be required to select a delay longer than the

Vcc rise time. If this is not possible, an internal or external Brown-Out Detection circuit should be

used. A BOD circuit will ensure sufficient Vcc before it releases the reset, and the time-out delay

can be disabled. Disabling the time-out delay without utilizing a Brown-Out Detection circuit is

not recommended.

Table 8-1.

Device Clocking Options Select

(1)

Device Clocking Option

CKSEL3:0

Low Power Crystal Oscillator

1111 - 1000

Full Swing Crystal Oscillator

0111 - 0110

Reserved

0101 - 0100

Reserved

0011

Calibrated Internal RC Oscillator

0010

External Clock

0000

Reserved

0001

Table 8-2.

Number of Watchdog Oscillator Cycles

Typ Time-out (V

CC = 5.0V)

Typ Time-out (V

CC = 3.0V)

Number of Cycles

0 ms

4.1 ms

4.3 ms

512

65 ms

69 ms

8K (8,192)

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